Chapter 14

Copyright © John Wiley & Sons, Inc. All rights reserved.
Chapter 14
The Brain and Cranial
Nerves
Lecture slides prepared by Curtis DeFriez, Weber State University

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

 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.

 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.

 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.

 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

 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.

 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
temporal
occipital
 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 NervesCranial Nerves

Cranial Nerves
Spinal and cranial nerves are compared in this table.
Spinal Cranial
Designation
C1-8, 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
root
Single root
Contents Mixed
Most mixed; some
sensory only
Target Limbs/Trunk
All 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.

End of Chapter 14
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