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Chp14

The Brain and Cranial Nerves
An Introduction to the Brain and Cranial Nerves

The adult human brain contains almost 97% of the body's neural tissue
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4kg (3lb) and has a volume of 1200ml (71in
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Brain size varies among individuals
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No correlation exists between brain size and intelligence
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14-1
The brain has several principal structures, each with specific
functions
An overview of the brain's major regions and landmarks
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Major Brain Regions and Landmarks
There are 6 major brain regions:
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the Cerebrum

•

the Cerebellum

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the Diencephalon

•

the Midbrain

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the Pons

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the Medulla Oblongata

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•

Cerebrum

Cerebrum) (the largest portion)
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The brain has an extensive area of
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Cerebral cortex) the neural cortex forming series of elevated ridges called gyri
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They increase the brain's surface area
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The cerebrum is the seat of most higher mental functions
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•

Cerebellum

Cerebellum) (2nd largest part of the brain)
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It is also
covered by a neural cortex called the cerebellar cortex
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•

Diencephalon

Diencephalon) the walls are composed of the left thalamus and right thalamus
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Hypothalamus) (is the floor of the diencephalon)
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Infundibulum) a narrow stalk, that connects the hypothalamus to the pituitary gland
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The hypothalamus and pituitary gland integrate the nervous and endocrine systems
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2

Brain stem) contains a variety of important processing centers and nuclei that relay info headed to
or from the cerebrum or cerebellum
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This region also contains centers that help
maintain consciousness
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In addition
to tracts and relay centers, it contains nuclei involved with somatic and visceral motor control
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Near the
pons, the posterior wall of the medulla oblongata is thin and membranous
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The MO relays sensory info
to the thalamus and to centers in other portions of the brain stem
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Pons

Medulla Oblongata

Embryology of the Brain
The embryonic origins of the brain can help us understand its internal organization
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This tube has a fluid-filled internal cavity, the
neurocoel
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3 prominent divisions of the Primary Brain Vesicles:
• prosencephalon (forebrain)
• mesendephalon (midbrain)
• rhombencephalon (hindbrain)

Secondary brain vesicles) formed by the subdivisions of the proencephalon and
rhombencephalon

Telencephalon) formed from the proencephalon and diencephalon
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Metencephalon) formed from the portion of the rhembencephalon adjacent to the
mesencephalon
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Myelencephalon) formed by the portion of the rhombencephalon closer to the spinal cord,
which becomes the medulla oblongata
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Cells of the ependmya line the ventricles
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3rd ventricle) what the diencephalon is called because there are 2 lateral ventricles
The 2 lateral ventricles are not directly connected, but each communicates with the 3 rd ventricle
of the diencephalon through an interventricular foramen (foramen of Monro)
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This passagway the 3rd ventricle with the 4th
ventricle

4th ventricle) the superior portion lies between the posterior surface of the pons and the anterior
surface of the cerebellum
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14-2
The brain is protected and supported by the cranial meninges,
cerebral spinal fluid, and the blood-brain barrier
The Cranial Meninges
The layers that make up the cranial meninges---the cranial dura mater, arachnoid mater, and pia
mater----are continuous with the spinal meninges
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The outer layer is fused with the periosteum of the
cranial bones
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This layer

covers the brain, providing a smooth surface that does not follow the
brain's underlying folds
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It extends into every fold
and fissure
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These dura folds provide additional stabilization and
support to the brain
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The 3 largest dural folds are called the:
1
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◦ The Superior sagittal sinus and the inferior sagittal sinus) are 2 large venous
sinuses that lie within this dura fold
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the tentorium cerebri: protects the cerebellum and separates cerebral
hemispheres from the cerebellum
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3
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The Protective Function of the Cranial Meninges
The meninges act like seat belts in a car (just like seat belts hold the driver in place, so do the
meninges hold the brain in position)
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Cranial trauma) a head injury resulting from impact with another object
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Cerebrospinal Fluid
CSF completely surrounds and bathes the exposed surfaces of the CNS
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A
human brain weighs about 1400g (3
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) in air, but only about 50g (1
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) when
supported by CSF
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The Formation and Circulation of CSF
Choroid plexus) (choroedies, like a membrane)
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The Blood Supply to the Brain
Neurons have a high demand for energy, but they have no energy reserves in the form of
carbohydrates or lipids
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Arterial blood reaches the brain through the internal carotid arteries and the vertebral arteries
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Cerebrovascular accident (CVA) (stroke) occurs when the blood supply to a certain
portion of the brain is shut off
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The Blood-Brain Barrier
Blood-brain barrier (BBB) isolates the neural tissue in the CNS from the general circulation
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These
junctions prevent materials from fusing between endothelial cells
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Blood-CSF Barrier) created by specialized ependymal cells
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Neurons have a constant need
for glucose that must be met regardless of concentrations in the blood and interstitial fluid
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Portions of the hypothalamus
2
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Capillaries of the pineal gland
4
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The medulla oblongata is a very busy
place
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The 3 groups of nuclei of the medulla oblongata:
1
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The reflex centers receive input from cranial
nerves, the cerebral cortex, and the brain stem
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•

The 2 subdivisions of the Cardiovascular Centers:
o Cardiac
o Vasomotor centers
Respiratory rhythmicity centers: set the basic pace for respiratory movements
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2
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3
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Decussation of pyramids) the site where tracts leaving the brain stem nuclei cross to the opposite side of the brain
before reaching their destination
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Olivary nuclei) relays information to the cerebral cortex about somatic motor commands as they are issued
by motor centers at higher levels
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14-4
The pons contains nuclei and tracts that carry or relay sensory
and motor information
The pons links the cerebellum with the midbrain, diencephalon, cerebrum, and spinal cord
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Sensory and Motor Nuclei of Cranial Nerves: These cranial nerves (V, VI, VII, and
VIII) innervate jaw muscles, the anterior surface of the face, 1 of the extrinsic eye muscles (lateral rectus),
and the sense organs of the internal ear (the vestibular and cochlear nuclei)

2
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These centers modify the activity of the respiratory rhythmicity centers in the medulla oblongata

3
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Nuclei and Tracts That Process and Relay Information Sent to or from
the Cerebellum: The pons links the cerebellum with the brain stem, cerebrum, and spinal cord
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The middle cerebellar peduncles are connected to the transverse fibers, which cross
the anterior surface of the pons
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14-5
The cerebellum coordinates learned and reflexive patterns of
muscular activity at the subconscious level
The cerebellum is an autonomic processing center
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Adjusting the Postural
Muscles of the Body:

The cerebellum coordinates rapid, autonomic adjustments
that maintain balance and equilibrium
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2
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(It performs this function indirectly by regulating activity along motor
pathways at the cerebral cortex, basal nuclei, and motor centers in the brain
stem
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)

The cerebellum has a complex, highly convoluted surface composed of neural cortex
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Are less prominent than the folds in the surfaces of the cerebral
hemispheres
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Vermis) a narrow band of cortex along the midline, which separates the cerebellar hemispheres
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Like the cerebrum, the cerebellum has a superficial layer of neural cortex
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Arbor vitae) (“tree of life”) internal white matter of the cerebellum resembling a tree
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Proprioceptive) position
The cerebellum receives proprioceptive information from the spinal cord and monitors all
proprioceptive, visual, tactile, balance, and auditory sensations received by the brain
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They also connect the
cerebellar hemispheres with sensory and motor nuclei in the pons
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The cerebellum can be permanently damaged by trauma or stroke
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THE RESULT IS

Ataxia) a disturbance in muscular coordination
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Is the region posterior to the cerebral aqueduct
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These nuclei, the superior and inferior
colliculi, process visual and auditory sensations
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The superior colliculi control the reflex movements of the eyes, head, and neck in response to
visual stimuli, such as a bright light
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The inferior colliculi control reflex movements of head, neck, and trunk in response to auditory
stimuli, such as a loud noise
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On each side it contains a
red nucleus and the substantia nigra
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This nucleus
receives information from the cerebrum and cerebellum and issues subconscious motor commands that
affect upper limb position and background muscle tone
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It lies lateral to the red nucleus
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Cerebral peduncles) nerve fiber bundles on the ventrolateral surfaces of the midbrain
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Descending fibers that go to the cerebellum by way of the pons
2
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Epithalamus) the roof of the diencephalon superior to the 3rd ventricle
Pineal gland) contained in the posterior portion of the epithalamus
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The Thalamus
On each side of the diencephalon, the thalamus is the final relay point for sensory information
ascending to the primary sensory cortex
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The 3rd ventricle separates the left and right thalamus
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Each thalamus extends from the anterior commissure to the inferior base of the pineal gland

Interthalamic adhesion) a projection of gray matter, that extends into the ventricle from the
thalamus on either side, but no fibers cross the midline
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continued on next page

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There are 5 major groups of thalamic nuclei and They are the anterior, medial, ventral, posterior,
and lateral groups:
1
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This system is involved
with emotion and motivation
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The medial

group: the nuclei provide an awareness of emotional states by connecting emotional centers
in the hypothalamus with the frontal lobes of the cerebral hemispheres
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3
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Ventral group nuclei also relay sensory info about touch, pressure, pain, temp, and
proprioception to the sensory areas of the cerebral cortex
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The posterior group: includes the pulvinar and geniculate nuclei
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• Lateral geniculate nucleus) of each thalamus receives visual info over the optic tract, which
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originates at the eyes
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Medial geniculate nucleus) relays auditory info to the appropriate area of the cerebral hemisphere
from specialized receptors of the internal ear
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The lateral

group: nuclei form feedback loops with the limbic system and the parietal lobes of the
cerebral hemispheres
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The Hypothalamus
The hypothalamus extends from the area superior to the optic chiasm, a crossover where the
optic tracts from the eyes arrive at the brain, to the posterior margins of the mammillary bodies
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Infundibulum) a narrow stalk immediately posterior to the optic chiasm, that extends inferiorly,
connecting the floor of the hypothalamus to the pituitary gland
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This area contains nuclei that are involved with the control of pituitary gland function
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Sensory info from the cerebrum, brain stem, and spinal cord
2
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Chemical stimuli in the circulating blood that move rapidly across highly permeable capillaries
to enter the hypothalamus (where there is no BBB)
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The hypothalamus performs the following functions:

1
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The Control of Autonomic Function:
3
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The Secretion of 2 Hormones: the hypothalamus secretes antidiuretic hormone (ADH, also
•
•

called vasopressin) and oxytocin (OXT)
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Paraventricular nucleus) produces oxytocin, which stimulates smooth muscle contractions in the uterus
and mammary glands of females and prostate gland of males
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For Example
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Thirst center) produces the sensation of thirst
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1
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The Regulation of Body Temp: the preoptic area of the hypothalamus is responsible for
thermoregulation
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The Control of Circadian Rhythms: the Suprachiasmatic nucleus coordinates daynight
cycles
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Includes nuclei and tracts along the border between the
cerebrum and diencephalon
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The
limbic system is also known as the motivational system because it makes you want to do
something
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establishing emotional states
2
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facilitating memory storage and retrieval
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It plays a role in regulating heart rate, in responding to fear and anxiety and
controlling the “fight or flight” response by the sympathetic division of the ANS, and linking emotions
with specific memories
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There are 3 gyri in the limbic lobe
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Cingulate gyrus) sits superior to the corpus callosum
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These gyri conceal the
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This nucleus is important in
learning, especially in the storage and retrieval of new long-term memories
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Is a tract of white matter that connects the hippocampus with the hypothalamus
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Much of
it is involved in processing sensory and motor information
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The white matter of the cerebrum lies deep to the cerebral cortex and around the basal nuclei
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5mm thick covers the paired cerebral hemispheres,
which dominate the superior and lateral surfaces of the cerebrum
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Central sulcus) is a deep groove on each hemisphere that divides the anterior frontal lobe from
the more posterior parietal lobe
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Insula) an “island” of cortex, that lies medially to the lateral sulcus
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3 points about the cerebral lobes:
1
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The 2 hemispheres have different functions, even though they look almost identical:
3
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We can classify the axons of white
matter as association fibers, commissural fibers, and projection fibers:

•

Association fibers:

Interconnect areas of cerebral cortex within a single hemisphere
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Longer association fibers are organized into discrete bundles, or fasciculi
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•

Commissural fibers:

Interconnect and permit communication between the cerebral
hemispheres
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The corpus callosum alone contains more than 200 million axons,
carrying some 4 billion impulses per sec
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All these fibers pass through the diencephalon
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The Basal Nuclei
This is where subconscious activities occur and are directed
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They are embedded in the white matter of the cerebrum
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The head of this nucleus lies anterior to the
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Functions of the Basal Nuclei
The basal nuclei are involved in the subconscious control of skeletal muscle tone and the
coordination of learned movement patterns
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As you begin voluntary movement, the basal nuclei control and adjust muscle tone, particularly
in the appendicular muscles, to set your body position
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Precentral gyrus) of the frontal lobe forms the anterior border of the central sulcus
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Neurons of the primary motor cortex direct voluntary movements by controlling somatic motor
neurons in the brain and spinal cord
These cortical neurons are called pyramidal cells because their cell bodies resemble little
pyramids
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Sensations of sight, sound, smell, and taste arrive at other portions of the cerebral cortex
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Auditory cortex and the Olfactory cortex) of the temporal lobe receive info about
hearing and smell, respectively
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Association Areas
The sensory and motor regions of the cortex are connected to nearby
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Somatic sensory association area) monitors activity in the primary sensory cortex
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Visual association area) monitors the patterns of activity in the visual cortex and interprets
the results
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Auditory association area) monitors sensory activity in the auditory cortex
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Premotor cortex or Somatic motor association area) coordinates learned
movements
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The neurons must be stimulated by neurons in other parts of the cerebrum
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They also perform analytical functions
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The General Interpretive Area
General interpretive area) (also called Wernike's area) Is present in only one hemisphere,
typically the left
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It plays an integral role in your personality by integrating sensory info and coordinating access
to complex visual and auditory memories
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The speech center regulates the pattern of breathing and vocalization needed for normal speech
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This
is the area of predicting consequences of events of actions
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Hemispherical Lateralization
Each of the 2 hemispheres are responsible for specific functions that are not ordinarily
performed by the opposite hemisphere
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Left Hemisphere

Right Hemisphere

In most people it contains the general
interpretive and speech centers and is responsible
for language-based skills
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For this reason the left hemisphere has been
called the Dominate hemisphere
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Interpretive centers allow you to recognize
things by touch, smell, sight, taste, or feel
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Monitoring Brain Activity: The Electroencephalogram
Electroencephalogram (EEG) a printed recording of electrical activity of the brain
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Alpha waves) occur in the brains of healthy, awake adults who are resting with their eyes closed
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Beta waves) higher-frequency waves active during stimuli or tasks that replace alpha waves
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Theta waves) may appear transiently during sleep in normal adults but are most often observed in
children and in intensely frustrated adults
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Normally seen during sleep in
individuals of all ages
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Seizure) a temporary cerebral disorder accompanied by abnormal movements, unusual sensations,
inappropriate behavior, or some combo of these symptoms
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There 12 pairs of cranial
nerves are visual on the ventral surface of the brain
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Destination: Olfactory bulbs

Olfactory nerves (I)

Optic nerves (II)
About 1million sensory nerve fibers
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They also change the shape of the

Oculomotor nerves (III)

lens for focusing
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Visceral motor: intrinsic eye muscles

Trochlear nerves (IV)

Primary function: Motor (eye movements)
Origin: Midbrain
Pass through: Superior orbital fissures of sphenoid
Destination: Superior oblique muscle

The smallest cranial nerve and it is pulleyshaped
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Trigeminal (V)

Primary function: Mixed (sensory and motor)
Origin: Optothalamic branch (sensory), Maxillary branch (sensory),

The largest cranial nerves are mixed
nerves
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sensory nuclei in pons; mandibular branch also innervates muscles of
mastication
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Facial nerves (VII)
Facial nerves are mixed nerves
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Motor: motor nuclei of pons
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Motor: motor nuclei of medulla
oblongata
Pass through: Jugular foramina between the occipital bone and the
temporal bones
Destination: Sensory: sensory nuclei of medulla oblongata
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Visceral motor:
parotid salivary gland by way of the otic ganglion

Vagus nerves (X)

Primary function: Mixed (sensory and motor), widely distributed in
the thorax and abdomen

Origin:
Pass through:
Destination:

Accessory nerves (XI)

Primary function:
Origin:
Pass through:
Destination:

Hypoglossal nerves (XII)

Primary function:
Origin:
Pass through:
Destination:

14-10
Cranial reflexes involve sensory and motor fibers of cranial
nerves
Cranial reflexes) are autonomic responses to stimuli that involve the sensory and motor fibers of cranial
nerves
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These reflexes are used to checked for damage to the cranial nerves

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