The diencephalon, which lies below the cerebrum, includes the thalamus, hypothalamus, and epithalamus (Fig. 43-2). The thalamus is the major “relay station,” or “central switchboard,” for the CNS. The hypothalamus plays a major role in autonomic nervous system control (controlling temperature and other functions) and intellectual function (cognition). The epithalamus contains the roof of the third ventricle and the pineal gland.

FIG. 43-2 The structures of the brainstem and diencephalon.

The cerebrum is the largest part of the brain and controls intelligence, creativity, and memory. The “gray matter” of the cerebrum is the central cortex—the center that receives information from the thalamus and all the lower areas of the brain. The cerebrum consists of two halves, referred to as the right hemisphere and the left hemisphere, which are joined by the corpus callosum. The left hemisphere is the dominant hemisphere in most people (even in many left-handed people). Within the deeper structures of the cerebrum are the right and left lateral ventricles. At the base of the cerebrum near the ventricles is a group of neurons called the basal ganglia, which help regulate the need for mobility.

The cerebral cortex is part of the cerebrum and is involved with almost all of the higher functions of the brain. This part of the brain processes and communicates all information coming from the peripheral nervous system (PNS). It also translates the impulses into understandable feelings and thoughts. The cerebral cortex is so complex that it is further divided into four lobes—the frontal lobe, parietal lobe, temporal lobe, and occipital lobe.

The frontal lobe is found at the front of the head near the temples and forehead. It processes voluntary muscle movements and higher functioning actions such as thought and speech. It also helps control mood, planning for the future, setting goals, and making judgments. The parietal lobe is found behind the frontal lobe. It processes spatial awareness and receives and processes information about temperature, taste, touch, and movement coming from the rest of the body. Reading and arithmetic are also processed in this lobe. The temporal lobes are located in the area of the brain parallel to the ears. They process hearing, memory, and language functions. The occipital lobe is located in the posterior of the brain and assists in the processing of visual information.

Located in the frontal lobe, the motor cortex controls voluntary movement. Corticospinal tracts, also called pyramidal tracts, begin in the motor cortex and travel through the brain before crossing in the medulla. This crossing explains how right motor cortex damage affects the movement in the left side of the body and vice versa, such as in many patients who have cerebral strokes. The cerebrum is divided into lobes by sulci (fissures). These lobes work together and are connected by nerve fibers. The name and main function of each lobe is listed in Table 43-1.

TABLE 43-1 CEREBRAL LOBE MAIN FUNCTIONS

Frontal Lobe

Parietal Lobe

• Understand sensation, texture, size, shape, and spatial relationships

• Three-dimensional (spatial) perception

• Important for singing, playing musical instruments, and processing nonverbal visual experiences

• Perception of body parts and body position awareness

• Taste impulses for interpretation

Temporal Lobe

• Auditory center for sound interpretation

• Complicated memory patterns

• Wernicke’s area for speech

Occipital Lobe

• Primary visual center

Limbic Lobe

• Emotional and visceral patterns connected with survival

• Learning and memory

Two important speech areas of the cerebrum are Broca’s area and Wernicke’s area. Broca’s area (speech area), also located in the frontal lobe, is responsible for the formation of words, or speech. Wernicke’s area (language area) is located in the temporal lobe and allows processing of words into coherent thought and understanding of written or spoken words.

The hypophysis (pituitary gland) has two lobes, each releasing specific hormones into the circulation under the regulation of the hypothalamus. The pituitary is often referred to as the “master gland” because of its control of numerous hormonal functions. However, the hypothalamus actually controls its functions.

The cerebellum receives immediate and continuous information about the condition of the muscles, joints, and tendons. Cerebellar function enables a person to:

• Keep an extremity from overshooting an intended target

• Move from one skilled movement to another in an orderly sequence

• Predict distance or gauge the speed with which one is approaching an object

• Control voluntary movement

• Maintain equilibrium

Unlike the motor cortex, cerebellar control of the body is ipsilateral (situated on the same side). The right side of the cerebellum controls the right side of the body, and the left cerebellum controls the left side of the body.

The brainstem includes the midbrain, pons, and medulla. The functions of these structures are presented in Table 43-2. Throughout the brainstem are special cells that constitute the reticular activating system (RAS), which controls awareness and alertness. For example, this tissue awakens a person from sleep when presented with a stimulus such as loud noise or pain or when it is time to awaken. The reticular formation area has many connections with the cerebrum, the rest of the brainstem, and the cerebellum.

TABLE 43-2 BRAINSTEM FUNCTIONS

Circulation in the Brain

Circulation in the brain originates from the carotid and vertebral arteries (Fig. 43-3). The internal carotid arteries branch into the anterior cerebral artery (ACA) and middle cerebral arteries (MCA), the largest ones. The two posterior vertebral arteries become the basilar artery, which then divides into two posterior cerebral arteries. The anterior, middle, and posterior cerebral arteries are joined together by small communicating arteries to form a ring at the base of the brain known as the circle of Willis.

FIG. 43-3 Cerebral circulation and the circle of Willis at the base of the brain.

The middle cerebral artery supplies the lateral surface of the cerebrum from about the mid-temporal lobe upward (i.e., the area for hearing and upper body motor and sensory neurons). The anterior cerebral artery supplies the midline, or medial, aspect of the same area (i.e., the lower body motor and sensory neurons). The posterior cerebral arteries supply the area from the mid-temporal region down and back (occipital lobe), as well as much of the brainstem. When blood flow is interrupted in any of these arteries (e.g., by a clot), the area of the brain being supplied is affected and may not function as it should.

The blood-brain barrier (BBB) seems to exist because the endothelial cells of the cerebral capillaries are joined tightly together. This barrier keeps some substances in the bloodstream out of the cerebrospinal circulation and out of brain tissue. Substances that can pass through the BBB include oxygen, glucose, carbon dioxide, alcohol, anesthetics, and water. Large molecules such as albumin, any substance bound to albumin, and many antibiotics are prevented from crossing the barrier.

Cerebrospinal fluid (CSF) also circulates, surrounds, and cushions the brain and spinal cord. While moving through the subarachnoid space, the fluid is continuously produced by the choroid plexus, reabsorbed by the arachnoid villi, and then channeled into the superior sagittal sinus. Expanded areas of subarachnoid space, where there are large amounts of CSF, are called cisterns. The largest one is the lumbar cistern, the site of lumbar puncture, from the level of the second lumbar vertebra to the second sacral vertebra (L2-S2).

Peripheral Nervous System: Structure and Function

The peripheral nervous system (PNS) is composed of the spinal nerves, cranial nerves, and autonomic nervous system.

There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal) exiting from the spinal cord. Each of the nerves has a posterior and an anterior branch. The posterior branch carries sensory information (sensation) to the cord (afferent pathway). The anterior branch transmits motor impulses (mobility) to the muscles of the body (efferent pathway).

Each spinal nerve is responsible for the muscle innervation and sensory reception of a given area of the body. The cervical and thoracic spinal nerves are relatively close to their areas of responsibility, whereas the lumbar and sacral spinal nerves are some distance from theirs. Because the spinal cord ends between L1 and L2, the axons of the lumbar and sacral cord extend downward before exiting at the appropriate intervertebral foramen. The area controlled by each spinal nerve is roughly reflected in the dermatomes. Dermatomes represent sensory input from spinal nerves to specific areas of the skin (Fig. 43-5). For example, the patient with an injury to cervical spinal nerves C6 and C7 has sensory changes in the thumb, index finger, middle finger, middle of the palm, and back of the hand.

FIG. 43-5 Dermatomes (cutaneous innervation of spinal nerves).

Sensory receptors throughout the body monitor and transmit impulses of pain, temperature, touch, vibration, pressure, visceral sensation, and proprioception. Sensory receptors also monitor and transmit the sensations of the special senses—vision, taste, smell, and hearing.

The cell bodies of the anterior spinal nerves are located in the anterior gray matter (anterior horn) of each level in the spinal cord. The anterior motor neurons are also referred to as lower motor neurons. As each nerve axon leaves the spinal cord, it joins other spinal nerves to form plexuses (clusters of nerves). Plexuses continue as trunks, divisions, and cords and finally branch into individual peripheral nerves.

The reflex arc is a closed circuit of spinal and peripheral nerves and therefore requires no control by the brain (Fig. 43-6).

FIG. 43-6 An example of reflex activity. Stimulation of skin results in involuntary muscle contraction (reflex arc).

Reflexes consist of sensory input from:

• Skeletal muscles, tendons, skin, organs, and special senses

• Small cells in the spinal cord lying between the posterior and anterior gray matter (interneurons)

• Anterior motor neurons, along with the muscles they innervate

There are 12 cranial nerves. Their name, number, origin, type, and function are summarized in Table 43-3. Cranial nerve function is an important part of the nursing assessment of the patient with a neurologic problem. For example, cranial nerves II, III, IV, and VI are important for assessment of the patient with a stroke.

TABLE 43-3 ORIGINS, TYPES, AND FUNCTIONS OF THE CRANIAL NERVES

Autonomic Nervous System: Structure and Function

The autonomic nervous system (ANS) is composed of two parts: the sympathetic nervous system (SNS), or the “fight or flight system”; and the parasympathetic nervous system. ANS functions are not usually under conscious control but may be altered in some people by using biofeedback and other methods.

The SNS cells originate in the gray matter of the spinal cord from T1 through L2 or L3. This part of the ANS is considered thoracolumbar because of its anatomic location. The SNS stimulates the functions of the body needed for “fight or flight” (e.g., heart and respiratory rate). It also inhibits those functions not needed in this situation.

The parasympathetic cells originate in the gray matter of the sacral area of the spinal cord (from S2 through S4) plus portions of cranial nerves III, VII, IX, and X (craniosacral). The parasympathetic nervous system can slow body functions when needed.

Parasympathetic fibers to the organs have some sensory ability in addition to motor function. Sensations of irritation, stretching of an organ, or a decrease in tissue oxygen are transmitted to the thalamus through pathways not yet fully understood. Because pain from internal organs is often felt below the body wall innervated by the spinal nerve, it is presumed that there are connections between the viscera and body structure that relay pain sensations.

Neurologic Changes Associated with Aging

Neurologic changes associated with aging often affect mobility and sensation. Motor changes in late adulthood can cause slower movement and response time and decreased sensation (Chart 43-1). Any problems that affect the nerves, bones, muscles, or joints also affect motor and therefore ADL ability.

Chart 43-1 Nursing Focus on the Older Adult

Changes in the Nervous System Related to Aging

PHYSIOLOGIC CHANGES	NURSING IMPLICATIONS	RATIONALES
Slower processing time	Provide sufficient time for the affected older adult to respond to questions and/or direction.	Allowing adequate time for processing helps to differentiate normal findings from neurologic deterioration.
Recent memory loss	Reinforce teaching by repetition and written teaching aids.	Greatest loss of brain weight is in the white matter of the frontal lobe. Intellect is not impaired, but the learning process is slowed. Repetition helps the patient learn new information and recall it when needed.
Decreased touch sensation	Remind the patient to look where his or her feet are placed when walking. Instruct the patient to wear shoes that provide good support when walking. If the patient is unable, change his or her position frequently (every hour) while he or she is in bed or the chair.	Decreased sensation may cause the patient to fall.
Change in perception of pain	Ask the patient to describe the nature and specific characteristics of pain. Monitor additional assessment variables to detect possible health problems.	Accurate and complete nursing assessment ensures that the interventions will be appropriate for the older adult (see Chapter 3).
Change in sleep patterns	Ascertain sleep patterns and preferences. Ask if sleep pattern interferes with ADLs. Adjust the patient’s daily schedule to his or her sleep pattern and preference as much as possible (e.g., evening versus morning bath).	Most older adults require less sleep than do younger adults. However, frequent rest periods are needed.
Altered balance and/or decreased coordination	Instruct the patient to move slowly when changing positions. If needed, advise the patient to hold on to handrails when ambulating. Assess the need for an ambulatory aid, such as a cane.	The patient may fall if moving too quickly. Assistive and adaptive aids provide support and prevent falls.
Increased risk for infection	Monitor carefully for infection.	Older adults often have structural deterioration of microglia, the cells responsible for cell-mediated immune response in the central nervous system (CNS).