Nearly 100 billion neurons compose the adult brain, which can be divided into the cerebrum (with two cerebral hemispheres), diencephalon, brain stem (which includes the midbrain, pons, and medulla oblongata), and cerebellum.
The Brain
Nearly 100 billion neurons
compose the adult brain, which can be divided into the cerebrum (with two
cerebral hemispheres), diencephalon, brain stem (which includes the midbrain,
pons, and medulla oblongata), and cerebellum (FIGURE
12-1). The largest part (cerebrum)
coordinates sensory and motor functions and higher mental functions such as
memory and reasoning. The diencephalon processes additional sensory
information. The nerve path-ways of the brain stem connect nervous system
components and regulate certain visceral activities. The cerebellum
coordinates voluntary muscular movements.
The brain is basically designed
as a central cavity surrounded first by gray matter and then by white matter.
The gray matter (cortex) consists mostly of neuron cell bodies, whereas the
white matter consists of myelinated fiber tracts. This pattern is different in
the spinal cord, in which the gray matter is located in the center with the
white matter outside. However, the brain also has extra regions of gray matter
not found in the spinal cord. The cerebral hemispheres and cerebellum have an
outer cortex, which is a layer of
gray matter. Male brains are typically larger compared to female brains.
Interconnected cavities known as ventricles exist
within the cerebral hemispheres and brain stem. They are continuous with the
spinal cord’s central canal and also contain CSF. The walls of the hollow
ventricular chambers are lined by ependymal
cells. The two large lateral
ventricles are located inside the frontal,
temporal, and occipital lobes. The third ventricle is under the corpus callosum in the
brain’s midline. The fourth ventricle is in the brain stem and a narrow cerebral aqueduct joins it
to the third ventricle.
Within each cerebral hemisphere
are large C-shaped chambers known as the paired lateral ventricles. They
demonstrate the pattern of cere-bral growth and lie close together anteriorly.
A thin median membrane called the septum pellucidum separates them. Each lateral ventricle is connected to the thin third ventricle, which is surrounded by
the diencephalon via a channel known as the interven-tricular
foramen. The third ventricle connects
to the fourth
ventricle via the cerebral aqueduct,
which runs through the midbrain. FIGURE
12-2 shows the ventricles
in the brain.
The cerebrum is divided into two
large cerebral hemispheres, one to the left and one to the right. They form the
superior part of the brain and are easily visualized, making up approximately
83% of the brain’s total mass. The corpus
callosum is a deep bridge of nerve fibers that connects the hemispheres,
separated by a layer of dura mater. It lies superior to the lateral
ventricles, deep inside the longitudinal fissure. The cerebrum’s surface is covered with gyri, which are separated
by shallow or deep grooves. Each shallow groove is called a sulcus and each deep groove is called a fissure. The fissures separate large regions of the brain. All these grooves
form distinct patterns in normal brains, with the gyri and sulci being more
prominent.
Several sulci divide each
hemisphere into the frontal, parietal, temporal, and occipital lobes (as well
as a structure called the insula). The insula is a brain lobe, but is buried deep within the lateral
sulcus forming a portion of the brain floor. The lobes of the cerebral
hemispheres refer to the skull bones they are positioned near. The cerebral
hemispheres are sepa-rated by the median longitudinal fissure, whereas the transverse cerebral fissure separates the cerebral
hemispheres from the cerebellum below
them. The lateral sulcus separates the temporal lobe from the frontal lobe.
The three basic regions of each cerebral
hemi-sphere are the cerebral cortex, white matter, and basal nuclei. The
cerebral cortex is a superficial gray matter and actually appears gray in
color in fresh brain tissue. The white matter is more internal, and the basal
nuclei are islands of gray matter located deep inside the white matter.
The cerebral
cortex, a thin layer of gray matter
comprising the outer portion of the cerebrum, is the center of the conscious mind. The adult human brain
contains almost 98% of all the neuron cell bodies of the nervous system. The
cerebral cortex is involved with awareness, communication, sensation, memory,
understanding, and the initiation of voluntary move-ments. Its gray matter
contains dendrites, neuron cell bodies, glia, and blood vessels. It lacks fiber
tracts but contains six layers in which there are billions of neu-rons. The
cerebral cortex is approximately 2–4 mm thick, yet it makes up approximately
40% of the over-all brain mass. Its surface area is nearly tripled by its many
convolutions.
Beneath the cerebral cortex is
white matter, com-prising most of the cerebrum. It contains myelinated axon
bundles, some of which pass from one cerebral hemisphere to the other. Others
carry impulses from the cortex to nerve centers of the brain and spinal cord (FIGURES 12-3 and 12-4).
The lobes of the cerebral cortex
are:
■■ Frontal lobe: Forms the
anterior portion of each cerebral hemisphere
■■ Parietal lobe: Lies
posteriorly to the frontal lobe
■■ Temporal lobe: Lies below the
frontal and parietal lobes
■■ Occipital lobe: Forms the
posterior part of each cerebral hemisphere
■■ Insula: Lies under the
frontal, parietal, and temporal lobes
In most people, one side of their
cerebrum acts as the dominant
hemisphere, controlling the use and
understanding of language. The left side of the cerebrum is usually responsible
for activities such as speech, writing, reading, and complex intellectual
functions. The nondominant hemisphere controls nonverbal functions and
intuitive and emotional thoughts. The dominant hemisphere controls the motor
cortex of the nondominant hemisphere.
Aside from sensory and motor
control, memory, and reasoning, the cerebrum also coordinates intelli-gence and
personality. It is the “executive suite” of the body. Functions overlap between
regions of the cere-bral cortex. The three functional areas of the cerebral
cortex are the motor, sensory, and association areas (FIGURE 12-5). All
neurons in the cerebral cortex are interneurons.
Each cerebral hemisphere controls the motor
and sensory functions of the contralateral (opposite) side of the body. The hemispheres are not exactly equal in
function, even though their structure is closely matched. Cortical functions
are specialized, which exhibits a phenomenon known as lateralization. No functional area acts individually and conscious
actions use the entire cortex in varying ways.
Most of the cerebral cortex motor areas are
located in the frontal lobes and are further defined as the primary motor cortex, premotor cortex,
Broca’s area, and frontal eye field. Impulses from large pyramidal cells in the motor areas
travel through the brain stem into the spinal cord via the corticospinal tracts that
form synapses with lower motor neurons. Their axons leave the spinal cord,
reaching the skeletal muscle fibers.
The primary motor cortex is also known as the somatic motor cortex. It is located in the precentral gyrus of the frontal lobe of both hemispheres. The mapping of the CNS structures of the body is referred to as somatotopy. The premotor cortex lies just anterior to the precentral gyrus in the frontal lobe and helps to plan movements. Broca’s area is found anterior to the inferior region of the premotor area and is more prevalent in the left hemisphere. It has a motor speech area, and also becomes active just before speaking or when planning other voluntary motor activities. The frontal eye field is superior to Broca’s area, located partly in and anterior to the premotor cortex. It controls voluntary eye movements. The central sulcus separates the primary motor areas from the somatosensory areas.
Sensory areas of the cerebrum interpret
impulses such as skin sensations,
which are picked up in the anterior portions of the parietal lobes. The
posterior occipital lobes affect vision, whereas the temporal lobes affect
hearing. Taste and smell receptors are located deeper within the cerebrum.
Sensory fibers also cross simi-larly to motor fibers. Additional sensory areas
include the insular and occipital lobes.
The primary
somatosensory cortex lies in the postcentral gyrus of
the parietal lobe. It is just pos-terior to the primary motor cortex and its
neurons receive input from the somatic sensory receptors of the skin. It also
receives input from position sense receptors in the joints, skeletal muscles,
and tendons. The somatosensory
association cortex is found just posterior to the
primary somatosensory cortex, is interconnected, and functions primarily to
inte-grate temperature, pressure, and related information. The primary visual cortex, also
called the striate cortex, is not only mostly
buried in the calcarine sulcus of the occipital lobe, but also extends to the extreme posterior occipital tip. It is the largest cortical sensory
area, receiving visual information from the retinas of the eyes. The visual association area uses
visual experiences from the past to interpret color, form, movement, and other
visual stimuli.
Each primary
auditory cortex lies in the supe-rior margin of
the temporal lobe and receives impulses from the inner ear, interpreting
location, loudness, and pitch. Posteriorly, the auditory
association area perceives sound stimuli such as
speech, music, and environmental noises. The vestibular
(equilibrium) cortex
controls balance and is located in the
posterior insula and the nearby parietal cortex. The primary (olfactory)
smell cortex is present on the
medial temporal lobe in the piriform
lobe area, which is sig-nified by its uncus,
a hook-like structure. The olfac-tory cortex is part of the rhinencephalon, a primitive structure that includes the orbitofrontal cortex, uncus, and related regions on or inside the
medial temporal lobe as well as the olfactory tracts and bulbs extending to the
nose.
The gustatory
(taste) cortex is located in the insula, deep
in the temporal lobe. The visceral sensory area controls visceral sensations and lies in the cortex of the
insula, just posterior to the gustatory cortex. Its sensations include bladder
fullness, stom-ach upset, and tightness in the lungs (such as from holding your
breath).
The internal cerebral white
matter controls commu-nication between areas of the cerebrum and between the cerebral
cortex and lower centers of the CNS. Myelinated fibers, bundled into large
tracts, make up most of this white matter. The fibers and tracts are classified
by the directions in which they run.
Association
fibers connect the various
parts of the same brain hemisphere. Adjacent gyri are connected by
short association fibers called arcuate
fibers. Different cortical lobes are
connected by long association fibers,
which are bundled into tracts. Corresponding gray areas of both hemispheres are
connected by commissural
fibers or commissures, which allow the hemispheres
to function together.
The corpus callosum is the
largest commissure, and there are also anterior
and posterior commissures. Projection fibers enter the cerebral cortex from
spinal cord or lower brain areas or
descend to lower areas from the cerebral cortex. They allow motor output to
leave the cerebral cortex and also sensory information to reach it. Projection
fibers are differ-ent from association and commissural fibers in that they run
vertically. Projection fibers at the top of the brain stem form a compact internal capsule, pass-ing between the thalamus and certain basal nuclei. They
then have a fan-like radiating pattern through the cerebral white matter and
are therefore referred to as the corona
radiata.
Several masses of gray basal nuclei (basal
ganglia) lie deep inside each cerebral hemisphere. The term basal nuclei refers
to the cerebral nuclei. These are the caudate
nucleus, globus pallidus, and putamen. The basal nuclei help to control skeletal muscle
activ-ities. It provides the general pattern and rhythm for movements such as
walking. They filter out inappro-priate responses as well as being involved in
cognition and emotion. The lack of dopamine released from the basal nuclei may
cause Parkinson’s disease. The cau-date nucleus arches superiorly over the
diencephalon, joining the putamen to form the corpus striatum, which has a striped appearance. The corpus striatum encompasses
the caudate and lentiform nuclei. The lentiform nucleus consists of a medial
globus palli-dus, and a lateral putamen. The basal nuclei are linked to the subthalamic nuclei of the diencephalon
and the substantia nigra of the midbrain. They receive input from all the cerebral
cortex, other subcortical nuclei, and each other. The globus pallidus and
substantia nigra relay information through the thalamus, reach-ing the premotor
and prefrontal cortices. Therefore, they influence muscle movements, as
controlled by the primary motor cortex. However, the basal nuclei do not
directly access the motor pathways. FIGURE 12-6 shows the basal nuclei and nearby structures.
1. List
the major divisions of the human brain.
2. Describe
the functions of the cerebrum.
3. What
are the locations of the central sulcus and the lateral sulcus?
4. Explain
the three basic regions of each cerebral hemisphere.
5. Describe
the components of the basal nuclei.
The diencephalon is mostly made up of the paired gray matter structures known as the thalamus, hypothalamus, and epithalamus
and forms the central core of the
forebrain. The diencephalon is surrounded by the cerebral hemispheres and
itself encloses the third ventricle.
The superolateral walls of the
third ventricle are formed by the egg-shaped bilateral nuclei of the thalamus. This
structure makes up 80% of the dien-cephalon and is found deep inside the brain.
The nuclei of the thalamus are interconnected (in most individuals) by an
intermediate mass known as the interthalamic
adhesion. The thalamic nuclei are mostly
named based on their location, each having functional specialties, with unique
fibers connected to certain regions of the cerebral cortex.
The thalamus processes and relays
all incoming and outgoing information between the cerebral cortex and the
spinal cord. The thalamus mediates motor activi-ties, sensation, cortical
arousal, learning, and memory. Related impulses are organized in groups through
the internal capsule of the thalamus to the correct area of the cerebral cortex
and association areas. Afferent impulses reaching the thalamus are basically
recog-nized as either pleasant or unpleasant. Specific stim-ulus discrimination
and localization actually occur in the cerebral cortex, not in the thalamus.
Nearly all other inputs ascending to the cerebral cortex are chan-neled through
the thalamic nuclei: inputs for memory or sensory integration projected to
areas such as the pulvinar, lateral dorsal, or lateral posterior nuclei, and inputs regulating
emotional and visceral function from the hypothalamus via the anterior nuclei.
Additionally, the thalamic nuclei interpret instructions aiding in direction of
motor cortical activity from the cerebel-lum (via the ventral lateral nuclei)
and the basal nuclei (via the ventral anterior nuclei).
The hypothalamus is the primary visceral control center of the body. It is crucial for the homeostasis of the body, affecting
nearly all body tissues. It is located below the thalamus, capping the brain
stem, and forming the inferolateral walls of the third ventricle (FIGURE 12-7). Paired, small, and round structures bulge anteriorly from the hypothalamus. Known as mammillary bodies, they act as relay stations in the
olfactory pathways. A stalk of hypothalamic
tissue known as the infundibulum lies between the mam-millary bodies and the optic chiasma.
The infundibu-lum connects the pituitary
gland to the base of the
hypothalamus.
The hypothalamus controls the autonomic nerous system (ANS) and endocrine system function. It also initiates physical responses to emotions. Other regulatory functions of the hypothalamus affect body temperature, intake of food, water bal-ance, thirst, and the sleep–wake cycle. Its control of ANS activities occur by control of brain stem and spinal cord activity. The hypothalamus is vital for the limbic system, which is the emotional part of the brain, and it acts through ANS pathways to initiate many physical expressions of emotion. The hypo-thalamus is also the body’s thermostat, controls hor-mone secretion from the anterior pituitary gland, and produces the hormones antidiuretic hormone and oxytocin.
The epithalamus is the most dorsal part of the diencephalon, forming the roof of the
third ventri-cle. The pineal gland extends from its posterior order. This gland secretes the hormone melatonin, which
helps regulate the sleep–wake cycle and also acts as an antioxidant. The caudal
border of the epithalamus is formed by the posterior
commissure. The major parts of the brain are summarized in TABLE 12-1.
1. Name
three major structures of the diencephalon.
2. What
are the main functions of the hypothalamus?
3. What
are the main functions of the thalamus?
4. What is
the function of the mammillary bodies and their location in the brain?
On the medial aspect of each
cerebral hemisphere and the diencephalon are a group of structures that
comprise the limbic system (FIGURE 12 -8). The upper part of the brain stem is encircled by the structures of
the limbic system. An almond-shaped nucleus on the tail of the caudate nucleus,
known as the amygdaloid
body, is part of the limbic system.
Other parts include the various sections of the rhin-encephalon. In the
diencephalon, the primary lim-bic structures are the anterior thalamic nuclei and the hypothalamus. Fiber tracts such as the fornix link all these regions. The rhinencephalon includes the cingulate
gyrus, septal nuclei, the C-shaped hippocampus, the dentate gyrus, and the
parahip-pocampal gyrus.
The emotional, feeling part of
the brain consti-tutes the limbic system. For emotions, the critical areas are
the anterior cingulate gyrus and the
amyg-daloid body, which is important for response to threats with either fear
or aggression. Emotions are expressed through gestures and frustration is
resolved by the cingulate gyrus. Much of the lim-bic system is involved with
the rhinencephalon and odors may trigger emotional reactions and memo-ries as a
result.
The limbic system can integrate and respond to many environmental stimuli because of extensive connections between it and both lower and higher brain regions. The hypothalamus relays most limbic system output. The hypothalamus processes emo-tional responses as well as autonomic (visceral) func-tion; as a result, acute or prolonged emotional stress may cause visceral illnesses such as heartburn or high blood pressure. Emotion -influenced illnesses are described as psychosomatic illnesses. The hippocampus in the limbic system is responsible for storage and retrieval of new long-term memories. The amygda-loid body also functions in memory processing.
The reticular
formation is mostly composed of white
matter and extends through the core of the brain stem. A section of this
formation, known as the reticular activating system, continually supplies impulses
to the cerebral cortex to enhance its
excitability (FIGURE 12-9). The reticular activating system also filters sensory
inputs. It is inhibited by sleep centers of the hypothalamus and other regions
and is affected by CNS depressants. The reticular formation also has a motor
section, projecting to the spinal cord via the reticulospinal tracts.
1. Explain
the amygdaloid body and the fornix.
2. What is
the function of the reticular activating system?
The most superior region of the brain stem is the midbrain , with descending regions
including the pons and medulla oblongata (FIGURE 12-10). The
entire brain stem only makes up about
2.5% of the total brain mass. The midbrain, pons, and medulla oblon-gata are
each about 1 inch in length. The brain stem is organized similarly to the
spinal cord, with deep gray matter surrounded by white matter fiber tracts. The
brain stem also has nuclei of gray matter that are embedded in its white
matter—this differs from the organization of the spinal cord.
Behaviors needed for survival are produced in the brain stem. These behaviors are automatic and highly controlled. The brain stem creates a pathway for fiber tracts that connect higher and lower neural centers. The brain stem nuclei are also linked to 10 pairs of the cranial nerves and are greatly involved with innerva-tion of the head.
Between the diencephalon and pons
is the portion of the brain stem known as the midbrain. It has two bulges (cerebral
peduncles) on its ventral aspect, which
support the cerebrum. Each pedun-cle has a crus
cerebri, a leg-like structure containing a large corticospinal (pyramidal)
motor tract that descends toward the spinal cord. Other fiber tracts, the superior cerebellar peduncles, connect the midbrain to the cerebellum in
its dorsal region. The roof of the midbrain is called the tectum. The cerebral aqueduct is the channel connecting the third
and fourthventricles.
In the midbrain, nuclei are also
located through-out the surrounding white matter. The largest mid-brain nuclei
are the corpora quadrigemina, which create four rounded protrusions on the dorsal mid-brain’s
surface. The superior colliculi are
two visual reflex centers coordinating
head and eye movements. The inferior colliculi relay impulses
from the audi-tory center. The tegmentum
is the area anterior to the cerebral aqueduct. Two pigmented nuclei are located
on each side of the white matter of the mid-brain: the substantia nigra and the red
nucleus. The substantia nigra has a high
amount of melanin pigment and appears dark in color. When dopamine- releasing
neurons of the substantia nigra degenerate, Parkinson’s disease results. The
red nucleus has a rich blood supply and iron pigment. It is a part of the
reticular formation.
Lying between the midbrain and
medulla oblongata, the pons is a bulge in the brain stem separated dorsally from the cerebellum by
the fourth ventricle. It primarily contains conduction tracts that run either
longitudinally, transversely, or dorsally. The pontine nuclei relay information between the motor cortex and
cerebellum. Three nerve pairs (the trigeminal
(V), abducens (VI) , and facial (VII) nerves) originate from the pontine nuclei. The pons also contains ascending, descending,
and transverse tracts, longitudinal tracts interconnected with other CNS
structures. The middle cerebellar peduncles are connected to the transverse fibers that cross the
anterior surface of the pons.
The most inferior part of the
brain stem is known as the medulla
oblongata or, more simply, the medulla . It joins the spinal cord
smoothly, at the level of the skull’s
foramen magnum. The cranial nerves known as the vestibulocochlear (VIII),
glossopharyngeal (IX), vagus (X), and hypoglossal (XII) nerves originate from
the medulla. It plays a vital role as a center of autonomic reflexes required
for homeostasis. Important functional groups of visceral motor nuclei are controlled by the medulla oblongata. Its cardiovascular center includes both the
cardiac center and vasomotor center. Its respi-ratory centers control
respiratory rhythm, rate, and depth. Various other centers of the medulla
influ-ence hiccupping, vomiting, coughing, swallowing, and sneezing. Motor
nuclei send motor commands to peripheral effectors.
Approximately, 11% of the brain
is made up by the cerebellum. It is the second largest portion of the brain (after the cerebrum) and
appears similar to the shape of a cauliflower. It is found dorsal to the pons,
medulla, and the fourth ventricle. The cerebel-lum protrudes under the
occipital lobes of the cere-bral hemispheres and is separated from these lobes
by the transverse cerebral fissure. The cerebellum processes inputs from the
cerebral motor cortex, brain stem, and sensory receptors. It then regulates
skeletal muscle movements for many different activ-ities such as driving a car,
playing a musical instru-ment, or using a computer. All cerebellar activity is
subconscious.
The surface of the cerebellum is highly convo-luted. It has fine gyri known as folia, which have a folded appearance and are transversely oriented. The cerebellum is bilaterally symmetrical, with a worm-like vermis connecting its two hemispheres. Each cerebellar hemisphere is divided into anterior, posterior , and flocculonodular lobes. The cerebel-lum has its own thin outer cortex of gray matter, internalized white matter (arbor vitae), and deep masses of gray matter. These paired masses include the dentate nuclei and neurons of the cerebellum contain large Purkinje cells, uniquely distributing axons through the white matter to synapse with the central cerebellar nuclei. Superior, middle, and inferior cerebellar peduncles connect the cerebellum with other brain structures.
1. What is
the function of the limbic system?
2. What
regions make up the brain stem?
3. What
are the functions of the medulla oblongata?
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