Chapter 14

The Brain and Cranial Nerves

The Brain• The Brain is part of Central Nervous

System (CNS)• It is divided into 6 major parts:

– The cerebrum– The diencephalon– The mesencephalon– The cerebellum– The pons– The medulla oblongata

Major divisions of the Brain

Fig.14.01b

The real thing

The Human Brain

• Ranges from 750 cc to 2100 cc• Contains almost 98% of the body’s

neural tissue• Average weight about 1.4 kg (3 lb)

Protection of the Brain

• The brain is protected by bone, meninges, and cerebrospinal fluid

• Harmful substances are shielded from the brain by the blood-brain barrier

Cerebrospinal fluid

• Protects by absorbing shock (it “floats” the brain.

• Maintains the electrochemical environment (opitmal pH & osmolality).

• Circulates nutrients and wastes.

The Ventricles

Formation of CSF

Choroid Plexuses

Figure 12.25a

Fig. 14.04c

Cirulaation of CSF

Meninges

• Three connective tissue membranes lie external to the CNS – dura mater, arachnoid mater, and pia mater

• Functions of the meninges– Cover and protect the CNS– Protect blood vessels and enclose venous

sinuses– Contain cerebrospinal fluid (CSF)– Form partitions within the skull

The Meninges

of the Brain

Dura Mater

• Three dural septa extend inward and limit excessive movement of the brain– Falx cerebri – fold that dips into the

longitudinal fissure– Falx cerebelli – runs along the vermis

of the cerebellum– Tentorium cerebelli – horizontal dural

fold extends into the transverse fissure

Figure 14–3bDural Folds

• Falx cerebri, tentorium cerebelli, and falx cerebelli

Arachnoid Mater• The middle meninx, which forms a loose

brain covering• It is separated from the dura mater by

the subdural space• Beneath the arachnoid is a wide

subarachnoid space filled with CSF and large blood vessels

• Arachnoid villi protrude superiorly and permit CSF to be absorbed into venous blood

Blood flow, O2 and the BBB• Though only 2% of body mass the brain

consumes 20% of the oxygen and glucoseof the resting body.

• No O2 for a minute = dizziness– 4 minutes = brain damage– 6 minutes = death

• The brain is protected by the blood-brain barrier.

Blood-Brain Barrier: Functions

• Selective barrier that allows nutrients to pass freely

• Is ineffective against substances that can diffuse through plasma membranes

• Absent in some areas (vomiting center and the hypothalamus), allowing these areas to monitor the chemical composition of the blood

• Stress increases the ability of chemicals to pass through the blood-brain barrier

The Blood-brain

barrier

The Brain Stem

• The brain stem is comprised of 3 main subdivisions:– The Medulla Oblongata– The Pons– The Mesencephalon

The diencephal

on & brainstem

Medulla Oblongata: 3 Groups of Nuclei

1. Autonomic nuclei:– control visceral activities

2. Sensory and motor nuclei:– of cranial nerves

3. Relay stations:– along sensory and motor pathways

Nuclei of the medulla oblongata

Table 14-2Summary: The Medulla Oblongata

Figure 14–6c

The Pons

• Links cerebellum with mesencephalon, diencephalon, cerebrum, and spinal cord

The Pons: 4 Groups

1. Sensory and motor nuclei of cranial nerves V, VI, VII, VIII

2. Nuclei involved with respiration:– apneustic center and pneumotaxic

center – modify respiratory rhythmicity

center activity

The Pons: 4 Groups

3. Nuclei and tracts:– process and relay information to and

from cerebellum

4. Ascending, descending, and transverse tracts:

– transverse fibers (axons) • link nuclei of pons with opposite cerebellar

hemisphere

Figure 14–7a

The Cerebellum• An autonomic & motor processing center

Functions of the Cerebellum

1. Adjusts postural muscles 2. Fine-tunes conscious and

subconscious movements

Figure 14–7b

Structures of the Cerebellum

Structures of the Cerebellum (1 of 2)

• Folia: – surface of cerebellum– highly folded neural cortex

• Anterior and posterior lobes:– separated by primary fissure

Structures of the Cerebellum (2 of 2)

• Cerebellar hemispheres:– separated at midline by vermis

• Vermis:– narrow band of cortex

• Flocculonodular lobe:– below fourth ventricle

Table 14-3

Summary: The Cerebellum

Figure 14–8aThe Mesencephalon

Midbrain nuclei

• Substantia nigra: dopamine release, control of subconscious muscle movents.

• Red nuclei: synapses between neurons of cerebellum and cerebrum. Also origin of oculomotor nerve (CN III) and trochlear nerve (CN IV).

Some major functions mesencephalon

• Cerebral peduncles: contain motor axons connecting cerebrum to brain stem and sensory axons that connect medulla to thalamus

• Tectum:– Superior colliculi: visual reflexes, including

pupillary reflexes.– Inferior colliculi: auditory path from ear to

thalamus. – Startle reflex

Table 14-4

Summary: The Mesencephalon

The Diencephalon• The Thalamus - switch board for the

cerebrum. Includes the:– Geniculate nuclei (part of the ventral group):

• Medial processes auditory information.• Lateral processes visual input.

– Other ventral nuclei connect motor & sensory areas of cortex to cerebellum & spinal cord

– Anterior & medial nuclei• Anterior connects to limbic system &

hypothalamus• Medial connects with cortex & limbic system

The Diencephal

on

The Thalamus:Filters ascending sensory information for primary sensory cortex.Relays information between basal nuclei and cerebral cortex

Figure 14–9

Fig. 14.09a-d

Those pesky

thalamic nuclei

Fig. 14.09e

Oblique section of brain showing the Thalamus and basal nuclei

Table 14-5

Summary: Thalamic Nuclei

The HypothalamusFunctions of the hypothalamus• ANS control

– , heart rate, respiration, sweating, digestion, urination, stress response

• Hormones - ADH, oxytocin, GnRH et al.• Basic emotions - rage, sex drive, fear,

pain pleasure.• Thirst & appetite• Temperature & sleep/wake (circadian)

cycles

Figure 14–10a

The Hypothalamus• Lies below thalamus

Table 14-6

Summary: The Hypothalamus

The Limbic System

• Is a functional grouping that:– establishes emotional states– links conscious functions of cerebral

cortex with autonomic functions of brain stem

– facilitates memory storage and retrieval

Figure 14–11aThe Limbic System

Figure 14–11bThe Limbic System

Limbic System• Structures located on the medial aspects of

cerebral hemispheres and diencephalon

• Includes the rhinencephalon, amygdala, hypothalamus, and anterior nucleus of the thalamus

• Parts especially important in emotions:

– Amygdala – deals with anger, danger, and fear responses

– Cingulate gyrus – plays a role in expressing emotions via gestures, and resolves mental conflict

• Puts emotional responses to odors – e.g., skunks smell bad

Limbic System: Emotion and Cognition

• The limbic system interacts with the prefrontal lobes, therefore:– One can react emotionally to

conscious understandings– One is consciously aware of emotion

in one’s life• Hippocampal structures – convert

new information into long-term memories

Table 14-7Summary: The Limbic System

The CerebrumMajor divisions of the Cerebrum• Hemispheres - left and right split down

the middle by the longitudinal fissure.• Lobes:

– Frontal, parietal, occipital, temporal and insula

– Delineated by the sulci (singular sulcus):• Central• Parieto-occipital• Lateral

Lobes of the Cerebrum

Landmarks of the

cerebrum

Figure 14–13

White Matter of the

Cerebrum

Myelinated fibers (axons)1. Association fibers2. Commissural fibers3. Projection fibers

Cerebral White Matter

Figure 14–14b, c

The Basal Nuclei

Functions of Basal Nuclei

• Are involved with:– the subconscious control of skeletal

muscle tone– the coordination of learned

movement patterns (walking, lifting)

Fig. 14.15Functional map of the cerebrum

Figure 14–15a

Motor and Sensory Areas of the Cortex

Figure 14–15b

Interpretive Areas of Cortex

Figure 14–16

Hemispheric Lateralization

The Left Hemisphere

• In most people, left brain (dominant hemisphere) controls:– reading, writing, and math– decision-making– speech and language

The Right Hemisphere

• Right cerebral hemisphere relates to:– senses (touch, smell, sight, taste,

feel)– recognition (faces, voice inflections)

Figure 14–17

The Electroencephalogram

Types of Brain Waves• Alpha waves – 8 - 13 Hz, regular and rhythmic,

low-amplitude, slow, synchronous waves indicating an “idling” brain

• Beta waves – 14 - 30 Hz, rhythmic, more irregular waves occurring during the awake and mentally alert state

• Theta waves – 4 - 7 Hz, more irregular than alpha waves; common in children but abnormal in adults

• Delta waves – 1 - 5 Hz, high-amplitude waves seen in deep sleep and when reticular activating system is damped

Brain Waves: State of the Brain

• Brain waves change with age, sensory stimuli, brain disease, and the chemical state of the body

• EEGs can be used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic lesions

• A flat EEG (no electrical activity) is clinical evidence of death

Figure 14–18Cranial Nerves

The 12 cranial nerves

1. On2. Old3. Olympus’4. Towering5. Top6. A7. Frenchman 8. Viewed 9. Greek10.Villagers11.At12.Home

Figure 14–19

The Olfactory Nerve (I)

Figure 14–20

The Optic

Nerves (II)

Figure 14–21

The Oculomotor Nerves (III)

Figure 14–21

The Trochlear Nerves (IV)

The Abducens Nerves (VI)

Figure 14–21

Figure 14–22The Trigeminal Nerves (V)

Figure 14–23

The Facial Nerves (VII)

Figure 14–24

The Vestibulocochlear Nerves (VIII)

Figure 14–25

The Glossopharyngeal Nerves (IX)

Figure 14–26

The Vagus Nerves (X)

Figure 14–27

The Accessory Nerves (XI)

Figure 14–27The Hypoglossal Nerves (XII)

Table 14-9 (1 of 2)

Summary: Cranial Nerves

Table 14-9 (2 of 2)

Summary: Cranial Nerves

Table 14-10

Examples of Cranial Reflexes

Clinical Stuff

• Epilepsy• CVA & TIA• Degenerative Diseases

– Alzheimers– Huntington’s– Parkinbson’s

Epilepsy

• A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking, characteristic of epileptic seizure

• Epilepsy is not associated with, nor does it cause, intellectual impairments

• Epilepsy occurs in 1% of the population

Epileptic Seizures

• Absence seizures, or petit mal – mild seizures seen in young children where the expression goes blank

• Grand mal seizures – victim loses consciousness, bones are often broken due to intense convulsions, loss of bowel and bladder control, and severe biting of the tongue

Control of Epilepsy

• Epilepsy can usually be controlled with anticonvulsive drugs

• Valproic acid, a nonsedating drug, enhances GABA and is a drug of choice

• Vagus nerve stimulators can be implanted under the skin of the chest and can keep electrical activity of the brain from becoming chaotic

Cerebrovascular Accidents (Strokes)

• Caused when blood circulation to the brain is blocked and brain tissue dies

• Most commonly caused by blockage of a cerebral artery

• Other causes include compression of the brain by hemorrhage or edema, and atherosclerosis

• Transient ischemic attacks (TIAs) – temporary episodes of reversible cerebral ischemia

• Tissue plasminogen activator (TPA) is the only approved treatment for stroke

Degenerative Brain Disorders

• Alzheimer’s disease – a progressive degenerative disease of the brain that results in dementia

• Parkinson’s disease – degeneration of the dopamine-releasing neurons of the substantia nigra

• Huntington’s disease – a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei