Chapter 14

Copyright © John Wiley & Sons, Inc. All rights reserved.

Chapter 14

The Brain and Cranial

Nerves


The Human Brain


The Human Brain The human brain, compared to all other animals’

brains, is marked by the highest ratio of brain to

body size - thought to directly correlate with our

higher level of intelligence.

Most of the expansion is manifest in man’s

large cerebral cortex. Especially expanded are

the frontal lobes which are associated with

higher (executive) functions

such as self-control, planning,

reasoning, and abstract thought.


Brain Development During the first 3 weeks of gestation, the human

embryo's neural tube flexes as it grows, forming

the three primary brain vesicles colloquially

called

the forebrain, midbrain, and

hindbrain. The 1st and 3rd

vesicles further divide forming

5 secondary brain vesicles in a

process called encephalization.


Brain Development The major parts of the adult brain are directly

derived from the 2o brain vesicles: From the

crescent-shaped cerebral hemispheres of the

telencephalon to the inferiorly placed brain stem

formed from the

metencephalon and

myelencephalon


Brain Development The brain grows at an amazing rate during

development; at times, as many as 50,000

neurons are added each second!

At birth, the neonatal brain looks very much like

that of an adult and almost all

the neurons the brain

will ever have are

already present.


Brain Organization The brain stem is the continuation of the

spinal cord and consists of the medulla

oblongata, pons and midbrain.

The cerebellum is the second largest part of

the brain.

The diencephalon gives rise to the thalamus

& hypothalamus.

The cerebrum is the newest (evolutionarily)

and largest part of the brain as a whole.

It is in the cerebral cortex that perception,

thought, imagination, judgment, and decision

making occur.


Brain Organization The major parts of the adult brain are shown here


Protective Coverings The cranial meninges are continuous with

the spinal meninges and mirror their structure

and function – they also bear the

same names: a tough outer dura mater a spidery arachnoid

mater and a thin,

delicate pia

mater


Protective Coverings The cranial dura mater, however, has two

layers – an external periosteal layer and an

internal meningeal layer; the spinal dura mater

has only one.

In the brain, extensions of the dura mater

form hard, non-compliant membranes that

divide the intracranial vault in various ways:.

The 3 important dural extensions are the falx

cerebri, the falx cerebelli, and the

tentorium cerebelli.


Protective Coverings The falx cerebri is a strong sickle-shaped fold

of dura mater which descends vertically in the

longitudinal fissure and separates the two

cerebral hemispheres.

The falx cerebelli is a

small triangular

process that separates

the two cerebellar

hemispheres.


Protective Coverings Clinically, the tentorium cerebelli is

important because brain tumors are often

characterized as supratentorial (above the

tentorium) and infratentorial (below the

tentorium). Most childhood tumors are

infratentorial, while most adult tumors are

supratentorial.

Since the tentorium is a hard structure, if

there is any brain swelling the brain can get

partly pushed down and herniate through the

tentorium, which becomes a life-threatening

event.


The brain represents only 2 percent of total body

weight, but receives about 20% of the body’s

blood supply and consumes 20% of the O2 and

glucose (even when resting).

Anteriorly, the internal carotid arteries

supply blood to the brain; the posterior blood

supply is

via the vertebral arteries.

The internal

jugular veins are

the venous return

Brain Blood Flow


The vascular endothelium around brain

capillaries differs from most other organs of the

body in that it forms tight junctions with the

end-feet of nearby astrocytes.

As a result of this unusual architecture, a blood

brain barrier (BBB) is formed

that serves to isolate the

parenchyma of the brain

from many substances in

the blood that would

normally be able to gain access.

Encyclopedia of Life Sciences, © Wiley

Brain Blood Flow


Brain Blood Flow The BBB protects the brain from some harmful

substances (like bacteria), but at a cost:

For one thing, certain molecules needed to

meet metabolic needs (such as glucose) must

be actively transported across the barrier

using specific transport proteins and energy.

Another aspect of the BBB is that if a brain

infection were to develop, antibiotics (and

many other drugs) have difficulty crossing into

the brain tissues and reaching therapeutic

levels.


Brain Blood Flow The BBB can be used advantageously in certain

pharmacological situations. For example, it is well

known that certain older antihistamine drugs readily

cross the BBB and cause sedation. This makes these

drugs not very good antihistamines, but very helpful in

cough and cold medications to induce sleep (NyQuil).

Drugs companies have devoted enormous resources

to the development of newer 2nd and 3rd generation

antihistamines that do not cross the BBB, and don’t

(usually) cause somnolence – they also charge you

accordingly.


Cerebral spinal fluid is a clear fluid that

circulates through the internal cavities in the

brain (called brain ventricles) and spinal cord

(the central canal) and also flows over and

around the brain and cord in the subarachnoid

space. In essence, the brain "floats" in it.

CSF absorbs shock and protects the brain and

the cord.

• It also helps transport nutrients and wastes

between blood and nervous tissues.

Production and Flow of CSF


The Irrigation System of the fluid filled brain showing the

circulating CSF.



Production and Flow of CSF The majority of CSF production - 80 to 150 mL at

any given time in an adult - comes from

ependymal cells in the choroid plexuses

(networks of blood capillaries that line the

ventricles).

The pathway CSF follows from the internal

ventricles to the SAS is given in the following

sequence:

• lateral ventricles interventricular foramina

third ventricle cerebral aqueduct fourth

ventricle median aperture (of Magendie )

and the lateral apertures (of Luschka ) SAS


Production and Flow of CSF The choroid plexuses can be seen in this

superior view of a transverse section through

the brain:


Once in the SAS, CSF flows continuously between the

pia mater covering the brain and the arachnoid that

is tightly

adhered to the outer dura.

Pressure remains

constant in the head

because the rate of

fluid reabsorption

closely matches fluid

formation at approx.

20 mL/h.



__ _________ __________ ____ ____ ___ _____

_______ ___ _________ _____ ______ ____

___________ ____ ______ ____ ___ _____ _______

_ ____ _______ ___

_________ _____ ___

____ __________

____ ___ ________

________ _____ _______ __

_________ _____ __ ______

__ _________ ___________

CSF is gradually reabsorbed back into the blood

through the arachnoid villi (finger-like

projections that extend into the dural sinuses).

In this graphic, the

arachnoid villi are

seen projecting

into the superior

sagittal sinus. A cluster of

arachnoid villi is called

an arachnoid granulation.



Failure of CSF to form and drain normally results

in a buildup of pressure called hydrocephalus.

Hydrocephalus occurs with congenital

abnormalities, head injury, meningitis,

and episodes of

bleeding into

the brain.

_________ ___ ___ __ Production and Flow of CSF


Parts of the Brain


The brain stem is superior to, but continuous

with, the spinal cord. Developmentally, it does

not represent a single structure, but rather a

group

of anatomical components

considered collectively.

It is made up of

the midbrain, pons,

and medulla oblongata.

The Brain Stem


The Medulla Oblongata


It has two external bulges called the pyramids

formed by the largest motor tracts in the body.

Axons from the left pyramid cross over to the

right and axons on the right cross over to

the left (decussation of

pyramids) – so that the left

hemisphere of the brain

controls the right

side muscles, while the right

hemisphere controls the left side.



The Medulla Oblongata Vital functional centers regulated by the

medulla include:

The cardiovascular center – controls the

rate and force of heartbeat, and the diameter

of blood vessels

The respiratory rhythmicity center –

controls the rate and rhythm of breathing

The vomiting, coughing, and sneezing

centers

The nuclei associated with 5 of the 12 cranial

nerves originates in the medulla (CN VIII – XII).

A portion of the 4th ventricle extends to the

medulla.



Three cranial nerves emerge from the

medulla.


The Pons The pons lies directly above the medulla and

anterior to the cerebellum (2.5 cm). It acts as a

bridge connecting the spinal cord with the brain

and

parts of the brain with

each other.


The Pons Together with the medulla, areas in the pons

help control breathing (inhalation and

exhalation).

The pontine respiratory group is normally

inactive during quiet breathing. This group of

neurons acts like an “off-switch” to terminate

medullary inspiratory activity.

Early termination of inspiration leads to an

increase in the rate of breathing (which is why

this center was formerly know as the

“pneumotaxic center” - taxic meaning fast).


The Pons

CN V exits the pons, while 3 others

come from the medullary pontine

angle.

The pons contains the nuclei associated with 4

pairs of cranial nerves: V - VIII

Cranial nerve V

emerges directly

from the pons.

VI, VII, and VIII

emerge from the

space between

the pons and

the medulla.


The Midbrain The midbrain extends from the pons to the

diencephalon.

The cerebral aqueduct passes through the

midbrain connecting the 3rd ventricle

above with the 4th

ventricles below

(both locations

of CSF formation

and circulation.)


On the anterior part of the midbrain are found

the “little feet” of the cerebral peduncles.

The peduncles contain axons of the

corticospinal, corticobulbar, and

corticopontine tracts which conduct nerve

impulses from motor areas in the cerebral

cortex to the spinal

cord, medulla,

and pons,

respectively.

The Midbrain


On the posterior part of the midbrain are four

rounded elevations known as the superior

and inferior colliculi which serve as reflex

centers for certain visual and auditory

reflexes,

and also the

startle reflex.

It is the origin of

cranial nerves III

and IV.

The Midbrain


The midbrain contains several other nuclei, including

the darkly pigmented substantia nigra. Neurons

that release dopamine, extending from the substantia

nigra, help control subconscious muscle activities;

loss of these neurons is associated with Parkinson

disease.

The red nucleus

helps control

voluntary movements

of the limbs.

The Midbrain


In addition to the well-defined nuclei already

described, much of the brain stem consists of a

netlike arrangement of neuronal cell bodies and

small bundles of myelinated axons known as the

reticular formation.

The ascending portion of this network is called

the reticular activating

system (RAS), and

consists of sensory

axons that project to

the cerebral cortex.

The Reticular Formation


The RAS functions to maintain

consciousness, a state of wakefulness in which

an individual is fully alert, aware, and oriented.

Inactivation of the RAS produces sleep, a state

of partial consciousness from which an individual

can be aroused.

It also prevents sensory

overload by filtering

out insignificant

information.

The Reticular Formation


The Cerebellum The cerebellum, or “little brain”, is the second

largest part of the brain and lies inferior to the

cerebrum and posterior to the brain stem.

It is separated from

the cerebrum by

the transverse

fissure (in which

the falx cerebelli

is located).


The Cerebellum The cerebellum’s central constricted area is the

vermis and the lateral “wings” or lobes are the

cerebellar hemispheres . The cerebellum compares intended

movements with what is happening with

skeletal muscles, and regulates posture,

equilibrium, and balance.


The Diencephalon The diencephalon is located near the midline of

the brain, above the midbrain. Like the cerebral

cortex, the diencephalon develops from the

forebrain vesicle (the prosencephalon) - yet it is

more primitive than the cerebral cortex, and lies

underneath it.

The diencephalon surrounds

the 3rd ventricle and

contains the thalamic

structures.


The Diencephalon The thalamus functions as a relay station for all

sensory impulses to the cerebral cortex (except

smell, which belong to the hypothalamus). Pain,

temp, touch, and pressure are all relayed to the

thalamus en route to the higher centers of the

cerebral cortex.

While not precisely localized here (that occurs in

the cerebral cortex), all of these peripheral

sensations are processed in the thalamus in

conjunction with their attendant memories and

the emotions they evoke.


The epithalamus is superior and posterior to

the thalamus.

It consists of the pineal gland (secretes

melatonin) and habenular nuclei (emotional

responses to odors).

More melatonin is

secreted in darkness

than light, and it is

thought to promote

sleepiness and help

regulate our

biological clocks.

thalamusepithalamus

The Diencephalon


The hypothalamus controls many homeostatic

functions:

It controls the Autonomic Nervous System (ANS).

It coordinates between NS and endocrine systems.

It controls body temperature (measured by blood

flowing through it).

It regulates hunger/thirst

and feelings of satiety.

It assists with the internal

circadian clock by

regulating biological activity.

thalamus

hypothalamus

The Diencephalon


The Cerebrum The cerebral cortex is the “seat of our

intelligence”– it’s because of neurons in the

cortex that we are able to read, write, speak,

remember, and plan our life.

The cerebrum consists of an outer cerebral

cortex, an internal region of cerebral

white matter, and gray

matter nuclei deep within

the white matter.


During embryonic development, the grey matter

of the brain develops faster than the white

matter - the cortical region rolls and folds on

itself. Convolutions and grooves are created in

the cortex during this growth process.

The folds are called gyri, the deepest of which

are known as fissures; the

shallower grooves between

folds are termed sulci.

The Cerebrum


The prominent longitudinal fissure separates

the cerebrum into right and left cerebral

hemispheres. The central sulcus further

divides the

anterior frontal

lobe from the

more posteriorly

situated parietal lobe.

Note the precentral gyrus

and postcentral gyrus of

those two lobes.

The Cerebrum


The precentral gyrus - located immediately

anterior to the central sulcus in the anterior

lobe - contains the primary motor area of the

cerebral cortex. Another major gyrus, the

postcentral gyrus, which is located

immediately posterior to the central sulcus in

the parietal lobe, contains the primary

somatosensory area of the cerebral cortex.

The parieto-occipital sulcus separates the

parietal lobe from the posterior-most

occipital lobe.

The Cerebrum


The lateral cerebral sulcus (fissure) separates

the frontal lobe from two laterally placed

temporal lobes, hanging like ear muffs off the

sides. A fifth part of the cerebrum, the insula,

cannot be seen at the surface of the brain

because it lies within the lateral

cerebral sulcus, deep to

the parietal, frontal,

and temporal

lobes.

The Cerebrum


frontalparietal

temporaloccipital

The lobes of the cerebrum correspond to the

bones of the braincase which bear the same

names.

The Cerebrum

frontal

occipital

parietal

temporal

frontalparietal

temporaloccipital


The Cerebrum Brodmann’s areas are numbered regions of

cortex that have been “mapped” to specific

cognitive functions.

Sensory areas of cerebral cortex are

involved in perception of sensory

information.

Motor areas control execution of voluntary

movements.

Association areas deal with more complex

integrative functions such as memory,

personality traits, and intelligence.


The Cerebrum

Brodmann’s areas: Numbered Regions of Cortical tissue.


The Cerebrum The primary somatosensory area (areas 1, 2, and

3 – located in the postcentral gyrus of each parietal

lobe) receives nerve impulses for, and consciously

perceives the somatic sensations of touch, pressure,

vibration, itch, tickle, temperature (coldness and

warmth), pain, and proprioception (joint and muscle

position).

Each point within the area “maps” impulses from

a specific part of the body (depending on the

number of receptors present there rather than on

the size of the body part).


The Cerebrum For example, a larger region of the

somatosensory area receives impulses from the

lips and fingertips than from the thorax or hip.

This distorted somatic sensory

map of the body is known as the

sensory homunculus (little man).

This allows us to pinpoint

(feel exactly) where somatic

sensations originate.


The Cerebrum The primary motor area (area 4 – located in

the precentral gyrus of the frontal lobe) controls

voluntary contractions of specific

muscles or groups of muscles.

The motor cortex also has a

homunculus map, with

more cortical area devoted

to muscles involved in

skilled, complex, or

delicate movement.


The Cerebrum The 1o visual area is located at the posterior

tip of the occipital lobe mainly on the medial

surface

The 1o gustatory area is located just inferior

to the 1o somatosensory area

The 1o auditory area is

in the superior part of the

temporal lobe

The 1o olfactory area is in

the inferomedial temporal lobe


The Cerebrum The cerebral white matter consists primarily

of myelinated axons in three types of tracts.

Association tracts contain axons that

conduct nerve impulses between gyri in the

same hemisphere.

Commissural tracts conduct nerve

impulses between corresponding gyri from

one hemisphere to another.

Projection tracts convey impulses to lower

parts of the CNS (thalamus, brain stem, or

spinal cord) or visa versa.


The corpus callosum is one of the three

important groups of commissural tracts (the

other two being the anterior and posterior

commissures) – it is a thick band of axons that

connects corresponding areas of the two

hemispheres.

Through the corpus callosum, the left motor

cortex (which controls the right

body) is linked to

the right motor

cortex (which

controls the left body).

The Cerebrum


The Cerebrum The outer surfaces of the gyri are not the only

areas of gray matter in the cerebrum. Recall

that the telencephalon consists of the cortex,

and also the basal nuclei.

The basal nuclei are conspicuous centers of

cell bodies deep in the cortex. The 3 basal

nuclei help initiate and terminate

movements, suppress unwanted movements,

and regulate muscle tone.


The Cerebrum The basal nuclei also control subconscious

contractions of skeletal muscles. Examples

include automatic arm swings while walking and

true laughter in response to a joke.


The Limbic System Encircling the upper part of the brain stem and

the corpus callosum is a ring of structures on

the inner border of the cerebrum

and floor of the

diencephalon that

constitutes the

limbic system.


The Limbic System The limbic system does not represent any one

part of the brain – it is more a functional

system composed of parts of the cerebral

cortex, diencephalon, and midbrain.

The limbic system is sometimes called the

“emotional brain” because it plays a primary

role in promoting a range of emotions, including

pleasure, pain, docility, affection, fear, and

anger.

Together with parts of the cerebrum, the

limbic system also functions in memory.


Hemispheric Lateralization Although the brain is almost symmetrical on its

right and left sides and shares performance of

many functions, there are subtle anatomical

and physiological differences between the two

hemispheres.

Each hemisphere specializes in performing

certain unique functions, a feature known as

hemispheric lateralization. Despite some

dramatic differences, there is considerable

variation from one person to another. Also,

lateralization seems less pronounced in

females.


Hemispheric Lateralization In most people, the left hemisphere is more

important for reasoning, numerical and

scientific skills, spoken and written language,

and the ability to use and understand sign

language.

Conversely, the right hemisphere is more

specialized for musical and artistic awareness;

spatial and pattern perception; recognition of

faces and emotional content of language;

discrimination of different smells; and

generating mental images of sight, sound,

touch, and taste.


Brain Waves The billions of communicating brain neurons

constantly generate detectable signals

called brain waves. Those we can more

easily measure are generated by neurons

close to the brain surface, mainly neurons in

the cerebral cortex.

Electrodes placed on the

forehead and scalp can be

used to make a record called

an electroencephalogram.


Brain Waves Summing waves of different frequency produces

some characteristic, and diagnostic patterns.

Alpha (10–12 Hz (cycles/sec) waves are

present when awake but disappear during

sleep.

Beta (14–30 Hz) waves are present with

sensory input and mental activity when the

nervous system is active.

Theta (4–7 Hz) waves indicate emotional

stress or a brain disorder.

Delta (1–5 Hz) waves appear only during sleep

in adults but indicate brain damage in an

awake adult.


Brain Waves Electroencephalograms are useful both in

studying normal brain functions, such as

changes that occur during sleep, and in

diagnosing a variety of brain disorders, such as

epilepsy, tumors, trauma, hematomas,

metabolic abnormalities, sites of trauma, and

degenerative diseases.

The EEG is also utilized to determine if “life” is

present, that is, to establish or confirm that

brain death has occurred.


The Twelve Cranial Nerves

Cranial Nerves


Oh! Oh! Oh! To Touch And Feel Very Good Velvet! Ah Heaven!

How to Remember CN I-XII


I Olfactory (oh)

II Optic (oh)

III Oculomotor (oh)

IV Trochlear (to)

V Trigeminal (1-3) (touch)

VI Abducens (and)

VII Facial (feel)

VIII Vestibulocochlear (very)

IX Glossopharyngeal (good)

X Vagus (velvet)

XI Accessory (ah)

XII Hypoglossal (heaven)


Cranial Nerves

Spinal and cranial nerves are compared in this

table.

Spinal Cranial

DesignationC1-8, T1-12, L1-5, S1-5,

Co1

Roman Numerals

I – XII

Number 31 pairs 12 pairs

Origin Spinal cord Brain

Number of

roots

2 - a dorsal and a ventral

rootSingle root

Contents MixedMost mixed; some

sensory only

Target Limbs/TrunkAll in the Head/Neck

(vagus n leaves)


Cranial Nerves

This is a picture of a masterful dissection, showing the cranial nerves in-situ (as they are “in place”).

The major functions of the 12 pairs of cranial

nerves are detailed in the

following

slides.


Cranial Nerves

CN I is the

olfactory nerve

(sense of

smell).


Cranial Nerves

CN II is the optic

nerve (sense of

sight).


CN III, IV, and VI innervate the extraocular

muscles that allow us to move our eyes.

CN III also supplies motor

input to our eyelid

muscles and

facilitates

pupillary

constriction.

Cranial Nerves


Cranial Nerves CN V is the trigeminal nerve (the major

sensory nerve of the face).

It has three large

branches, each of

which supplies an

area of the face:

• ophthalmic

• maxillary

• mandibular


CN VII is the facial nerve. It has 5 large somatic

branches which innervate the muscle of

facial expression. It also carries some

taste sensations

(anterior 2/3 of tongue).

Paralysis of

CN VII is called

Bell’s Palsy and leads

to loss of ability to close

the eyes and impairment of

taste and salivation.

Cranial Nerves


Cranial Nerves CN VIII is the vestibulocochlear nerve. From

the inner ear, the vestibular component carries

information on balance, while the cochlear

component enables hearing.

Damage of CN VIII causes vertigo, ringing in

the ears, and/or deafness.


Cranial Nerves CN IX is the glossopharyngeal nerve. This

nerve carries some taste sensations as well as

ANS impulses to salivary glands and the

mechanoreceptors of the carotid body and

carotid sinus (senses changes in BP).


Cranial Nerves

CN X is the vagus

nerve (“the

wanderer”), which

carries most of the

parasympathetic

motor efferents to

the organs of the

thorax and

abdomen.


Cranial Nerves CN XI is the spinal accessory nerve. This nerve

supplies somatic motor innervation to the

Trapezius and Sternocleidomastoid muscles.


Cranial Nerves CN XII is the glossopharyngeal nerve. This is a very

large nerve (a lot of resources) to be devoted solely

to the tongue – it takes a lot more coordination than

you might guess to chew, talk, and swallow without

injuring our tongue.

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Chapter 14