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Laboratory Manual

Penelope L. Kuhn

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TABLE OF CONTENTS

PAGE

INTRODUCTION 3

MODULE 1: The Basics

1A: Nervous System Organization

1B: Anatomical Orientation

1C : Terminology

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4

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MODULE 2: The Meninges

MODULE 3: Dorsal View

MODULE 4: Lateral View

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11

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MODULE 5: Cerebellum 15

MODULE 6: Cranial Nerves 18

MODULE 7: Ventral View 22

MODULE 8: Sagittal View 24

MODULE 9: Horizontal View 30

MODULE 10: Coronal View 35

MODULE 11: Systems

11A: Basal Ganglia System 45

11B: Circle of Willis 47

11C: Limbic System 48

11D: Reticular Formation 51

11E: Ventricular System 53

MODULE 12: Spinal Cord 55

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INTRODUCTION

eBrain is a neuroanatomy tutorial using the sheep brain for dissections to identify

structures. Associating brain structure with a known or hypothesized function is the study of

behavioral or functional neuroanatomy. The sheep brain is a low-cost alternative to using human

brain and yet is close enough in comparative anatomy to provide a good understanding of basic

human brain structure. Ovine brains are readily available as a by-product of the food industry

and have a safer profile than bovine brain tissue. The size makes them easy to manipulate and

dissect, with features and structures large enough to examine and identify. Dissecting the sheep

brain is typically a comparative anatomy exercise, therefore discussions on structure function

relationships in this tutorial focus on general human functions associated with neural structures

and systems.

This laboratory manual is meant to accompany the interactive eBrain software. It

contains all of the text in the Modules and several key figures. Always refer to the software

module for the complete set of figures and use the NeuroGlossary for additional information on

specific structures and the terminology.

MODULE 1: THE BASICS

A. Nervous System Organization

The study of neuroanatomy requires a basic understanding of nervous system

organization. There are several strategies available; a few are presented here. One advantage of

using these strategies is that the material can be studied and memorized in "chunks." Specific

anatomical terms can then be added to each corresponding chunk as your study progresses

throughout the tutorial. This method makes it easier for you to assimilate the information so that

you can understand it, organize it conceptually, and retrieve it later for use.

Nervous System Divisions There are two branches of the nervous system, the central

nervous system (CNS) and peripheral nervous system (PNS). This tutorial focuses primarily on

the central nervous system. The central nervous system includes the brain and spinal cord.

Brain Divisions The brain can be divided into three general areas; forebrain, midbrain,

and hindbrain. Each contains specific structural elements that interact with each other. This

strategy is further defined by embryonic divisions.

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Embryonic Divisions Another organizational strategy for studying the brain is based on

embryonic development. Specific segments of developing neural tissue give rise to groups of

structures. This system overlaps well with the basic brain divisions. For example, the forebrain

consists of the telencephalon and diencephalon. Structures here include the cerebral cortex, basal

ganglia, limbic system, hypothalamus and thalamus. The midbrain consists of the

mesencephalon, so structures in this area include the tectum and tegmentum. The hindbrain

consists of the metencephalon and myelencephalon. These areas include structures such as the

cerebellum, pons, and medulla.

Take advantage of the NeuroGlossary for more information on the Greek and Latin

origins of these terms, and the brain structures included in each division.

B. Anatomical Orientation

The study of anatomy requires fluency with terminology that directs your attention

toward a particular position in three-dimensional space. You might initially think that the sheep

brain appears to be a rather monochromatic, amorphous blob. But as you will see, some

landmarks and structures will be easily identifiable. For other structures, understanding relative

spatial positions and identifying landmarks will be crucial for locating the anatomy.

Planes of Dissection The brain is spatially oriented in the three-dimensional planes for

either dissection or medical imaging. The coronal plane is parallel to the face, whereas the

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horizontal plane is parallel to the floor. The sagittal plane divides the two hemispheres and is

parallel to the midline surface. Midsagittal refers to the exact midline whereas parasagittal refers

to any section within the plane that is parallel to the midline.

Directional Orientation Relative

direction is described by pairs of terms within a

plane that describe movement along the

longitudinal axis of the CNS. Rostral refers to

movement toward the "beak" or nose, caudal

refers to movement toward the tail. Dorsal

refers to movement toward the top or back, such

as a dorsal fin on a dolphin and ventral refers to

the front or stomach. Medial refers to

movement toward the midline and lateral refers to movement toward the side. These terms are

relative; that is, your elbow is medial to your hand but lateral to your shoulder.

Nomenclature in Humans Compared to quadrupeds, standing upright results in a 120°

angle in the longitudinal axis. Therefore, additional terms such as superior, inferior, anterior, and

posterior are used to describe directional orientation in bipeds like humans. Sometimes these

terms are used in naming brain structures, such as the inferior colliculus, which is ventral to the

superior colliculus.

C. Terminology

There are a variety of recurring themes in the conventions used to name brain structures

such as nuclei, tracts, and systems. The nomenclature used is often a clue as to the place of origin

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and termination, or the shape, direction, or function of the anatomical feature. Several examples

are provided here, but notice these naming conventions as you progress through the tutorial.

Nuclei Neuroanatomical structures often consist of nuclei. In the brain, a nucleus is a

collection of neuronal cell bodies and dendrites (and associated glia). Typically, neurons in one

nucleus have a similar target and/or related function. The names of many nuclei have Greek or

Latin origins, often based on perceived shape or color. For example, the hippocampus [Greek;

seahorse] has a folded shape that resembles a seahorse, and the locus coeruleus [Latin; blue

place] is named for a particular shade of blue.

Many neuroanatomical structures in the brain are actually a collection of functionally

related nuclei. For example, the hypothalamus is composed of several specific nuclei that each

participate in different features of autonomic nervous system and endocrine regulation.

The anatomy in the peripheral nervous system analogous to a nucleus is a ganglion

(ganglia, plural).

Tracts Another structural feature of the brain is the tract. A tract is a collection of axons

that have a shared neuronal origin and project to a similar target. Tracts are often myelinated, and

can merge or diverge with each other on route to a target. Tract names are based on point of

origin and termination. For example, the mammillothalamic tract originates primarily from

neurons in the mammillary bodies (a nucleus in the hypothalamus) that project axons to the

thalamus. Tracts are also identified by their direction of travel; for example, there are afferent

(projecting toward the brain, as in sensory inputs), efferent (projecting away from the brain, as in

motor outputs), ipsilateral (same sided), and contralateral (opposite sided) projections that

decussate (cross over to the opposite side). The anatomy in the peripheral nervous system

analogous to a tract is called a nerve.

Commissures There are several large tracts called commissures that specifically connect

functions between the right and left hemispheres. The largest of these is the corpus callosum.

You will study commissures in Module 8: Sagittal View because the midsagittal section is the

best perspective from which to view these. The labeled anatomy here will be discussed further in

that module.

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Systems Systems are collections of nuclei and tracts that provide a specific function; for

example, motor, visual, or somatosensory systems. Systems are also named based on location.

The limbic system [Latin; border] is involved in emotional regulation and includes several nuclei

and structures that border the thalamus. There are many functional systems in the CNS and they

often share structures. For example, the amygdala is often considered part of both the basal

ganglia and limbic system. Interestingly, the basal ganglia system is in the brain, NOT in the

peripheral nervous system as the term "ganglia" would suggest.

Matter It will become apparent as you dissect the sheep brain that there are two visually

distinct types of tissue, commonly called gray matter and white matter. Gray matter consists of

nuclei and cortices (the cerebral and cerebellar cortex)[Greek; outer layer or bark], whereas

white matter primarily consists of myelinated axons, tracts, or commissures. Myelin is a

membrane provided by oligodendrocytes that speeds the conduction of electrical signals down

the neuronal axon. Myelinated axons appear white. As you dissect the tissue, you will notice

patterns. The white matter pattern in the cerebrum is called the corona radiata, and the white

matter pattern in the cerebellum is called the arbor vitae.

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Glial cells, typically astrocytes and oligodendrocytes, are a vital part of the nervous

system. They are located profusely throughout the brain and spinal cord. Unfortunately, they are

not a subject for this tutorial.

MODULE 2: THE MENINGES

Sheep brains with the meninges intact can be purchased from vendors. This membrane

network is an important component to studying neuroanatomy.

The meninges are a major source of protection for the brain and spinal cord; these

membranes encase the entire central nervous system. Dissecting the meninges is of course

necessary in order to study the underlying neuroanatomy in detail. The dissection exercise also

provides an examination of the meningeal anatomy and understanding of its functional

significance.

Notice the initial appearance of the meninges. The thick white outer layer you can

immediately see is called the dura mater. Your sheep brain may also have the remnants of the

eyes still attached; cut these as shown in the Video Dissection. Before proceeding with the rest of

the video, browse the information in this module.

The meninges serve to protect the brain and spinal cord in terms of shock absorption,

architectural stability, and as a chemical barrier. A layer of cerebrospinal fluid is directly under

the meninges, providing a fluid environment to protect delicate central nervous system tissues.

Under normal conditions, the fluid prevents the brain and spinal cord from hitting the hard bony

surfaces surrounding them.

The sheep brains purchased from biological supply vendors have undergone tissue

fixation with a preservative. Typically this is some form of formaldehyde solution. If brain tissue

is not "fixed" and the meninges are removed, architectural stability is destroyed and the brain

would appear to melt and spread out into a sticky mass. By fixing the tissue, we can examine

structures more closely and even cut into the tissue, retaining the natural tissue shape and

architecture. In the living brain, the architecture and separation between tissue structures is

maintained by the meninges.

Layers The meninges (meninx, singular) consist of three layers of membrane; the dura

mater, arachnoid, and pia mater. The outer-most layer closest to the cranium is the dura mater,

the toughest layer of the meninges. This is a very strong and resilient type of tissue. Beneath the

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dura is the subdural space. A subdural hematoma is a collection of blood in this space, typically

due to mechanical injury.

The middle meninx is called the arachnoid membrane, named for its spider web-like

appearance. The subarachnoid space is filled with blood vessels. The choroid plexus is a

specialized vascular structure that extends from the arachnoid into the ventricles (see Module 11:

Systems) and filters blood for making cerebrospinal fluid (CSF). The subarachnoid space,

ventricles, and cisterns are filled with CSF. In later dissections you will see a lot of choroid

plexus material in the lateral ventricles and third ventricle. It has a dark, tufted appearance.

Cerebrospinal fluid circulates throughout the brain and spinal cord and is recycled

through vascular spaces such as the superior sagittal sinus. This vascular structure is encased in

dura. The arachnoid membrane and has clusters of specialized cells called granulations or villi

that extend from the arachnoid into the superior sagittal sinus in order to return CSF to the

venous blood supply. This enables the CSF to continuously circulate.

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The pia mater is the innermost, delicate layer of the meninges most closely associated

with the neural surface. When dissected from the brain, the depths of the sulci can be explored. If

not dissected, the sulci and fissures often have overlying blood vessels that have a dark

appearance. The pia has not been dissected or removed in eBrain.

Folds The dura mater has specific folds that contribute to architectural stability. The

tentorium is a large fold of dura that separates the cerebellum and cerebrum. The falx cerebri is

another large fold of dura in the longitudinal fissure that separates the cerebral hemispheres. You

can see these clearly on the dorsal surface as thicker, whiter areas of dura.

The Sella Turcica The sella turcica [Latin; Turkish saddle] is a portion of the sphenoid

bone, called the body, at the base of the brain that protects the pituitary gland. Some of the

cranial nerves enter or exit the sella (see Module 6: Cranial Nerves), some of which you can

clearly see on the lateral aspect of the sella from the ventral surface.

The sella is encased in dura. When you dissect the sella turcica free of the brain, notice

the two arching bony structures embedded on the lateral aspects of the pituitary gland. Once

dissected, portions of several cranial nerves, the pituitary gland, the infundibulum, and part of the

tuber cinereum tend to remain in the sella. Look at the underside for these structures. Check the

NeuroGlossary for more information on these structures that will be discussed in detail later.

Pituitary Gland The pituitary gland sits in the center of the sella. It is suspended from

the hypothalamus at the base of the brain by a stalk called the infundibulum. This is cut when the

meninges are dissected, so on the ventral surface of the sheep brain all you may see is a small

hole. Look at the underside of the dissected sella for remnants.

The pituitary gland is also called the hypophysis and is actually two distinct lobes. If you

remove the dura from the pituitary you can see these. The adenohypophysis (anterior pituitary

lobe) is glandular tissue that secretes two protein hormones, vasopressin and oxytocin, in

response to signals from the hypothalamus. The neurohypophysis (posterior pituitary lobe) is not

a separate gland, but an actual extension of the hypothalamus. It is composed largely of the

axons of hypothalamic neurons which extend downward through the infundibulum to the

pituitary. Six different hormones are synthesized and released here. The dark, tufted tissue lateral

to the pituitary in the sella is a capillary (vascular) bed.

Cisterns There are specific areas where the dura does not fold but separates somewhat

from the arachnoid and pia, which remain against the brain tissue, but the subarachnoid space is

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increased in size, creating cisterns [Latin; box]. The cisterns are full of cerebral spinal fluid.

There are several cisterns. The cisterna magna is located at the caudal end of the cerebellum

where dura meets the medulla on the brainstem, and the superior cistern is located between the

occipital lobes and cerebellum where the tentorium sits. The interpeduncular cistern is between

the cerebral peduncles. You will see this more clearly in Module 7: Ventral View.

MODULE 3: DORSAL VIEW

Several anatomical features are characteristic of the dorsal surface of the brain. After

working on this module, look at an unlabeled photo of the sheep brain, or your tissue in the lab,

and try to name all of the structures discussed here.

Divisions From the dorsal surface you can see several superficial divisions of the central

nervous system, such as the cerebrum, cerebellum, and spinal cord. The brainstem is on the

ventral surface.

The cerebrum is the largest portion of the brain, consisting of two hemispheres. It is

covered by the cerebral cortex, an outer layer of gray matter that is considered neocortex.

Neocortex is the most recently evolved brain tissue and is composed of six horizontal layers of

neurons and many columnar arrangements of cells. The neocortex is divided into four functional

lobes in this view.

The cerebellum is also covered by cortex, but there are three layers of neurons rather than

six, and the folds of the cerebellum are called folia [Latin; leaves]. The cerebellum mediates

smooth motor coordination, posture, balance, learning and motor memory, processing novelty,

and shifting/orienting attention. More on the cerebellum later (see Module 5).

Convolutions The cerebral cortex of a human, stretched out flat, is the size of an extra

large pizza. In order to fit inside the bony cranium, it has developed convolutions, or folds. The

valley of each fold is called a sulcus (sulci, plural) and the crest or hill of each fold is called a

gyrus (gyri, plural). The distribution and shape of convolutions are surprisingly similar within

each species but do vary between individuals. They form several anatomical landmarks that are

conserved across higher vertebrates. For example, the longitudinal fissure is a very deep sulcus

that separates the left and right cerebral hemispheres. Running perpendicular to the longitudinal

fissure is the ansate sulcus in sheep, which is often considered analogous to the central sulcus in

humans (see the NeuroGlossary).

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Lobes The cerebral cortex from the dorsal perspective has four functional lobes.

The frontal lobe is rostral to the central sulcus in humans (ansate sulcus in sheep) and it mediates

motor processes and "executive" functions such as cognition, language, planning, and judgment.

The parietal lobe is caudal to the central sulcus in humans and mediates somatosensory (body

sensation) processing, attention, spatial awareness, proprioception, and language comprehension.

The temporal lobe is located laterally and mediates auditory processing, language, and memory.

The occipital lobe is the caudal-most lobe and it mediates visual processing.

Homunculus There are of course many differences in neuroanatomy between the sheep

and primate brain. The ansate sulcus in sheep is often considered analogous to the central sulcus.

In humans, the central sulcus divides the frontal and parietal lobes. There is a gyrus on either

side of the central sulcus, and these have specific and very important functions in the human. The

corresponding areas in sheep brain have not been described as well.

The pre-central gyrus in the frontal lobe in primates mediates motor activity. A

somatotopic motor homunculus [Latin; little man] maps onto the cortex indicating the relative

amount of cortical tissue dedicated to moving a particular body part.

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A similar type of organization occurs along the post-central gyrus in the parietal lobe. This area

mediates somatosensory processing. Notice in the figure below the cross section of cortex from

the left hemisphere depicting the somatosensory homunculus.

Other than the primary sensory and motor

areas, a significant portion of the cerebral

cortex is dedicated to integrating

information from a variety of areas. This is

called the association cortex. Many inputs

are received from a variety of sources and

are “associated” or combined such that

important information can be gleaned from

the inputs and relevance can be interpreted

or a behavioral response can be initiated.

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MODULE 4: LATERAL VIEW

Several dorsal structures continue toward the lateral aspect of the brain. You might be

able to identify them on an unlabeled sheep brain figure. This module includes some items for

review, and some new structures. There are also several structures that will be covered more

thoroughly in later modules; they are labeled here for initial identification only.

Lobes Some of the structures that you should already recognize are several of the

cerebral lobes. Notice that you can see much more of the temporal lobe from the lateral view.

An additional lobe is now visible. The limbic lobe is composed of allocortex. This type of

cortex has three, instead of six, horizontal layers of cells forming its gray matter surface. Readily

visible in the sheep, the limbic lobe (also called the pyriform lobe) is smaller in the human and is

obscured by the greatly expanded, overlapping temporal lobe. The limbic lobe mediates

emotions and olfaction. The limbic lobe also includes the cingulate gyrus (see Module 8: Sagittal

View).

Anatomy The rhinal fissure separates the temporal and frontal lobes from the limbic

lobe. The limbic lobe contains an area called the uncus that overlies a deep structure, the

amygdala that is an important part of the limbic system (see Module 11: Systems). The uncus

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participates in olfactory processing. The caudal area of the limbic lobe is considered the

entorhinal cortex. This area sends inputs to a deeper structure, the hippocampus, and mediates

memory processing.

Notice another landmark, the Sylvian fissure, that separates the frontal and temporal

lobes. This is also called the Lateral fissure. Within the frontal lobe, an area called the

orbitofrontal cortex is associated with integrating emotional and cognitive processing, especially

with regard to reward. The orbitofrontal cortex also participates in impulsivity.

An area just rostral to the temporal lobe in sheep (behind the temporal lobe in humans)

called the insula mediates gustatory, visceral sensory, and emotional processing. In humans it

also has a potential role in cravings.

From this view, you can see the medulla, an oblong structure that regulates life-sustaining

functions such as heart rate, blood pressure, and respiration, among other autonomic processes.

The other features in this image such as cranial nerves will be covered in Module 6. They are

identified here as an introduction.

MODULE 5: CEREBELLUM

The cerebellum [Latin; little brain] is located on the dorsal surface of the brainstem. It is

a hindbrain structure that mediates smooth movement, coordination, postural balance, learning

and motor memory, mental imagery, attention to novelty, and shifting and orienting attention. It

is attached to the brainstem by two peduncles near the lateral aspect of the brainstem, each

containing three branches. At the base of the cerebellum, paired on either side of the midline in

the deep tissue, are clusters of four cerebellar nuclei (dentate, emboliform, globose, and

fastigial).

Given its relatively small size, the cerebellum is surprisingly dense with neurons and

connections. The human cerebellum comprises only 10% of brain volume in weight but has more

neurons than the remaining CNS. All of these neurons are organized in a highly intricate manner

conserved across species.

Anatomy There are several strategies for organizing the many divisions of the

cerebellum for study. These are based on structural, physiological, and molecular mapping

techniques. It has two hemispheres, several lobes, lobules, zones, microzones, flocculi, stripes,

and patches. A minimalist approach is taken here.

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From the dorsal aspect, the cerebellum is organized in three paired longitudinal zones.

There is the medial (or vermal) zone, the intermediate zone, and the lateral (or hemispheral)

zone. On the sheep brain, you can see a large midline portion that protrudes called the vermis

[Latin; worm]. The vermis on the sheep brain is considerably larger than on the human brain, in

which case it does indeed resemble a worm. In this diagram, you can also see the intermediate

zone (paravermis) and lateral zone (hemisphere). These areas can also be divided into lobes,

lobules, and stripes.

Anatomically the cerebellum is also divided into three lobes in the rostrocaudal direction,

the anterior, posterior, and flocculonodular lobes. At the rostral end of the cerebellum in the

vermis you can see the anterior lobe, which is separated from the posterior lobe by the primary

fissure. The flocculonodular lobe is difficult to see, except for tissue visualized on the lateral

aspect (see below). Shallow folds subdivide the lobes into lobules, and each lobule consists of

many folia [Latin; leaves].

The 3/4 view of the cerebellum identifies the vermis, and the flocculus, of the

flocculonodular lobe. For fun, some lobules have been labeled as well.

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The cerebellum regulates neuronal populations in other areas of the brain that control

movement, autonomic function, and cognition. It learns by comparing inputs and outputs for

error detection and coordinates functions accordingly.

Sagittal View In a sagittal section, you can clearly see the many folds of the cortex,

which is organized in three layers of neurons, in contrast to the six layers of the cerebral cortex.

Notice the fine organization of leaves, or folia, on the cerebellar cortex in the sagittal view. The

intricate pattern of white matter is called the arbor vitae, or tree of life. This sagittal section also

reveals the intricately organized lobules. This view provides an additional perspective on the

primary fissure and the anterior and posterior lobes.

Peduncles The cerebellar peduncles are the only source of connections between the

cerebellum and the rest of the brain and spinal cord; therefore, they carry all axonal inputs and

outputs. Peduncles are large bundles of fibers, and once the cerebellum has been dissected from

the brainstem (see the Video Dissection), you can see three branches in each.

The superior cerebellar peduncle (brachium conjunctivum) connects the cerebellum to the

midbrain and contains mostly efferent axons from the deep cerebellar nuclei that send feedback

to the motor cortex in the frontal lobe. Afferent fibers traveling in this peduncle take

proprioceptive information to the cerebellum from the lower body.

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The middle cerebellar peduncle (brachium pontis) contains afferents from the pons.

Motor information originating in the frontal lobe and projecting to the spinal cord is relayed back

through this connection to the cerebellum for comparison, error detection, and fine-tuning.

The inferior cerebellar peduncle (restiform bodies) contains afferent fibers from the

brainstem and spinal cord, and cerebellar efferent fibers projecting to the vestibular nuclei,

mediating postural balance.

The cerebellum in part forms the roof of a large cavity in the brainstem called the fourth

ventricle. The ventricular system (see Module 11: Systems) conveys cerebrospinal fluid through

a series of hollow spaces called ventricles that are distributed through the center of the brain and

down the central canal of the spinal cord. The ventricles are numbered from rostral to caudal;

therefore, the space you see beneath the cerebellum is the fourth ventricle. You can see a small

opening that leads to the central canal.

MODULE 6: CRANIAL NERVES

Neuroanatomical studies of the brain also include important structures in peripheral

systems, such as the peripheral nervous system cranial nerves (C.N.). These are the only nerves

in the PNS that do not enter or exit the spinal cord. Most of them enter or exit the brainstem, as

you will see.

At the end of this module, you should be able to refer back to an unlabeled image and

identify correctly all of the cranial nerves.

Cranial Nerves Cranial nerves are PNS nerves that enter or exit a foramen in the

cranium to make direct connections between the brain and head or neck areas (generally). All

other PNS nerves make their connections with the brain through the spinal cord. Cranial nerves

can have either sensory or motor functions; however, many of them have both.

Cranial nerves are paired, that is, they are bilateral, and are identified by a Roman

numeral indicating a numbering progression from rostral to caudal. The best view of the majority

of cranial nerves is from the ventral surface.

Cranial nerves on the hindbrain have a tendency to be torn during removal of the sheep

brain from the cranium. It is rather difficult to find a specimen with all 12 pairs intact. The

photograph below includes a few outlines of torn cranial nerves.

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Cranial nerve I is the olfactory nerve and its function is purely sensory. On the sheep

brain you will not be able to locate the olfactory nerve because it is torn when the brain is

removed from the cranium. You can, however, clearly see the olfactory bulbs which are the

targets of the olfactory nerves. C.N. I mediates olfaction, and the sensory nerves that synapse

here in the olfactory bulbs continue on toward the brain after splitting into branches, the lateral

and medial olfactory tracts. The area between the olfactory tracts is called the olfactory tubercle.

Together, the tubercle and the amygdala are considered the ventral striatum, and are associated

with reward behavior.

C.N. II is the optic nerve which is purely sensory and mediates vision. Axons from the

retina project toward the brain and partially decussate at the optic chiasm. Images in the left

visual field are received by the right portion of the retina, and those axons are projected to the

right side of the brain, therefore some of these axons must cross the midline through the chiasm.

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The opposite is true for images in the right visual field. Once the axons are past the chiasm, they

are considered by convention to be optic tracts. The tracts then continue to project toward

primary visual cortex.

C.N. III is the oculomotor nerve which is purely motor and mediates eye movement. It

also has parasympathetic fibers of the autonomic nervous system that regulate pupil constriction.

C.N. IV is the trochlear nerve, the only cranial nerve that does not enter/exit the ventral

surface of the brain. You can see it exiting the brainstem from the dorsal surface, underneath the

cerebellum. The trochlear nerve is purely motor and mediates movement of the eyes by

innervating the superior oblique muscles.

C.N. V is the trigeminal nerve, by far the largest cranial nerve. It has several branches,

and mediates both sensory and motor functions. The initial branches include the mandibular

(jaw), and maxillary (face), and ophthalmic (eyes) divisions. The trigeminal nerve enters/exits

the lateral aspect of the pons and is immediately enveloped in dura that encases the sella turcica.

When removing the dura, the trigeminal nerve branches typically tear and remain with the sella.

Motor functions mediated by the trigeminal nerve include jaw movements such as

chewing. The trigeminal nerve mediates many sensory functions and contains afferents that bring

sensory information from the meninges, face, and teeth. The trigeminal nerve is typically the

target for local anesthesia when you are in the dentist's chair.

C.N. VI is the abducens nerve which also mediates eye movement. It innervates the

lateral rectus muscle of the eye, and therefore causes abduction of the eye, hence its name.

C.N. VII is the facial nerve which has both sensory and motor functions. It innervates

muscles that mediate facial expressions and innervates glands of the head (except the parotid)

such as lacrimal (tear) glands. It also has sensory afferents from the tongue, mediating taste

perception.

C.N. VIII is the vestibulocochlear nerve, sometimes called the acoustic nerve. This is

misleading because the vestibulocochlear nerve mediates both sound and inputs from the

vestibular system that participate in regulating postural balance.

C.N. IX is the glossopharyngeal nerve, which has mixed sensory and motor functions. It

mediates sensation from the tongue (taste), tonsils, pharynx, and middle ear, and innervates

muscles of the pharynx for swallowing.

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C.N. X is the vagus nerve, and it has many branches that innervate the heart, stomach,

and other thoracic and abdominal organs. It is a mixed sensory and motor nerve, providing motor

parasympathetic fibers to all organs except the adrenal glands and motor control to some skeletal

muscles. It mediates such diverse functions as heart rate, sweating, muscles for speech and

breathing, swallowing, outer ear sensation, and sensory properties of the meninges.

C.N. XI is the spinal accessory nerve, which is purely motor and mediates movement of

the upper trapezius and sternocleidomastoid muscles of the neck and shoulders; the "shrug"

muscles in humans.

C.N. XII is the hypoglossal nerve, and it is purely motor and mediates tongue movement.

If you are wondering what kind of study strategy will help you remember all of this,

consider using a mnemonic device. Over the years, many have been devised for the cranial

nerves. There are mnemonics for the cranial nerve names as well as their sensory, motor, or

sensory and motor (“both”) functions. Look at examples in the table below, but by making up

your own mnemonic, you significantly enhance your memory by creating associations that mean

something to you. Try a rhyming strategy.

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MODULE 7: VENTRAL VIEW

In addition to the cranial nerves in Module 6, the ventral view reveals an increasing

number of review structures, such as the cerebral lobes, and many new structures to identify.

When the meninges were dissected, you also removed the sella turcica from the ventral surface.

Structures associated with the sella will not be included in this module (refer back to Module 2:

The Meninges). New terminology will be introduced in chunks based on embryonic brain

divisions.

Telencephalon In this portion of the forebrain you can of course still identify the

cerebral lobes. You can see the limbic lobe more clearly from the ventral view.

The limbic lobe has specific areas that mediate various functions. The uncus is at the

rostral end of the limbic lobe and it mediates olfaction. This makes sense after looking at the

cranial nerves associated with this area.

The entorhinal cortex is at the caudal end of the limbic lobe and it provides primary

inputs to the hippocampus, therefore mediates memory processing.

A very small area of cortex just rostral to the temporal lobe in sheep (underneath the

temporal lobe in humans) is the insula. The insular cortex mediates gustatory, visceral sensory,

and emotional processing. It also has a potential role in cravings.

A rather large sulcus, the rhinal fissure, separates the frontal and temporal lobes from the

limbic lobe.

Diencephalon The other portion of the forebrain, the diencephalon, contains the

thalamus and hypothalamus. The hypothalamus and related structures are apparent in the ventral

view. The border of the hypothalamus is approximated in. There are many nuclei in the

hypothalamus that mediate many vital autonomic nervous system and endocrine system

processes such as eating, thirst, growth and reproduction, body temperature, and sleep, among

others.

When the sella turcica was removed, you had to cut through the infundibulum. The term

refers to any cone shaped opening, but in this case it is a stalk that connects the hypothalamus to

the pituitary gland. On the ventral surface, you may see a hole; that is what is left of the

infundibulum. It originally projects from the tuber cinereum, a structure that will appear to be

loosely connected gray matter at the midline.

23

The tuber cinereum is part of the hypothalamus and contains the tuberomammillary

nucleus. This nucleus is the sole source of the neurotransmitter histamine which participates in

regulating circadian rhythms.

The mammillary bodies are nuclei at the caudal end of the hypothalamus that are

involved in memory processes.

Mes-, Met-, and Myelencephalon There are several midbrain (mesencephalon) and

hindbrain (met- and myelencephalon) features on the ventral surface. Most of the midbrain

structures are not visible from the ventral view, other than the cerebral peduncles which contain

many fiber tracts connecting the cerebrum with distal parts of the CNS. Later dissections will

reveal structures and tracts contained in the peduncles. The interpeduncular cistern is simply a

space of arachnoid and pia separation, situated between the peduncles, that contains

cerebrospinal fluid. There are several cisterns in the CNS.

The hindbrain structures include the pons that mediates various aspects of the sleep cycle.

The pons has inhibitory outputs to spinal cord motor neurons so that only small muscles such as

24

in the face, hands, and feet are capable of movement during sleep. A very characteristic type of

activity in the pons, along with two other structures, is indicative of REM sleep. Caudal to the

pons is the medulla.

The medulla oblongata mediates survival mechanisms such as heart rate, respiration, and

blood pressure. The medulla has several notable features such as the olivary nucleus (or olive)

which forms a bump on the lateral aspect. The olive is closely associated with the cerebellum. It

mediates the control and coordination of movement and has a role in sensory processing and

cognitive tasks such as the timing of sensory inputs.

A band of transverse fibers located on the ventral surface of the medulla, adjacent to the

pons, carries auditory inputs from the right ear to the left auditory cortex of the temporal lobe

and vice versa, resulting in bilateral inputs from each ear. This is the trapezoid body.

The pyramids are long tracts that run parallel to each other, lateral to the ventral median

fissure. They convey efferents from the motor cortex down the spinal cord.

MODULE 8: SAGITTAL VIEW

This module begins by identifying structures seen in the midsagittal section. Refer to the

dissection video for cutting the tissue. Structures are organized here for review by embryonic

divisions. A subsequent parasagittal section will only label a few structures, with the intent of

helping you visualize the dimensionality of each structure.

Telencephalon This is a midsagittal view of the telencephalon and associated structures.

The cerebral lobes are evident, as are many new underlying structures.

The cingulate cortex is a midline gyrus that is considered part of the limbic lobe (see

Module 7: Ventral View) and limbic system (see Module 11: Systems). It is very large and as a

component of the limbic system the cingulate mediates many different functions such as

emotions, motivated responses, and cognitive processing. The anterior portion is involved in pain

perception.

Just ventral to the cingulate gyrus is the corpus callosum, the largest commissure in the

CNS. A commissure is a tract that connects the left and right hemispheres. Look in the

NeuroGlossary for an interesting dissection of the corpus callosum.

Depending on the accuracy of your midsagittal section, you may not see an opening just

ventral to the corpus callosum. Often, there is a bit of membrane covering this large cavity. The

25

membrane is called the septum pellucidum (a septum is just tissue that separates two things...you

have a septum dividing your two nostrils). In this image, the septum pellucidum is partly torn

and you can see an opening into a large cavity. This cavity is the lateral ventricle that holds

cerebrospinal fluid. It is part of the ventricular system (see Module 11: Systems). The septum

pellucidum separates the left and right lateral ventricles.

You can see another more ventral band of myelinated axons that also connects the two

hemispheres, this time the tissue is emanating from the hippocampus, which is not in view yet.

The band of fibers below the corpus callosum is called the fornix, or hippocampal fornix. It too

is a commissure. It projects axons from the hippocampus to the septal nuclei and hypothalamus.

Notice that the lateral ventricle has commissures on both the roof and floor.

There is yet another commissure; however this one is rather small in comparison. It is the

anterior commissure and it is just rostral to a structure that will be discussed in the next section,

the thalamus. The anterior commissure is also just caudal to the septal area, a midline area of

gray matter that is composed of the septal nuclei, part of the limbic system, mediating emotions.

Diencephalon The diencephalon contains the thalamus and hypothalamus, along with

other associated structures.

The thalamus is a large, round-appearing structure in the center of the brain. Like most

structures it is bilateral and is connected by an area of tissue called the massa intermedia. The

thalamus has many nuclei mediating numerous functions. It is considered the major relay center

for all inputs and outputs. For example, all sensory inputs arrive at the thalamus (except for

olfactory) in order to be projected to their various targets.

On your tissue, the massa intermedia may appear to be a plateau area of tissue that has a

cut edge. The thalamus will be visible around the edge of the massa intermedia as smooth gray

matter that disappears laterally into the hemisphere. The third ventricle entirely encases the

thalamus and typically contains a considerable amount of choroid plexus, the dark tufted

vascular material that creates cerebrospinal fluid (see Module 11: Systems). The third ventricle is

difficult to see in the midsagittal section, as it is a hollow space in the exact plane of section.

Ventral to the thalamus is the appropriately-named hypothalamus. This is another area of

gray matter that contains many nuclei mediating a wide-range of functions. In this case, the

hypothalamus controls many autonomic (fight or flight) and endocrine (hormone) functions in

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the nervous system. It also mediates many motivated behaviors such as eating, sleeping, and

reproduction.

At the caudal end of the hypothalamus is a cluster of nuclei called the mammillary

bodies. These structures are associated with memory processing.

Moving caudally, the dorsal portion of the diencephalon contains the pineal gland (or

pineal body). This single structure located in the midline of the brain participates in regulating

circadian rhythms as it releases melatonin. Inputs come from the stria medullaris and from the

habenula.

The habenula consists of a group of many distinct subnuclei that mediate the release of

the monoamines dopamine, norepinephrine, and serotonin. It mediates behaviors such as pain

processing, reproductive behavior, feeding, sleep-wake cycles, stress responses, and learning. An

area in the lateral habenula is associated with encoding negative feedback or negative rewards.

The stria medullaris is a fiber tract projecting from the amygdala (see Module 10:

Coronal View) that runs along the surface of the thalamus and enters the habenula on each side.

27

Just as the telencephalon has an anterior commissure, the diencephalon contains a

posterior commissure connecting the right and left hemispheres.

Mesencephalon The mesencephalon, or midbrain, includes the corpora quadrigemina.

This is the general term for four bodies, a pair of superior and inferior colliculi. The superior

colliculi mediate directing eye movement, saccades, and tracking objects. The inferior colliculi

contain auditory relays to mediate determining the location of auditory stimuli in the

environment. These dorsal structures are also called the tectum. In non-mammals, the superior

colliculus is called the optic tectum.

The cerebral aqueduct, or aqueduct of Sylvius, conveys cerebrospinal fluid from the third

to fourth ventricles. It conveniently divides the tectum from the tegmentum.

The tegmentum is part of the midbrain cerebral peduncles located ventral to the cerebral

aqueduct. It contains neurons involved in many unconscious homeostatic and reflexive

pathways, including the rostral end of the reticular formation (conscious awareness, arousal), the

periaqueductal gray matter (pain), the red nucleus (motor), the substantia nigra (motor, part of

the basal ganglia system), and the ventral tegmental area (VTA). The VTA contains

dopaminergic neurons involved in motivated behaviors. It is studied extensively in addiction

research.

Met- and Myelencephalon The metencephalon and myelencephalon contain some

review structures. You have already studied Module 5: The Cerebellum, but note again the fine

structure of the folia and arbor vitae in the cerebellum, and the primary fissure separating the

anterior and posterior lobes.

The cerebellum forms the roof of the fourth ventricle, a hollow cavity in the ventricular

system that contains cerebrospinal fluid (see Module 11: Systems). The floor of the fourth

ventricle is formed by the pons and medulla.

The pons [Latin; bridge] relays information to and from the cerebral cortex, cerebellum

and spinal cord. The pons contains, among others, neurons of the reticular formation. In addition,

specific EEG spikes from the pons are associated with entering REM sleep and cortical

activation. The pons also sends inhibitory signals down axons of the spinal cord to motor

neurons. This inhibits large muscle groups from moving during REM sleep.

The myelencephalon consists of the medulla oblongata. The medulla is the caudal portion

of the hindbrain that contains sensory and motor pathways, the reticular formation, and regulates

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vital autonomic processes such as heart rate, blood pressure, and respiration.

Not part of the embryonic brain divisions, but part of the CNS distinctly different from

the brain, the spinal cord is also visible from the midsagittal view. You might be able to see the

central canal, a long, thin hollow that enables CSF to circulate throughout the spinal cord. CSF

enters the central from the fourth ventricle. Module 12: Spinal Cord provides more information

on this part of the CNS.

Commissures (above) Studying the midsagittal section is not complete without a review

of the commissures. Commissures are typically myelinated tracts, masses of axons, that

functionally connect structures between the right and left hemispheres of the brain. There are six

noted commissures, the corpus callosum is by far the largest. Refer to the NeuroGlossary for an

interesting dissection of this structure.

You can also see the anterior and posterior commissures, the hippocampal commissure

(the fornix), the optic chiasm, and the thalamic commissure (massa intermedia) which in this

case contains gray matter but connects the left and right thalamic nuclei.

29

Parasagittal (below) Parasagittal sections provide somewhat of an idea about the

dimensionality of structures, but also reveal many new structures. Only one parasagittal section

is introduced here to get you started. The number of structures diminishes as you dissect

laterally. This section is approximately 1 cm from the midline.

The main components in an entire system, the basal ganglia (see Module 11: Systems)

are now revealed. The caudate nucleus receives inputs from many cortical areas in mediating

movement and various features of cognition. Along with the putamen, these inputs are conveyed

to the globus pallidus and substantia (not in view). The lentiform nucleus, which you see labeled

here, consists of the globus pallidus and putamen.

You can also see the hippocampus, which mediates memory processes. The fimbria and

fornix are hippocampal outputs. Notice that the caudate and hippocampus are on the "floor" of

the lateral ventricle.

Another structure is revealed in this dissection, the cerebellar peduncle. This is the only

source of connections between the cerebellum and other parts of the CNS. There are two

30

peduncles on the lateral aspect of the brainstem, and each peduncle has three branches (see

Module 5: The Cerebellum).

MODULE 9: HORIZONTAL VIEW

Horizontal sections provide a way to view many structures. After you have completed

this module you will have a better understanding of the location of structures relative to each

other, and when compared to the sagittal section you will begin to discern their size, shape, and

3D qualities.

In this series, every section will not have every element labeled, so try to discern which

structures are still apparent from view to view; this is a good exercise. For the most part, any

structure that does not change its basic shape or direction may not be continuously labeled

throughout the series.

Horizontal sections are cut somewhat differently than dissections in the other planes. In

this case, there are no simple landmarks to get you started. You'll notice in the video that

beginning the dissection consists of a simple series of gently removing thin dorsal layers until the

hippocampus is exposed. As you do this, try to begin in a plane parallel to the longitudinal axis

of the brain and use this as your guide to continue the dissections. Once you reveal the

hippocampus or caudate, continue with cuts made approximately 0.5 cm thick. This produces a

sequence of serial sections, so the discussions will be identified by section number, running

dorsal (#1) to ventral (#7). The lab manual provides images of just a few of these sections. Refer

to the digital module for all sections. The figure below shows an approximation of the sections.

31

Section 1 (not shown) This section is not quite at the level of the caudate or

hippocampus. The image is included to give you an idea of what the first few cuts will reveal.

You can of course identify the longitudinal fissure. Note also the cingulum, a white matter tract

running parallel to the longitudinal fissure that is within the cingulate gyrus.

Section 2 (below) The hippocampus and caudate nucleus are now exposed. Notice that

you have revealed the structures on the "floor" of the lateral ventricle. This includes the caudate

nucleus and the hippocampus. The caudate nucleus is part of the basal ganglia system (see

Module 11: Systems) which mediates movement, learning and memory, and some features of

cognitive processing. The hippocampus is an important structure in memory processing.

Between them lies a large cluster of choroid plexus. Adjacent to the hippocampus you may be

able to see the fimbria (in this case under the choroid). These are efferents projecting from the

hippocampus, often lying underneath choroid, that converge to form the fornix.

The

corpus callosum in horizontal sections gives you a different perspective on the genu, body, and

splenium (rostral to caudal, respectively).

32

Section 3 As the plane of dissection moves deeper, many structures appear larger, and

the thalamus is now in view. You can also more clearly see the septum pellucidum, the

membrane that separates the lateral ventricles.

Section 4 (below) Many structures are now revealed, and when compared with the

sagittal view you can begin to get an appreciation for the three-dimensional shapes. The caudate

is still in view, and will be throughout, and in the next few sections you will see other nuclei that

are part of the basal ganglia system, the putamen and globus pallidus. The basal ganglia mediate

movement, affective behavior, learning, and some cognitive features.

At this level, the thalamus is a predominate structure. This is the main relay area for

sensory afferents projecting to the appropriate targets throughout the brain. Throughout the

subsequent dissections, the thalamic nuclei change shape dramatically. Tracing these in each

section and compiling an image stack gives you an excellent way to visualize the 3D structure.

At the midline, notice the size of the fornix, or hippocampal commissure. Caudal to this

you can see the stria medullaris, a tract that projects from the amygdala that runs along the

thalamus on its way to the habenula.

The habenula mediates the release of the monoamines dopamine, norepinephrine, and

serotonin involved in behaviors such as pain processing, reproductive behavior, feeding, sleep-

33

wake cycles, stress responses, and learning. An area in the lateral habenula is associated with

encoding negative feedback or negative rewards.

The pineal gland has been revealed at this level. It secretes melatonin and participates in

regulating circadian rhythms. Lateral to the habenula and pineal gland are the hippocampus and

dentate gyrus. The dentate gyrus provides inputs to hippocampal layers and these structures

mediate memory processing.

Most caudal is the superior colliculus, a structure of the corpora quadrigemina in the

midbrain. This structure mediates directing eye movement, saccades, and tracking objects.

Nearby, the pulvinar participates in mediating visual processing.

Section 5 Although partially present in previous sections, you'll notice here that the

white matter capsules, external and internal, have been labeled. They are convenient landmarks

for identifying basal ganglia structures. The capsules contain many white matter tracks that

provide connections between the basal ganglia and cortical neurons. The caudate and putamen

have internal capsule fibers coursing through the area, giving it a striped or striated appearance.

This is the derivation of the term "striatum." Together, the caudate and putamen are considered

the dorsal striatum. The external capsule forms the lateral border of the putamen. The caudate

and putamen are the sites for almost all inputs coming from the cortex.

The thalamus has expanded in size and you can now clearly see the lateral geniculate

nucleus of the thalamus. This area receives afferents from the optic tract. You can see the white

matter tract along the outer border of the lateral geniculate. Adjacent to the lateral geniculate is

the pulvinar, a nucleus that participates in mediating visual processing.

The septal area is in view. These gray matter structures on either side of the septum

pellucidum provide connectivity between limbic system structures. This area includes the medial

septal nuclei involved in memory processing and lateral septal nuclei involved in motivated

behaviors.

Section 6 At this level you can see the caudate nucleus, putamen, and globus pallidus of

the basal ganglia system. This is a system of subcortical nuclei that regulates movement,

affective behavior, learning, and some cognitive processes. In spite of its name, ganglia, it is a

CNS system that also includes the substantia nigra and subthalamic nuclei (see Module 11:

Systems).

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As the dissections progress ventrally, we are below the level of the superior colliculi and

have barely revealed the inferior colliculi, which receive auditory afferents and mediate locating

auditory stimuli in the environment. You can also see a hint of cerebral aqueduct. The cerebral

aqueduct separates the tectum (corpora quadrigemina) from the tegmentum. Surrounding the

aqueduct is the periaqueductal gray, which mediates pain processing.

Section 7 This dissection is ventral to the cerebral aqueduct, so we now have the

tegmentum in view. The tegmentum is a midbrain area of the cerebral peduncles that contains

neurons involved in many unconscious homeostatic and reflexive pathways, including the rostral

end of the reticular formation (awareness, arousal) (see Module 11: Systems), the periaqueductal

gray matter (pain), the red nucleus (motor), the substantia nigra (motor), and the ventral

tegmental area (VTA). The VTA contains dopaminergic neurons involved in motivated

behaviors and addiction.

Near the midline, there are several structures apparent. The mammillothalamic tract, seen

here in cross-section, is a fasciculus of white matter fibers connecting the mammillary bodies

35

with the thalamus. Additional fibers from the fornix merge into this tract and an additional

termination for this tract is with tegmental nuclei.

Also near the midline, caudal to the septal area, is the nucleus accumbens. This structure

mediates anticipation of rewards associated with motivated behaviors and appears to play a role

in emotions. It is studied extensively in addiction research. The nucleus accumbens and the

olfactory tubercle collectively form the ventral striatum, considered part of the basal ganglia.

MODULE 10: CORONAL VIEW

This set of dissections in the coronal plane will give you an idea of the dimensionality

and shape of brain structures. Serial sections can be difficult to follow because the shape of a

structure can change in seemingly odd ways between sections. Refer to previous planes of

dissection for comparison.

In this series, every section will not have every element labeled, so try to discern which

structures are still present from view to view; this is a good exercise. For the most part, any

structure that does not change its basic shape or direction may not be continuously labeled

throughout the series.

The Dissection Video will guide you through the coronal sectioning of the sheep brain,

but basically after beginning at

Bregma, all sections are approximately

0.5 cm apart. This produces a sequence

of serial sections, so the discussions

will be identified by section number,

running rostral (#1) to caudal (#12).

The image below provides anatomical

landmarks for cutting

the coronal sections. Each section is

approximately 0.5 cm apart. These of

course are approximations, and may differ depending on the age, size, and strain of the sheep.

The accuracy of the cuts also depends on maintaining the alignment of the blade with the axes of

the brain.

36

Section 1 (below) The dissection begins with a coronal section using the ansate sulcus

and Bregma as the landmark. Bregma is the intersection of the parietal and frontal bone sutures.

On the sheep brain, it is approximated by the intersection between the ansate sulcus and

longitudinal fissure.

Several structures that you already know are visible on this first section. The longitudinal

fissure and corpus callosum are obvious points of reference. Locate the lateral ventricles and

notice their shape. The septum pellucidum dividing these is clearly visible. You can see a white

matter tract descending ventrally from the area around the septum pellucidum and it projects to

the hypothalamus in later sections. By convention, this is called the septohypothalamic tract.

This section is at the rostral end of the sheep brain, therefore the ventral surface will

contain olfactory tracts (not labeled) and the olfactory tubercle.

There is another white matter structure coursing through the deeper tissue called the

external capsule. This contains many fibers connecting the cerebral cortex with the basal ganglia.

The basal ganglia system regulates movement, affective behavior, learning, and other cognitive

37

processes. Two basal ganglia structures are seen here. The caudate nucleus appears on the "floor"

of the lateral ventricle and its shape will change as the coronal dissections progress. The putamen

is lateral to the caudate. Together, these structures are also called the striatum. Further

dissections will demonstrate why.

Section 2 (below) A landmark structure is apparent in this section. The rhinal fissure

separates the frontal and temporal lobes from the limbic lobe. You can also see white matter

structures that are in many of these coronal sections and are labeled here, the corona radiata and

cingulum. The cingulum is a tract within the cingulate gyrus that projects to the entorhinal cortex

of the limbic lobe.

In this section, notice the change in size and shape of the caudate nucleus on the floor of

the lateral ventricle (which has also changed shape). Also notice the putamen, and another

structure that is part of the basal ganglia, the globus pallidus. These three structures always

maintain their relative position with one another. The term striatum is used because the caudate

and putamen are interspersed with myelinated fibers of the internal capsule, giving the area a

striped appearance.

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The septum pellucidum now has a cluster of nuclei on either side of it, called the septal

area. These nuclei provide connectivity between limbic system structures, mediating emotions,

learning and memory, and motivated behaviors.

A structure near the septal area and at the point where the basal ganglia meet medially is

called the nucleus accumbens. This structure is also part of the limbic system and is studied

extensively in motivated behaviors such as drug addiction. The nucleus accumbens and olfactory

tubercle together are considered the ventral striatum, whereas the caudate and putamen are

considered the dorsal striatum.

Section 3 On this section are structures that were labeled on the previous cut, but notice

the change in size and shape as the dissections continue caudally. You can also clearly see that

the external capsule borders the putamen and globus pallidus whereas the internal capsule runs

through the caudate and putamen. Use these landmarks to help you identify structures.

Section 4 (below) In this section you are at the level of the optic chiasm. Basal ganglia

structures are still apparent but the caudate and putamen are smaller in size. There are several

changes medially. Notice the anterior commissure, a white matter tract that connects cortical

regions between hemispheres in the temporal lobes.

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Although it is not visible yet, the hippocampus is a structure associated with memory. It

has fibers that project from it called fimbria, which converge to form the fornix. These are the

major outputs from the hippocampus. The fornix is also known as the hippocampal commissure

as its fibers project toward the septal area and hypothalamus, and to the opposite hemisphere.

Section 5 (below) In this view, the ventricles have changed shape considerably and the

third ventricle is in view. All that is left of the caudate is a small "tail" lateral to the lateral

ventricles. The fornix is still very apparent in the lateral ventricles.

The basal ganglia nuclei are replaced visually by the thalamus, a paired, deep brain

structure subdivided into numerous nuclei that convey sensory relays to and from many other

structures. Along the dorsal surface of the thalamus is the stria medullaris, a tract projecting from

the amygdala to the habenula (seen later). The amygdala contains a group of nuclei that mediate

specific behaviors such as determining the emotional salience of stimuli.

40

The mammillary bodies are part of the hypothalamus and mediate memory processing.

The hypothalamus is a paired structure below the thalamus subdivided into numerous nuclei that

regulate a variety of vital autonomic nervous system and endocrine processes such as eating,

thirst, body temperature, and sleep, among others.

Section 6 (below) Moving further caudally, the thalamus still dominates the space. An

area called the lateral geniculate nucleus of the thalamus mediates visual processing as it receives

inputs from the optic tract, and the medial geniculate nucleus mediates auditory processing. The

pulvinar mediates visual processing and eye movement. There are many more subdivisions of the

thalamus that are not included here.

The mammillothalamic tract (you can see it in cross section) is also associated with the

thalamus. The mammillary bodies receive inputs from the hippocampus through the fornix and

then project to the thalamus through the mammillothalamic tract.

The habenula is now visible as a small but important structure that mediates the release of

monoamine neurotransmitters involved in behaviors such as pain processing, reproductive

behavior, feeding, circadian rhythms, stress responses, and learning. If you remember from the

previous section, the stria medullaris is a tract projecting from the amygdala that runs along the

dorsal thalamus to enter the habenula.

41

The hippocampus is now in view; it mediates memory processing. It is a large structure

with an interesting shape [Greek; seahorse] that follows the lateral ventricles as they expand.

Section 7 Notice the change in shape of the hippocampus, and the lateral ventricle. The

third ventricle is still present, and actually occupies a great deal of space, but it is difficult to see

in coronal sections (see Module 11: Systems, The Ventricular System).

At the base of the brain the hypothalamus and mammillary bodies are no longer present

but you can see the bilateral cerebral peduncles. These are large clusters of tracts mediating

motor functions.

You saw the habenula on the previous section, now the habenulopeduncular tract is

apparent. This tract projects from the habenula to the cerebral peduncles where many fibers

make their way to and from cortical areas. The cerebral peduncles and tegmental areas contain

many monoamine neurons that release neurotransmitters like dopamine, norepinephrine, and

serotonin. Efferents from the habenula play a role in modulating the activity of these neurons.

The pulvinar was mentioned in the previous section. It is located adjacent to other visual

processing structures, the lateral geniculate and superior colliculus (seen in later sections).

Section 8 (below) The third ventricle now contains the pineal gland (or body), an

42

unpaired endocrine gland that secretes melatonin for regulating circadian rhythms. It is part of a

cascade of information from other nuclei that receive and transmit light signals from specialized

cells in the retina. At this level many structures from previous sections remain. A new tract is

visible, the posterior commissure, which not only connects structures between hemispheres but

is also important for bilateral pupillary reflexes.

Section 9 (below) Due to the angle of this cut, a new basal ganglia structure is barely

visible in this view, the substantia nigra. These are paired tegmental nuclei containing two

regions, one of which (pars compacta) contains dopaminergic projections to the striatum and

cerebral cortex. The pars reticulata sends projections to the thalamus and superior colliculus.

You can see a hint of the superior colliculus in this view.

Several structures in this view participate in visual processing and eye movement. The

posterior commissure connects right and left hemispheres, and is also important in mediating

bilateral pupillary reflexes. The pulvinar, the lateral geniculate nucleus of the thalamus, and the

superior colliculus, which is just beginning to be revealed, also participate.

43

Section 10 (below) In this view, you can see tracts called optic radiations that are

primary visual afferents projecting to the occipital cortex to mediate visual processing. You can

also see the colliculi in this view.

The superior and inferior colliculi (colliculus, singular) are collectively called the corpora

quadrigemina [Latin; four twins] as they are paired structures. This area of midbrain is also

called the tectum. The superior colliculi mediate directing eye movement, saccades, and tracking

objects visually. The inferior colliculi mediate locating auditory stimuli in the environment.

The substantia nigra is now clearly visible. This nucleus is part of the basal ganglia

system. On close inspection this area has a dark appearance due expression of the pigment

melanin by some of these neurons.

The presence of the cerebral aqueduct identifies the midbrain. Surrounding the cerebral

aqueduct is the appropriately named periaqueductal gray. This gray matter area mediates several

functions, including modulation of pain transmission in the spinal cord, regulation of certain

visceral motor activities, and autonomic regulation. The tectum is dorsal to the cerebral aqueduct

of Sylvius, whereas the tegmentum is ventral to the aqueduct. Both terms refer to a very

generalized tissue area.

44

The midbrain tegmentum contains many nuclei and structures of the reticular formation,

a complex system that includes an ascending or afferent branch that projects to the cortex, and a

descending or efferent branch that projects to the spinal cord. This system mediates many varied

functions (see Module 11: Systems). The raphe nucleus is in the medial area of the reticular

formation and it is the source of neurons that synthesize and release the monoamine

neurotransmitter serotonin.

Section 11 (below) At this level in the dissection it is very difficult to retain cortical

material, but the focus is on the brainstem at this point so it is not a problem if during the

dissection the caudal pole of the cerebrum becomes disconnected.

You continue to see elongated structures such as the tegmentum, reticular formation and

its raphe nucleus, and the periaqueductal gray. The colliculi, or corpora quadrigemina, have not

been dissected squarely so you can see these structures are lacking on the right hemisphere (your

left) but they have been drawn in.

Section 12 (above) Notice that the cerebral aqueduct has led to the fourth ventricle. You

can also clearly see the middle cerebellar peduncle (brachium pontis), the anterior cerebellar lobe

and other lobes and zones of the cerebellum, as well as the vermis. The four deep cerebellar

nuclei are located bilaterally at the base of the cerebellum.

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The level of this dissection is at the pons, a brainstem structure that relays information to

and from the cerebral cortex, cerebellum and spinal cord. The pons contains, among others,

neurons of the reticular formation. It mediates many functions, including the initiation of specific

electrical spikes that signal the beginning of REM sleep, and inhibiting spinal cord motor

neurons during sleep.

More nuclei of the reticular formation are in this area, such as the locus coeruleus which

is the site of neurons that synthesize and release the monoamine neurotransmitter

norepinephrine. The pedunculopontine nucleus is involved in many functions, including arousal,

attention, learning, reward, and locomotion. It is also implicated in the generation and

maintenance of REM sleep.

MODULE 11: SYSTEMS

A. Basal Ganglia System

As you have progressed through this tutorial, several modules have mentioned structures

that are part of the basal ganglia system and involved in mediating movement. This module

focuses on the basal ganglia system.

It used to be thought that the basal ganglia system mediated movement. But due to the

extent of connectivity with other structures, it is now known to mediate many more functions. It

is a system of subcortical nuclei that also regulates affective behavior, learning, motivation, and

some higher-order cognitive processes. In spite of its name, ganglia, it is a CNS system. To

correct this discrepancy, the system is sometimes referred to as the basal nuclei.

The basal ganglia system is composed of nuclei in the forebrain, including the caudate

nucleus, putamen, globus pallidus, and subthalamic nuclei. The system also involves the

substantia nigra in the brainstem. The substantia nigra is composed of the pars compacta and pars

reticulata, as seen in this figure. Some include the nucleus accumbens, ventral tegmental area,

and amygdala in this system due to the close association with these and other limbic structures.

The motor functions of the basal ganglia are perhaps the best described in terms of the

interconnectivity, neurotransmitters involved and functional outcomes.

Together, the caudate and putamen are considered the dorsal, or corpus, striatum. The

name describes the striped appearance in coronal sections due to the white, myelinated fibers of

the internal capsule that course through these nuclei.

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Neurons in all areas of the neocortex, except for the primary visual and auditory areas,

project axons to neurons in the dorsal striatum. The association cortex in the frontal and parietal

lobes provides the most inputs. For motor function, the precentral gyrus of the frontal lobe and

thalamus provide the inputs and these are excitatory.

After receiving inputs from the motor cortex, striatal neurons of the caudate and putamen

then project their axons to the globus pallidus and substantia nigra pars reticulata. These inputs

are inhibitory, releasing the inhibitory neurotransmitter GABA.

The globus pallidus and pars reticulata provide all of the efferents from the basal ganglia

system. These are also inhibitory outputs that are projected to thalamic nuclei and then relayed

back to cortical neurons. This forms an inhibitory circuit that effectively controls unwanted

movements. The basal ganglia enforce a constant inhibitory influence on neurons in order to

control their activity. The system can then exert disinhibition, or switch off the inhibition, and

allow movement to occur. This is of course an over-simplification of a system that has very

complicated connectivity. For example, the substantia nigra pars compacta provides dopamine

inputs back to the striatum to modulate activity in the caudate.

There are other circuits in addition to motor systems that involve the basal ganglia, for

example the limbic system. In humans, the interaction of the basal ganglia with other circuits and

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nuclei mediates brain functions such as the cognitive processes underlying speech, learning,

conscious perception, mood, planning, memory, and attention. The basal ganglia system has

inhibitory control in these circuits similar to the type of control in motor systems. This is

sometimes referred to as "behavioral switching," the ability to shift from inhibition to

disinhibition to effect functional outcomes.

B. Circle of Willis

Neuroanatomical studies of the brain also include important structures in peripheral

systems, such as the specialized vascular structure called the circle of Willis. The circle of Willis

is an interconnected assembly of blood vessels at the base of the brain formed by several arteries.

This structure mixes blood ascending from both the left and right sides of the body for supply to

the entire brain. If blood from one side of the body was blocked and unable to reach brain tissue,

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the blood supply from the other side of the body would reach the deprived hemisphere through

the circle of Willis.

This configuration provides convenient and life-saving redundancy should there be an

obstruction in blood flow. However, the system is also prone to anatomical variations that may

be reflected in the development of aneurysms.

In sheep brains that are purchased from a vendor for dissection, there is often not enough

blood in the vessels to easily visualize the circle of Willis. If you look closely though, you can

see the arteries as thin tubes. In the figure below, the main arteries that contribute to the circle of

Willis have been traced and superimposed on the anatomy, alongside the original, unlabeled

photo. As you dissected the sheep brain, you may have noticed empty spaces, or holes in the

tissue. These are often the lumen, or interior, of empty blood vessels. The brain has a vast array

of vessels and capillaries throughout the gray and white matter.

C. Limbic System

You have already identified the limbic lobe in several modules. The limbic lobe includes

the cingulate gyrus which forms a rim around the corpus callosum. It also includes the area on

the ventrolateral surface of the brain that is also called the pyriform lobe. This area is subdivided

into the entorhinal cortex and uncus. The image below depicts the human limbic lobe.

In contrast, the limbic system [Latin; border, belt] is a group of structures that borders the

thalamus and forms a functional system mediating emotions and motivation. The limbic system

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has a vast array of connections with the basal ganglia, cortex, and many other circuits to mediate

a wide range of behaviors such as emotions, cognition, judgment, impulsivity, learning, memory,

pain perception, motivation, reward, and others. It also has connectivity with the autonomic

nervous system through interactions with the hypothalamus.

Historically, the concept of the limbic system was originated to describe the neural

systems and mechanisms underlying emotions. Over time, a variety of structures have been

included and excluded from the list that comprises the limbic system. The thalamus,

hypothalamus, mammillary bodies, septal area, amygdala, hippocampus, anterior nucleus of the

dorsal thalamus, orbital and medial prefrontal cortex, nucleus accumbens, ventral basal ganglia,

mediodorsal nucleus of the thalamus and cingulate gyrus are included. However, even the

current list of structures varies. Some consider the idea of a limbic system obsolete. Much

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research has focused on the amygdala and hypothalamus, and their interaction with cerebral

cortical structures.

Studies have confirmed that the amygdala mediates processes that determine the

emotional significance of stimuli. Further, the amygdala forms associations between a stimulus

and potential reinforcement value. That is, the amygdala mediates associative learning. The

amygdala has direct connectivity to the hippocampus (learning) and hypothalamus (autonomic

nervous system activation) as well as with neocortical areas for higher order cognitive processes.

The septal area contains medial septal nuclei involved in memory processing and lateral

septal nuclei involved in motivated behaviors. Through the septohypothalamic tract it projects to

the hypothalamus. This tract also contains fibers from the fornix, conveying hippocampal

efferents. The fornix also merges its fibers with the mammillothalamic tract and one of the points

of termination is the tegmental nuclei. The mammillary bodies mediate memory, and are part of

the limbic system. The nucleus accumbens mediates anticipation of rewards associated with

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motivated behaviors and appears to play a role in emotions. It is studied extensively in addiction

research. The nucleus accumbens and the olfactory tubercle collectively form the ventral

striatum, which is part of the basal ganglia. This short module on the limbic system gives you an

introduction to the variety of structures participating in such a complex system, and an

appreciation for the degree of connectivity involved in mediating functions.

D. Reticular Formation

The reticular formation (RF) is a system of interconnected nuclei located along the

central core of the brainstem, including the medulla, pons, and midbrain. The number of different

nuclei and diffuse pattern of connectivity makes this system difficult to describe. The RF

mediates diverse functions through its interconnections in the brain and spinal cord. It can be

organized into divisions and zones, and contains many nuclei and tracts.

The reticular formation can be anatomically described as having two divisions. The

ascending reticular formation, also called the reticular activating system or RAS, participates in

mediating the sleep-wake cycle, conscious awareness, alertness, and habituation to stimuli.

Efferents from neurons in this division project to the thalamus and the cerebral cortex. The

descending reticular formation mediates posture, equilibrium, modulates sensory motor reflexes

and pain, and mediates various autonomic nervous system functions such as cardiovascular and

respiratory control. Neurons in this division receive afferents from the hypothalamus and have

connections with the cerebellum and spinal cord.

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Neurons in the reticular formation can also be organized into three longitudinal zones

along the brainstem core. Medial to lateral these are the raphe nuclei, the medial zone and lateral

zone. The raphe nuclei form a ridge adjacent to the midsagittal plane and have many long-

distance connections throughout the brain and spinal cord. Neurons in the medial zone are the

source of long projections from the reticular formation. The lateral zone neurons interact

primarily with cranial nerves.

The reticular formation contains nuclei of considerable importance. Bilateral damage to

the reticular formation can cause coma and death. The dorsal tegmental nuclei of the reticular

formation are in the midbrain, whereas the central tegmental nuclei are in the pons. The central

and inferior nuclei are found in the medulla. A few specific nuclei will be discussed here.

The raphe nucleus has already been mentioned. Given the number and distribution of

axonal projections throughout the brain, the behaviors modulated by the raphe nuclei are diverse

but include feeding, sleep, and mood. Neurons in the raphe nucleus are the

main source of the monoamine neurotransmitter serotonin.

The locus coeruleus is a nucleus that is the main source of neurons that synthesize and

release norepinephrine, another monoamine neurotransmitter. These neurons are in the dorsal

area of the pons and have wide projections to the thalamus, hippocampus, and cerebral cortical

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areas. LC neurons modulate diverse behaviors as well, such as mood, the sleep-wake cycle,

arousal, stress, attention, emotion, motivation, decision making and learning and memory.

The pedunculopontine nucleus is a main component of the reticular activating system. It

contains a variety of neurons, including those that synthesize and release acetylcholine,

glutamate, and GABA. It sends projections to the thalamus, basal ganglia, cerebral cortex and

lower brainstem to mediate functions such as arousal, attention, learning, reward, and

locomotion. The figure below depicts some components of the reticular formation in sheep brain.

E. Ventricular System

The ventricular system is an integrated network of hollows, aqueducts, and cisterns that

circulates cerebrospinal fluid throughout the central nervous system. Cerebrospinal fluid is a

watery, clear, and colorless liquid derived from blood. The average adult human has

approximately 150-200 ml total volume of CSF, and it is replaced approximately every three

days. CSF is a product of blood that has been filtered to remove various proteins, hormones, and

other chemicals. It is produced by a specialized vascular structure called the choroid plexus that

extends from the arachnoid membrane into the ventricular spaces.

The CSF provides nutrients and carries away waste products from the brain and spinal

cord. CSF also provides an interface between the environment and delicate brain and spinal cord

tissues. Under normal conditions, the fluid prevents the brain and spinal cord from hitting the

hard bony surfaces surrounding them. In addition, a hit to the head or sudden movement

produces mechanical waves that can damage tissue. To a degree, the fluid dissipates potentially

injurious forces.

CSF also helps maintain intracerebral pressure. Changes in pressure or disease states can

cause the ventricular spaces to either increase or decrease in size, and the ventricular system can

increase or decrease the production of CSF accordingly. The fluid also has diagnostic value. An

examination of proteins and other molecules in a CSF sample can determine the health of the

central nervous system.

Cerebrospinal fluid circulates throughout the brain and spinal cord and is recycled

through vascular spaces such as the superior sagittal sinus. This vascular structure is encased in

dura. The arachnoid membrane has clusters of specialized cells called granulations or villi that

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extend from the arachnoid into the superior sagittal sinus in order to return CSF to the venous

blood supply. This enables the CSF to continuously circulate throughout the ventricles.

The ventricles are identified in a rostral to caudal sequence. The lateral ventricles (first

and second ventricles) are the largest. They are located in the telencephalon of the cerebral

hemispheres. Cerebrospinal fluid is conveyed from the lateral ventricles down the

interventricular foramen/foramen of Magendie into the third ventricle. This ventricular space

surrounds the thalamus and extends toward the superior cistern. CSF from the third ventricle is

conveyed down the cerebral aqueduct/aqueduct of Sylvius to the fourth ventricle, which lies

beneath the cerebellum. All of the ventricular spaces contain choroid plexus material which

produces CSF. From the fourth ventricle, CSF is conveyed down the central canal to reach spinal

cord tissues.

There are specific areas where the dura does not fold but separates somewhat from the

arachnoid and pia, which remain against the brain tissue, but the subarachnoid space is increased

in size, creating cisterns [Latin; box]. The cisterns are full of cerebral spinal fluid. There are

several cisterns; a few are identified in this sagittal section of sheep brain.

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The cisterna magna is located at the caudal end of the cerebellum where dura meets the

medulla on the brainstem, and the superior cistern is located between the occipital lobes and

cerebellum where the tentorium sits. The interpeduncular cistern is between the cerebral

peduncles. You will see this more clearly in Module 7: Ventral View. The image above provides

a comparison between the sheep and human ventricular systems.

MODULE 12: SPINAL CORD

The spinal cord is an elongation of the central nervous system from the level of the

caudal medulla that extends through the foramen magnum at the base of the cranium. It is

encased in bony vertebrae and protective meninges and terminates at the lumbar level as the

filum terminale, which is actually pia mater.

There are several functional levels of the cord, two of which are enlarged due to the

amount of tissue required for innervation of the forelimbs (cervical enlargement) and hindlimbs

(lumbar enlargement). The thoracic level is narrow in comparison. The number of segments in

each region varies by species and is determined by the number of vertebrae. In humans, there are

8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal spinal cord segments. Mammals with

tails have additional coccygeal segments.

The spinal cord conveys all neural information between the brain and the periphery, and

also has local circuits that mediate reflexes.

When dissected in axial (or transverse) sections, there are several fissures and sulci that

provide convenient landmarks and delineate functional areas of tissue in the spinal cord.

However, as you can see on the figure below, without staining it is very difficult to identify

structures. The tracing yields a little more information. Sulci, fissures, and commissures provide

landmarks for identifying the anatomy. Notice that the spinal cord has anterior and posterior

commissures as does the cerebrum.

After a simple histochemical stain, the white matter and gray matter are distinguishable

and enable further study. The white matter areas contain many myelinated axon tracts organized

in well-defined columnar arrangements, called funiculi (funiculus, singular). The core of the

spinal cord contains an H-shaped arrangement of neuronal somas (and associated glia) that

surrounds the cerebrospinal fluid-filled central canal. These are also organized in columnar

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arrangements as well as segmental. Tracts from the white matter make connections with neurons

in the gray matter at each level of the cord.

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The white matter is organized in funiculi, which contain many columnar bundles of

myelinated axons that extend longitudinally throughout the cord. There are three main funiculi;

dorsal, lateral, and ventral. The funiculi are further subdivided into fasciculi. For example, the

dorsal columns contain the fasciculus gracilis and fasciculus cuneatus, which mediate sensory

function from the lower body and arms, respectively. The spinal cord funiculi contain many

tracts, each performing a specialized function.

The gray matter is organized into dorsal, lateral (in some areas) and ventral horns, and an

intermediate gray area with gray matter commissures that are an area of decussation. Nuclei in

the gray matter are organized in columns and segmentally in lamina.

The spinal cord is a highly organized system of both long-distance and segmental

connectivity, containing all inputs from the periphery to the brain and all outputs from the brain

to the periphery. In humans, the spinal cord is the size of your pinky finger.

Each spinal cord segment

includes a pair each of dorsal and

ventral roots. The dorsal root afferents

convey sensory information whereas

the ventral root efferents convey

outputs to muscles, blood vessels,

organs, or glands. Other than the 12

pairs of cranial nerves, all nerves of the

peripheral nervous system that interact

with the brain do so through spinal

cord connections.

The junction of dorsal and

ventral roots forms a pair of spinal

nerves bilaterally at each segment of

the spinal cord. Cell bodies for the

sensory dorsal roots are outside of the

spinal cord, in the dorsal root ganglia.

Cell bodies for the ventral motor roots

are inside the spinal cord in the gray

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matter ventral horn. Sensory and motor fibers at each segment join ascending and descending

tracts within the cord.

The spinal cord typically terminates at the first lumbar level, which results in a large

number of roots extending further down the vertebral column before exiting at the appropriate

level. This bundle of fibers is called the cauda equina, or horse's tail.

The Reflex A circuit formed from a sensory neuron interacting with a muscle is called a

reflex arc. The sensory afferents enter the dorsal root entry zone and synapse in the dorsal horn

on interneurons. Interneurons then convey the inputs to motor neurons in the ventral horn which

send fibers through the ventral root exit zone and project outputs to synapses on muscle fibers.

The interneurons excite a motor neuron for the flexor muscle while simultaneously inhibiting the

motor neuron for the extensor muscle. Reflex activity occurs in spinal cord segments and does

not require interaction with the brain. This is one of the simplest circuits in the nervous system.