The Brain

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Chapter: Anatomy and Physiology for Health Professionals: Central Nervous System

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

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.



Ventricles

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.



Cerebral Hemispheres

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.


Cerebral Cortex

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.

Function

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.


Motor Areas

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

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).


Cerebral White Matter

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.


Basal Nuclei

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.


Diencephalon

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.

Thalamus

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).

Hypothalamus

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.

Epithalamus

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?


Limbic System

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.

Reticular Formation

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?


Brain Stem

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.

Midbrain

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.

Pons

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.

Medulla Oblongata

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.


Cerebellum

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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