neuro basics

7/22/2019 Neuro Basics

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Chapter 15 - The BrainDevelopment of the Brain

The central nervous system (CNS) begins as a hollow neural tubewith a fluid-filled internal cavity. In thefourth week of development the cephalic (head) portion of the tube expands to form three primary brain vesicles:

1. Prosencephalon forebrain

2. Mesencephalon midbrain 3. Rhombencephalon hindbrain

As development proceeds, the prosencephalondivides into two new expanded portions called secondary brainvesicles:

a.Telencephalon This portion develops into the cerebral hemispheres.

b.Diencephalon This portion retains the name diencephalon and subdivides into the epithalamus,thalamus and hypothalamus. The eyes will also develop from optic vesicles that extend laterally from thediencephalons.

The mesencephalon does not divide and is also called the midbrain.

The rhombencephalon undergoes subdivision into the:

a.Metencephalon The portion is next to the midbrain. The ventral portion of the metencephalon developsinto the pons and the dorsal portion expands to form the cerebellum (little cerebrum).

b. Myelencephalon The portion closest to the spinal cord forms the medulla oblongata.

The adult brain has 6 major landmarks: 1. cerebrum, 2. diencephalon, 3. mesencephalon, 4. pons, 5.cerebellum, 6. medulla oblongata. The mesencephalon, pons and medulla oblongata are often collectivelyreferred to as the brainstem.


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Ventricles of the Brain

The fluid-filled interior of the embryos neural tube becomes the fluid-filled ventricles of the adult brain. Theventricles are lined by ependymal cells and are filled with cerebrospinal fluid (CSF). There are four ventricles ithe brain:

The first and second ventricles are contained within the cerebral hemispheres and have a complex shapedue to the expansion of the cerebral hemispheres. These ventricles are called the lateral ventricles becausethey are side-by-side, separated in the middle by a thin partition called the septum pellucidum. Each lateralventricle communicates (in the sense of being interconnected) with the third ventricle by theinterventricular foramina (sing. foramen).

The third ventricle is within the diencephalons and communicates with the fourth ventricle by a narrow

passage within the mesencephalon called the aqueduct of the midbrain (a.k.a. aqueduct of Sylvius, or,cerebral aqueduct).

The fourth ventricle is between the pons and the cerebellum and within the superior portion of the

medulla oblongata. The fourth ventricle communicates with the central canal of the spinal cord.


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Cranial Meninges

The brain is protected from damage by the cranium but also needs protection from the hard interior of the skulduring sudden changes in movement. The cranial meninges provide this protection by securing the brain inposition inside the fluid-filled space created by the cerebrospinal fluid.

The cranial meninges have three layers:

Dura Mater

The dura mater is the outermost layer and consists of two fibrous layers:


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1.Endosteal Layer fused to the periosteum lining the cranial bones.

2.Meningeal Layer the innermost layer.

In certain areas the endosteal and meningeal layers separate and are lined by endothelium to form the durasinuses.

At four locations the dura mater extends into the cranial cavity to provide greater support for the brain:

1. Falx cerebri projects in between the cerebral hemispheres along the longitudinal fissure. The

superior sagittal sinus and inferior sagittal sinus are found within this fold.

2.Tentorium cerebelli separates and protects the cerebellar hemispheres from those of the cerebrum.The transverse sinus is found within the tentorium cerebelli.

3. Falx cerebelli extends from the midsagittal line and divides the cerebellar hemispheres.

4. Diaphragma sellae is an extension of the dura mater from the rim of the depression (hypophysealfossa) formed by the sella turcica. It surrounds the stalk that attaches the pituitary gland to thehypothalamus.


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Arachnoid Mater

The outermost part of the arachnoid mater lines the inner surface of the dura mater. From this lining a web-like mesh of collagen and elastic fibers extend across a fluid-filled space, called the subarachnoid space,and attach to the pia mater. Blood vessels are found in the subarachnoid space.

Along the superior sagittal sinus, arachnoid extends through the meningeal layer of the dura mater andprojects into the sinus. These projections are called arachnoid granulations and are places where CSFflowinto the sinus.

Pia Mater

The pia mater is a membrane that is tightly attached to the surface of the brain by astrocytes. The pia materfollows the contour of the brain and forms the floor of the subarachnoid space and supports the cerebral


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vessels found there.

The Blood-Brain Barrier (BBB)

The BBB protects the nervous tissue from substances in the blood that would interfere with its normal activityFor example, glycine, which is a neurotransmitter, is selectively transported out of the nervous tissue into theblood.

At the same time the BBB protects the proper working of the nerves by selectively transporting materials fromthe blood into the nervous tissue. An example of this is glucose, the preferred fuel for nerve cells.

The BBB is created by the tight junctions between the endothelial cellsand the association of the endothelialcells with astrocytes.

Cerebrospinal Fluid (CSF)

The CSF completely surrounds and bathes the surfaces of the central nervous system. Its functions include:

1. Cushioning the delicate neural tissue.

2.Supporting the physical mass of the brain as the brain essentially floats in a fluid-filled container.

3.Transporting nutrients, chemical messengers and waste products.

Formation of CSF

The CSF originates from the choroid plexus. The choroids plexus consists of permeable capillaries associatedwith specialized ependymal cells that have tight junctions between them.

Choroid plexuses are found in:

a.The roof of the third ventricleand floors of the lateral ventricles. Folds that originate in the thirdventricle extend in the lateral ventricles through the interventricular foramina.

b.The roof of the fourth ventricleand extending between the cerebellum and pons.

The ependymal cells of the choroids plexus control the composition of the CSF by active transportmechanisms.


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Circulation of CSF

CSF is continually produced in the choroids plexus at the rate of 500 ml/day. The CSF that reaches the fourthventricle enters the subarachnoid space by passing through the lateral apertures(openings) and median

apertureof the roof of the 4thventricle. The CSF circulates through the subarachnoid space surrounding the braiand spinal cord. The CSF finally reenters the circulation through the arachnoid granulations.


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Cerebrum

The cerebrum is the largest region of the brain.

The many functions of the cerebrum include:

1. Controlling thought processes and intellectual functions

2. Processing somatic sensory and motor information

3. Exerting voluntary and involuntary control over somatic motor neurons

Cerebral Hemispheres The cerebrum consists of paired cerebral hemispheres. The gray matter on the surface is called cerebralcortex. The cerebral cortex is where the most complicated higher level functions of the brain are performed. The

surface area of the cerebral cortex is about 2200 cm2(2.5 ft2) and fits into the cranium because of its crumpledappearance. This results in the cortex being thrown into folds and creases.

The folds of the cortex are called gyri (sing. gyrus) and the creases or grooves are called sulci (sing. sulcus)The deeper sulci are called fissures.


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Cerebral Lobes A deep longitudinal fissure separates the two cerebral hemispheres and each cerebral hemisphere is dividedinto lobes whose names relate to the overlying skull bones:

1.Frontal lobe is delimited by the central sulcus and the lateral sulcus.

2. Temporal lobe is inferior to the lateral sulcus.

3. Parietal lobe extends posteriorly from the central sulcus to the parieto-occipital sulcus.

4.Occipital lobe is the region posterior to the parieto-occipital sulcus.

Surfaces of the cerebral cortex that are hidden from view include the:

Insula This region of cortex is concealed inside the lateral sulcus by overlying folds of the temporal, frontaand parietal lobes (It is almost as if the surface were sucked inward.)

Cingulate lobe The cingulated lobe borders the corpus callosum and is only visible in the hemisected,sagittal view of the brain.


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Three important points about the function of the cerebral hemispheres are:

1. Each cerebral hemisphere is concerned with the sensory and motor functions of the opposite side of thebody.

2. Some functions are present predominantly in one cerebral hemisphere. This phenomenon is described aslaterality.

3. The assignment of a specific function to a specific region of the cortex is imprecise.

Motor and Sensory Areas

The central sulcus separates the primary motor and sensory areas of the cortex.

The primary motor cortex is found on the precentral gyrus of the frontal lobes. Neurons called pyramidacells that direct muscle contraction originate here.


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The primary somatosensory cortex is found on the postcentral gyrus of the parietal lobes. Neurons herereceive somatic sensory information including, touch, pressure, tissue damage (pain) and temperature.

The visual cortex of the occipital lobes processes visual information.

The auditory cortex and olfactory cortex is found in the temporal lobes and processes hearing and smell,respectively.

The gustatory cortex lies in the anterior portion of the insula and adjacent parts of the frontal lobes andprocesses taste information.

Association Areas

The association areas are where the individual bits of information or individual commands are broughttogether (associated) for interpretation and coordination.

The somatosomatic sensory association areas in the parietal lobes use the somatic sensory input received bthe somatic cortex to interpret size, form and texture.

The somatic motor association areas (a.k.a. premotor cortex) in the frontal lobes plan and coordinatemovements involving different muscle groups.

The visual association areas in the occipital lobes interpretthe individual points of light as lines, contoursand colors.

Integrative Centers

The integrative centers brings together (integrates) information and commands from different association areato perform higher order analytical functions. For example, the prefrontal cortex of the frontal lobe enables you tpredict the consequences of different behaviors, plan future behaviors and to perform tasks using workingmemory.

The functions of integrative centers include those devoted to language production (Brocas area), languagecomprehension (Wernickes area), mathematical computation, and spatial awareness. These centers arelocated predominantly in one cerebral hemisphere. For example, language production and comprehension arelocated in the left cerebral hemisphere in the vast majority of people.

Central White Matter

Different regions of the surface cerebral cortex communicate with each other and with other regions of the

CNS by myelinated fibers that form the central white matter of the cerebrum. The myelinated fibers form bundles of fibers that can be divided into three types:

1.Association fibers interconnect portions of the cerebral cortex with those in the same cerebralhemisphere.

2.Commissural fibers cross from one hemisphere to the other and permit communication between them.The corpus callosum and the anterior commissure are examples.

3. Projection fibers connect the cerebral cortex with other parts of the CNS including diencephalon, brainstem, cerebellum and spinal cord.


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Basal Ganglia (a.k.a. Basal Nuclei)

The basal ganglia are actually nuclei that are found in the cerebral hemispheres, diencephalon andmidbrain. The basal ganglia include:

1. Caudate Nucleus (tailed nucleus) 2. Putamen (husk)

3. Globus Pallidus (pale globe)


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4. Subthalamus (below the thalamus)

5. Substantia Nigra (black substance)

The caudate nucleusandputamen are similar histologically and are together called the striatum because ofthe myelinated fibers that run through them.

The putamenand globis pallidusare together referred to as the lentiform (lenticular) nucleus becausetogether they reminded the anatomist of a lens.

The functions of the basal ganglia are not completely understood but with the cerebral cortex and thalamusthey form circuits which influence affect (emotions), memory, cognitive(thinking) functions and sensorimotorfunctions

The best know diseases associated with the basal ganglia are:

Parkinsons disease A disease manifested by involuntary tremors, bradykinesia and abnormalities in

muscle tone and caused by death of cells in the substantia nigra that secrete the neurotransmitter dopamine. Huntingtons chorea A disease whose signs include involuntary movements and dementia.

Limbic System

The limbic system consists of cerebraland diencephalic componentswith theirinterconnecting tracts. Thename limbic (border) derives from the fact that these components are located around the border between thecerebrum and diencephalon.


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The functions of the limbic system include:

1. Establishment of emotional states and behavioral drives.

2. Linking the conscious, intellectual functions of the cerebral cortex with unconscious and autonomicfunctions.

3. Facilitating memory storage and retrieval.

Diencephalon

The diencephalon connects the cerebral hemispheres to the midbrain and includes the:

Epithalamus

The epithalamus forms the roof of the third ventricle. The anterior portion is membranous and is the locationof the choroid plexus which protrudes into the third ventricle and extends by way of the interventricularforamina into the lateral ventricles.

The posterior portion of the epithalamus contains the pineal gland which is involved with the control ofsleep-wake cycles.

Thalamus

The thalamus consists of a pair of egg-shaped concentrations of nuclei that flank the third ventricle. Thepaired right and left thalami are often connected by a bridge of neural tissue called the interthalamic adhesion.

The thalami have both sensory and motor functions. All sensory information, with the notable exception ofolfaction, is processed by the thalami before being finally relayed to the cerebral cortex. In this position thethalami may act as information filters. The thalami also participate with the sensorimotor cortex and basalganglia in the initiation, control and coordination of movement.


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Hypothalamus

The hypothalamus forms the floor of the third ventricle and extends from the area superior to the opticchiasm to the posterior margins of the mammillary bodies. Posterior to the optic chiasm, the floor of the thirdventricle forms a funnel-shaped extension called the infundibulum which connects the pituitary gland to thehypothalamus.

Functions of the hypothalamus include:

1. Control of Autonomic Function

The hypothalamus regulates heart rate, blood pressure, respiration and digestive functions

2. Control of Pituitary Gland Function Chemical messengers released by neurons in the tuberal area (between the optic chiasm and mammillarybodies) control secretion of hormones by the anterior pituitary.

3. Secretion of Hormones

Neurons of the hypothalamus secrete two hormones, antidiuretic hormone and oxytocin, which arereleased at the posterior pituitary.

4. Production of Emotions and Drives

Regions of the hypothalamus are associated with emotions such as aggression and contentment anddrives such as hunger and thirst.

5. Regulation of Body Temperature

The "thermostat" that enables the nervous system to produce physiological responses that adjust bodytemperature is in the hypothalamus.

6. Control of Circadian (Daily) Rhythms

The hypothalamus receives input from the eyes that enable it to adjust daily rhythms.

Mesencephalon


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The cerebral aqueduct (a.k.a. mesencephalic aqueduct or aqueduct of Sylvius) runs through themesencephalon, or midbrain. The region dorsal to the cerebral aqueduct is called the tectum ("roof") and theregion below the aqueduct is called the tegmentum ("covering").

Tectum

The tectum contains two pairs of sensory nuclei that are collectively called the corpora quadrigemina (four"twin" bodies) that are functionally divided into two pairs:

Superior colliculi (sing. colliculus)

The superior colliculi receive visual inputsfrom the lateral geniculate nuclei of the thalamus andgenerates reflexive responses to this information that particularly involve eye movements.

Inferior colliculi

The inferior colliculi receive auditory inputsfrom the medial geniculate nuclei of the thalamus andgenerates reflexive responses to this information.

Tegmentum

The important nuclei found in the tegmentum include:

Red nuclei

The red nuclei appear red because of the numerous blood vessels the course within it. These nuclei areassociated with the control of skeletal movements.

Substantia nigra

The substantia nigra ("black substance") consists of gray matter containing darkly pigmented cells thatgive rise to its name. The substantia nigra, as part of the basal nuclei, plays a role in the initiation andcontrol of skeletal movement.


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Cerebral Peduncles (Crus Cerebri)

The cerebral peduncles are located on the ventrolateral surfacesof the midbrain and contain primarilydescending fibers including motor commands descending to the spinal cord and sensory information going tothe thalamus.

Pons

The pons is a prominent bulge of the anterior surface of the brain stem. The pons is ventral to the anterior parof the 4th ventricleand is attached to the cerebellum by the middle cerebellar peduncles.

The pons contains:

1. Nuclei for Cranial nerves V, VI, VII and VIII

2. Nuclei involved with the control of respiration

3. Nuclei that process and transfer cerebral inputs to the opposite side of the cerebellum by means oftransverse fibers.

4. Ascending and descending tracts.

Cerebellum

The cerebellum consists of two cerebellar hemispheres that contain neural cortex folded into a smaller spaceThe smaller folds are called folia ("leaves") deeper folds include the primary fissure that separates the cerebellar


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hemispheres into anterior and posterior lobes. The cerebellum is attached to the pons and the rest of the brain steby the superior, middle and inferior cerebellar peduncles.

The functions of the cerebellum include:

1. Maintaining equilibrium

2. Controlling eye movements

3. Adjusting the postural muscles

4. Programming and fine tuning movements

Medulla Oblongata

The medulla oblongata is the part of the brainstem that connects to the spinal cord.

The medulla oblongata includes:

Nuclei that serve as relay stations along sensory and motor pathways. These nuclei include:

Nucleus gracilis and nucleus cuneatus which pass somatic sensory information from the spinal cord to

the thalamus.

Olivary nuclei which are located deep to prominent bulges along the ventrolateral surface of the


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medulla called the olives. These nuclei relay information from the spinal cord, diencephalon, cerebralcortex and brain stem to the cerebellar cortex.

Nuceli of cranial nerves N VIII toN XII.

Autonomic nuclei that include:

Cardiovascular centers that adjust and control heart functions and blood pressure.

Respiratory rhythmicity center that affects the respiratory rate.

Fibers that descend from higher centers to the spinal cord are found on the ventral surface of the medulla in

structures called pyramids.

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