13. Nervous System: Brain and Cranial Nerves
The brain can be divided into four major regions (Fig. 13.1):
A. The cerebrum is the largest and most readily visible part of the brain. Conscious thoughts,
sensations, intellect, memory, and complex movements originate in the cerebrum.
B. The diencephalon lies below the cerebrum and forms critical connections between the
cerebrum and the brainstem. The diencephalon contains the thalamus and the hypothalamus.
C. The brainstem is the “lower” portion of the brain, and it connects the “upper” portions of the
brain to the spinal cord. The brainstem consists of the midbrain, the pons, and the medulla
oblongata.
D. The cerebellum lies inferior to the posterior portion of the cerebrum. The cerebellum
coordinates movements of the body based on sensory information that it receives from the body.
I. Protection and Support of the Brain
Cranial meninges
The cranial meninges are similar to the spinal meninges (Fig. 13.5). In fact, the cranial
meninges are contiguous with the spinal meninges.
A. The pia mater is the innermost layer, and it is tightly associated with the surface of the brain.
B. The arachnoid mater is the middle layer. A subarachnoid space filled with CSF lies between
the arachnoid and the pia mater. Note that the arachnoid is smooth, and it does not follow the
folds of the cerebrum.
C. The dura mater is the outermost and toughest layer. There are two major differences
between the dura of the brain and the dura of the cord: (1) The dura of the brain consists of two
layers; the periosteal layer is attached directly to the inner surface of the skull and the
meningeal layer is deep to the periosteal layer. In places, the two layers are fused together; in
other places, dural sinuses lie between the two layers. (2) There is no epidural space associated
with the brain.
Brain ventricles
The brain contains cavities, called ventricles (Fig. 13.7). Each cerebral hemisphere contains a
large lateral ventricle. The third ventricle lies within the diencephalon. The fourth ventricle
lies between the pons and the cerebellum.
The ventricles are connected to each other, and the fourth ventricle is continuous with the central
canal of the spinal cord. Like the central canal of the spinal cord, the ventricles of the brain are
filled with cerebrospinal fluid (CSF). The CSF circulates from the ventricles and central canal
into the subarachnoid space of the surrounding meninges via openings at the fourth ventricle.
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Cerebrospinal fluid
Cerebrospinal fluid forms a liquid cushion around the brain and spinal cord. CSF is also found
in the ventricles of the brain and central canal of the spinal cord. It is similar in composition to
blood plasma, but with less protein and different concentrations of various ions.
CSF is produced from blood in structures called the choroid plexuses (Fig. 13.8), which are
located in each of the ventricles. The choroid plexuses contain specialized ependymal cells,
which secrete the CSF, remove waste products from the CSF, and regulate its composition.
Structures called the arachnoid villi absorb CSF back into the bloodstream.
CSF is produced at a rate of about 500 ml per day, although the total volume of CSF at any one
time is just 150 ml. Thus, there is rapid turnover of CSF. Hydrocephalus may result in infants
that have problems with reabsorption of CSF.
Blood-brain barrier
Unlike capillaries in most other parts of the body, capillaries within the CNS are lined with
epithelial cells that are tightly connected to prevent movement of materials between the blood
and the interstitial spaces (Fig. 13.10). Astrocytes also wrap around capillaries to restrict
movement between the blood and brain tissue. Together, the epithelial cells of the capillaries
and the astrocytes form what is called the blood-brain barrier. Although lipid soluble
molecules (e.g., oxygen, CO2, lipids, and small alcohols) are able to diffuse freely between the
CNS and the blood, movement of other molecules is restricted.
II. Cerebrum
Cerebral hemispheres
The surface of the cerebrum is called the cerebral cortex, and it is composed of 2-4 mm of gray
matter with billions of neuron bodies (Fig. 13.11). Underneath the cortex lies the cerebral
white matter. The cerebral cortex has lots of folds, called gyri, which increase the surface area
of the cortex. The grooves between the gyri are called sulci. Particularly deep grooves are called
fissures. One major landmark of the brain is the longitudinal fissure, which divides the cortex
into left and right hemispheres.
Lobes of the cerebrum
Each hemisphere can be divided into five major regions (Fig. 13.12):
A. The frontal lobe is anterior.
B. The parietal lobe is dorsal and in the middle.
C. The temporal lobe is lateral and in the middle.
D. The occipital lobe is posterior.
E. The insula lies deep near the intersection of the frontal, temporal, and parietal lobes.
The central sulcus forms a border between the frontal and parietal lobes (Fig. 13.11). The
parieto-occipital sulcus forms a border between the parietal and occipital lobes. The lateral
sulcus forms a border between the frontal and temporal lobes.
The central sulcus separates the motor and sensory areas of the cortex (Fig. 13.11). The
precentral gyrus is located immediately anterior to the central sulcus. This is the primary motor
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generating area of the cortex. The movements of muscles associated with speech are generated
in Broca’s area. A portion of the brain known as Wernicke’s area also is involved in speaking
and understanding language.
The postcentral gyrus is located immediately posterior to the central sulcus. This is the primary
somatosensory processing area of the cortex. It receives information from receptors for touch,
pressure, pain, vibration, and temperature. The sensations of sight, sound, smell, and taste are
processed in other parts of the cortex, specifically, the visual cortex (occipital lobe), primary
auditory cortex (temporal lobe), olfactory cortex (temporal lobe), and the gustatory cortex
(insula), respectively. (See Table 13.3.) The prefrontal cortex (frontal lobe) is a very
complicated region of the cerebrum. It is involved with intellect, cognition, and personality.
Central white matter
The central white matter of the cerebrum consists primarily of myelinated axons, which
generally fall into three categories (Fig. 13.14):
A. Association tracts connect areas of cortex within a single hemisphere
B. Commissural tracts connect the two hemispheres. The corpus callosum is the largest
structure containing commissural fibers.
C. Projection tracts link the cerebrum to other parts of the brain and the spinal cord.
III. Diencephalon
The diencephalon is important for integrating conscious and unconscious sensory information
and motor commands.
Thalamus
The thalamus consists of two egg-shaped regions of the brain (Fig. 13.18). It is the principle
relay station for directing sensory information from the spinal cord, medulla, and cerebellum to
the cortex. The thalamus processes and relays auditory information, visual information, taste,
and somatic sensory information.
Hypothalamus
The hypothalamus is a small region situated below the thalamus (Fig. 13.19). It has many
functions, including control and integration of a wide variety of autonomic functions. Some
specific functions of the hypothalamus are as follows:
Process and relay olfactory information
Release various hormones, including oxytocin and antidiuretic hormone (ADH)
Integration of the autonomic nervous system
Control over heart rate
Control of digestive tract activity
Rage and aggression
Regulation of body temperature
Hunger and satiety centers
Thirst
Sleep patterns
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IV. Brainstem
The brainstem consists of three specific regions of the brain: the midbrain, the pons, and the
medulla oblongata (Fig. 13.20). All regions of the brainstem contain fibers that connect the
higher portions of the brain (cerebrum and diencephalon) to the cerebellum and the spinal cord.
The cerebral aqueduct, which connects the third ventricle to the fourth ventricle, passes
through the brainstem.
Midbrain
The midbrain is located inferior to the diencephalon (Fig. 13.21). It has reflex centers that
control eye, head, and neck movements in response to visual and auditory stimuli. On the
ventral side of the midbrain are the paired cerebral peduncles. They contain motor fibers that
extend from the cortex to the lower parts of the CNS. The midbrain also contains the
spinothalamic sensory tracts, which come from the spine and lead to the thalamus. The superior
cerebellar peduncles connect the midbrain to the cerebellum.
Pons
The pons is a swelling of the brainstem between the medulla and the midbrain (Fig. 13.22). The
pons contains longitudinal fibers that connect the medulla with higher parts of the brain.
Transverse fibers link the cerebellar hemispheres with the midbrain. Respiratory centers that
help control breathing are located in the pons.
Medulla oblongata
The medulla oblongata is the part of the brain that links the brain to the spinal cord (Fig. 13.23).
There is not a clear anatomical distinction between the medulla and the spinal cord, but the
foramen magnum marks the division between what is considered brain and what is considered
cord. The medulla contains ascending and descending tracts that run between the brain and the
cord. On the ventral side of the medulla are two prominent bulges called the pyramids. They
contain the largest motor nerve tracts. Just superior to the spinal cord, these tracts cross over
from right to left and vice versa. This crossing over of the motor tracts is known as the
decussation of the pyramids. This is why motor signals that control the left side of the body
originate from the right side of the cortex. Likewise, sensory information crosses from one side
of the body to the other in the dorsal side of the medulla oblongata. The medulla has a
cardiovascular center, which regulates cardiac output (heart rate and stroke volume) and the
diameter of blood vessels. There is also a respiratory center, which regulates breathing rate.
V. Cerebellum
Cerebellum
The cerebellum has two primary functions: (1) adjusting posture of the body to maintain balance,
and (2) fine-tuning movement. When viewed dorsally, the cerebellum is shaped somewhat like a
butterfly (Fig. 13.24). The portion along the midline is called the vermis, and the wings are the
cerebellar hemispheres.
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VI. Cranial nerves
The cranial nerves originate from the brain (Fig. 13.30), and they exit the skull through various
foramina. Each cranial nerve may carry sensory information, motor information, or a mix of the
two. Individual cranial nerves are numbered beginning with the most anterior nerve.
Learn the names (in order) of the twelve cranial nerves found in humans, learn at least one
function of each, and know whether it is sensory, motor, or mixed.
I. Olfactory—smell (sensory)
II. Optic—vision (sensory)
III. Oculomotor—movement of the eye (motor)
IV. Trochlear—movement of the eye (motor)
V. Trigeminal—sensory and motor to the face (sensory and motor)
VI. Abducens—movement of the eye (motor)
VII. Facial—sensory and motor to the face (sensory and motor)
VIII. Vestibulocochlear—balance and hearing (sensory)
IX. Glossopharyngeal—sensory and motor to head and neck (sensory and motor)
X. Vagus—sensory information from esophagus, respiratory tract, visceral organs; motor
commands to heart, glands, stomach, and more (sensory and motor)
XI. Accessory—movement of neck and upper back (motor)
XII. Hypoglossal—movement of tongue (motor)
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