Chapter 15
The Brain and Cranial Nerves
The Brain
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The average male adult brain weighs about 3.5 lbs.
Composed of 3 divisions:
Cerebrum = Cerebral cortex and Basal nuclei
 Cerebellum – posterior and inferior to cerebrum
 Brainstem: Diencephalon (Thalamus, Hypothalamus and
Epithalamus), Midbrain (superior and inferior colliculi),
Pons and Medulla oblongata
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Brain Meninges
Arachnoid villi
Absorbs CSF back into venous sinuses and blood
Brain Ventricles
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Cavities within the cerebrum and brain stem where cerebrospinal fluid
(CSF) is manufactured and circulated.
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Two lateral ventricles over thalamus and roll out into temporal lobes,
medial third and fourth ventricles joined by cerebral aqueduct.
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Lateral ventricles look like a Rocky Mountain sheep Rams horns and
communicate to third ventricle via interventricular foreamen.
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CSF is formed by the ependymal cells of choroid plexus in all four
ventricles; is an ultrafiltrate of blood- no cells present; high in Na and K.
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CSF circulates through ventricular system as well as central canal of
spinal cord and subarachnoid space around brain and spinal cord.
Brain Ventricles
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CSF formation and
circulation into and from
blood vascular system.
CSF provides protection
and nourishment to the
brain and spinal cord.
Has higher levels of Na
and K than blood.
CSF reduces weight of
brain (1400 gm to 50 gm).
Cerebrum
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Cerebrum composes the vast majority of the brain
mass (80%). It is divided into 2 halves or hemispheres, a
right and a left hemisphere.
Longitudinal fissure- midsagittal groove dividing the two
hemispheres into a right and left side.
Consists of an outer layer of gray matter over white
matter (like crust on white bread).
Sulci and gyri = valleys and ridges on surface of
cerebral hemispheres.
Five lobes: Frontal, parietal, occipital, temporal and
insula.
Brain blood flow and barrier
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Blood flow to the brain is provided by the internal
carotid arteries and the vertebral arteries.
The blood brain barrier (BBB) is thought to be due to
specialized endothelial cells that are influenced by the
glial astrocytes.
In the choroid plexus there is also a CSF-BBB formed
by the ependymal cells.
The BBB is absent in some places of the 3rd and 4th
ventricles at patches called circumventricular organs
where some substances may pass into the brain tissue.
Cerebral Cortex
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The cortex is the outer layer of the cerebrum is
composed of gray matter and is where our
“consciousness” is located.
It allows us to be who we are, perceive sensations,
comprehend and understand, as well as learn and,
communicate with the ability to act and react at a
conscious level.
Sulcus or Sulci
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sulcus/sulci – the valleys or grooves found
between the gyri. Some of these also have
specific names.
 central sulcus – separates the frontal and
parietal lobes..
 lateral sulcus – separates the temporal lobe
from the parietal/ frontal lobes.
 parieto-occipital sulcus – separates the
parietal and occipital lobes.
Gyrus or Gyri
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gyrus/gyri – the elevated ridges of tissue in the cortex
1. precentral gyrus- motor cortex area 4
2. post central gyrus- sensory cortex areas 1-3
- homunculus (little man)- portrays motor and
sensory areas of body overlaying precentral and
postcentral gyri of the cortex
Sulci and Gyri
Humunculus “little man”
Functional areas of the Cortex
Three important areas of cortex
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Motor areas control voluntary movement.
Sensory areas provide consciousness and awareness of
sensations.
Association areas act mainly to integrate diverse
information into decisive and meaningful actions. This
is done via interneurons or association neurons.
Cortical motor, sensory and association areas
Motor Areas of Cerebral Cortex
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1° (primary) Motor Cortex – located in the precentral
gyrus. Also known as Broadmann’s area # 4.
-Contains large Pyramidal neurons that allow us to
perform precise and skilled movements with our skeletal
muscles.
-Motor innervation is contralateral i.e. the left side of the
brain controls the right side of the body and vice versa.
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Broca’s Area – located superior to lateral sulcus and anterior to
precentral gyrus. Associated with motor speech and is usually
located in the left hemisphere
Premotor cortex anterior to precentral gyrus (Broadmann 6).
thought to be site of memory bank for fine motor skils.
Sensory Areas of Cerebral Cortex
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1° (primary) Somatosensory Cortex – located in
the postcentral gyrus of the parietal lobe. Areas 1-3
Visual Sensory Area – Located on the extreme
posterior tip of the occipital lobe. Receives input
from retina; is largest cortical sensory area.
Auditory Area – medial aspects of the temporal
lobes.
Gustatory Area –parietal lobe deep to the temporal
lobe at the tip of the tongue of humunculus.
Cerebral anatomy to know
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Longitudinal and transverse fissures
Lateral, Central and Parieto-occipital sulci
Precentral (motor #4) and Postcentral (sensory #1-3)
gyri
Lobes: Frontal, Parietal, Temporal, Occipital and
Insula
Basal nuclei: Caudate nucleus, Putamen, Globus
pallidus
Corpus callosum, septum pellucidum, fornix, and
internal capsule
Cerebral anatomy to know
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White matter is myelinated neuronal tissue in brain
and spinal cord and contains neuronal tracts.
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Gray matter is unmyelinated and contains mostly
neuronal nuclear groups or nuclei and unmyelinated
tracts.
Coronal section
Cerebral White Matter
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3.
is composed of myelinated processes. Within the white
matter are “roads” of ordered groups of neuron
processes called tracts. There are three major types
of tracts in the cerebral cortex:
Commissural fibers – connect the gray matter
between the two hemispheres. e.g. corpus callosum
Association fibers – connect adjacent gyri in same
hemisphere. e.g. visual and auditory association
areas.
Projection fibers – these connect to regions outside
of the cerebrum e.g. internal capsule; corona radiata
Communication fibers
Communication fibers
Basal Nuclei “Corpus striatum”
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Masses of cerebral gray matter buried deep in each
hemisphere in the white matter and lateral to the
thalamus.
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Caudate nucleus, globus pallidus and putamen
Communicate with cortex in controlling movement
 Complete function is ??, but may regulate start, stop and
intensity of voluntary body movements from cortical levels.
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Basal Nuclei “Corpus striatum”
The Limbic System
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Concerned with sense of smell in association with
long term memory and emotional responses (anger,
fear, rage, sex and hunger).
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Includes cingulate gyrus, amygdala, hippocampus,
septum, fornix, thalamus, hypothalamus and olfactory
bulbs.
The Limbic System
Diencephalon
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Thalamus is sensory relay nucleus from spinal cord and
brainstem regions to the cerebral cortex. Located at top of
brainstem, each half is joined by intermediate mass.
Hypothalamus nuclear group just inferior to thalamus
consisting of ~ 12 nuclei. Third ventricle is roof.
-Main visceral control center for many systems: regulates
ANS, GI, cardiovascular centers, sleep/wake, emotions,
behavior, hunger/thirst, body temp, memory, sex drive and
endocrine system.
Epithalamus pineal gland and habenula produce melatonin for
sleep/wake.
Diencephalon, Midbrain, Pons and Medulla
Hypothalamus
Epithalamus
-Pineal gland
-Habenular nuc.
Cerebral and Diencephalon
Functions
Brain stem (see Figure 15.7; p. 422)
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Extends from posterior border of diencephalon to the
beginning of spinal cord at foreamen magnum.
Consists of Midbrain, Pons, and Medulla oblongata
-controls automatic behavior of systems necessary for life
(e.g. cardiovascular, respiratory, emetic, etc.)
-passageway for all tracts to cerebrum and spinal cord
-contains nuclei of many cranial nerves to face and neck
 Has same pattern of gray/white matter as spinal cord (i.e.
white matter surrounds gray matter)
Brain Stem (Lateral view)
Brain Stem
(Diencephalon, Midbrain and Hindbrain)
Midbrain
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Lies between a plane that extends from behind pineal gland
down to posterior end of mamillary bodies and caudally to the
rostral part of the pons.
Central cavity is cerebral aqueduct with tectum (roof) dorsally
and cerebral peduncles ventrally.
Periaqueductal gray is involved in fight or flight (Sympathetic)
Contains copra quadrigemina (sup & inf colliculi) which are
relay nuclei for vision and hearing, respectively.
Other nuclei include substantia nigra and red nucleus
Contains nuclei for occulomotor (CN-III) and trochlear (CNIV).
Midbrain/Pons (X-section)
Midbrain/Hindbrain (lateral view)
Reticular Formation
Composed of 100’s of small gray nuclei that extend
from medulla, pons and midbrain and project up to
the diencephalon, cerebellum and cerebrum.
 It receives diverse input and sends a continuous
stream of information to cerebral cortex keeping it
alert to all activity during sleep and consciousness. It is
often referred to as the reticular activating system
(RAS).
 The motor portion sends neurons to the spinal cord
via reticulospinal tracts.
 Functions include cardiovascular control, pain control,
breathing, swallowing, and habituation to environment.
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Reticular Formation
Pons
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Bulging region on inferior surface of brain stem.
Lies between the midbrain and medulla oblongata.
Forms a bridge between left and right halves of
cerebellum.
Fourth ventricle separates pons from cerebellum.
Cranial nerves V (trigeminal), VI (abducens), and VII
(facial) arise from pontine nuclei.
Middle cerebellar peduncles carry fibers from Pons to
cerebellum.
Pons
Pons (lateral view)
Medulla oblongata
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Extends from Pons to spinal cord
Pyramids (pyramidal tracts) decussate (crossover) at
caudal border and are motor corticospinal tracts.
Cranial nerves VIII (vestibulocochlear), IX
(glossopharyngeal), X (Vagus), XI (accessory) and XII
(hypoglossal) arise from medullary nuclei near fourth
ventricle.
Visceral centers for cardiac, vasomotor and respiratory
centers are in medulla.
Medulla oblongata
Cerebellum
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Cauliflower-like looking mass at posterior part of
brain and dorsal to the pons and medulla.
Coordinates and smoothes body movements, and
maintains posture and equilibrium.
Two lateral hemispheres are connected by vermis
Hemispheres subdivided into 3 lobes:
1. anterior, 2. posterior and 3. flocculonodular
Anterior and posterior coordinate movements
Flocculonodular adjusts posture and equilibrium
Cerebellum
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Has three regions: Outer cortical gray matter, Inner
white matter (arbor vitae) and Deep cerebellar nuclei.
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Cerebellar cortical nuclei function to smooth body
movements.
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White matter contains tracts that carry information to
and from cerebral cortex.
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Deeper nuclei relay instructions to other areas of
brain from cerebellar cortex to refine body
Cerebellum
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Cerebellar cortex receives 3 forms of information :
1). Equilibrium, 2). Position of muscles in limbs,
neck and trunk and 3). Info from cerebral cortex.
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Cerebellar peduncles are fibers tracts that connect
cerebellum with different parts of brain.
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Unlike cerebral motor outflow which is contralateral,
cerebellar outflow is ipsilateral (same side).
Cerebellum
Cerebellum
Cranial Nerves
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Twelve pairs of cranial nerves arise from nuclei in
brain and migrate out of cranium to innervate
peripheral organs.
First 2 pairs arise from forebrain, the remainder come
from brainstem nuclei
They are numbered in Roman numerals I thru XII.
All cranial nerves except for X innervate head and
neck.
Cranial nerve X (Vagus) innervates head, neck, thorax
and abdomen.
Cranial Nerves
I Olfactory nerve (S)
VII Facial (B)
II Optic nerve (S)
VIII Vestibulochochlear (S)
III Occulomotor (M)
IX Glossopharyngeal (B)
IV Trochlear (M)
X Vagus (B)
V Trigeminal (B)
XI Accessory (M)
VI Abducens (M)
XII Hypoglossal (M)
Oh, Oh, Oh To Touch And Feel Velvet, Good
Vibrations And Harmony.
I. Olfactory nerves (S)
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Strictly sensory for smell.
Nerve fibers are in cribriform plate of ethmoid bone
I Olfactory nerve fibers
II. Optic nerves (S)
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Strictly sensory for vision
Enters cranium via optic foramen anterior to pituitary
gland in sella turcica.
II Optic nerves
III. Occulomotor nerves (M)
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Primarily motor nerve to extrinsic eye muscles
(inferior oblique, sup., inf. and medial rectus muscles)
III Occulomotor to eye muscles
IV. Trochlear nerves (M)
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Primarily motor to superior oblique eye muscles
IV Trochlear nerve to sup. oblique
V. Trigeminal nerves (B)
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Mixed nerve with 3 divisions to face; largest cranial
nerve.
V Trigeminal to face and jaw
VI. Abducens nerves (M)
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Primarily motor to lateral rectus muscle of eye
VI Abducens to lateral rectus
VII. Facial nerves (B)
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Mixed nerve with 5 branches to neck and face.
VII Facial nerve distribution – five branches to face & neck
VIII. Vestibulocochlear nerves (S)
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Purely sensory for hearing and equilibrium
VIII Vestibulocochlear to inner ear
IX. Glossopharyngeal nerves (B)
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Mixed nerve to pharynx, post. tongue, parotid glands
IX Glossopharyngeal to tongue and salivary glands
X. Vagus nerves (B)
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Mixed nerve extends to thoracic and abdominal
cavities; parasympathetic to heart, lungs and abdomen
XI. Accessory nerves (M)
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Motor nerves to larynx, pharynx and neck muscles for
swallowing
XI Accessory to neck muscles
XII. Hypoglossal nerves (M)
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Motor to muscles of tongue
XII Hypoglossal to tongue muscles