10 The nervous system
Neuroglia: The supporting structure of nervous tissue that consists of a fine web of tissue composed of neuroglia or glia cells. It performs supportive and nutritive functions for the nerve network but is not directly involved in nerve impulse transmission.
Postural Reflexes: Reflexes that are basically proprioceptive, concerned with the position of the head in relation to the trunk and with adjustments of the extremities and eyes to the position of the head.
The primary goal of anesthesia, whether general anesthesia, neuraxial anesthesia (spinal and epidural anesthesia), or regional anesthesia, is the alteration of the normal functioning of the nervous system in the body. General anesthesia achieves this primarily by interacting with the central nervous system, whereas regional anesthesia affects the peripheral nervous system. The effects of neuraxial anesthesia bridge both the central nervous system and the peripheral nervous system. Regardless of the type of anesthesia used, patients in the postanesthesia care unit (PACU) will have some alteration in their nervous system functioning. Consequently, the perianesthesia nurse must have an understanding of the basic anatomic and physiologic principles of the nervous system. This chapter provides the perianesthesia nurse with a comprehensive review of both the anatomy and the physiology of the central and peripheral nervous system.
The nervous system can be broadly divided into two components: the central nervous system (CNS) and the peripheral nervous system (PNS). Although these divisions are commonly used, the boundaries between them can be somewhat arbitrary. The flow of sensory information and motor control signals between the two elements of the nervous system is critical for its normal functioning and the health of the individual.
The CNS comprises the brain and spinal cord and is exceedingly complex, both anatomically and physiologically. None of the structures in the CNS function in an isolated manner. Neural activity at any level of the CNS always modifies or is modified by influences from other parts of the system, which accounts for the unique nature and extreme complexity of the CNS, much of which remains to be clearly understood.
The human brain serves both structurally and functionally as the primary center for control and regulation of all nervous system functions. As such, it is the highest level of control and integration of sensory and motor information in the entire body.
The brain (encephalon) is divided into the following three large areas based on its embryonic development: (1) the forebrain (prosencephalon) contains the telencephalon (cerebrum) with its hemispheres and the diencephalon; (2) the midbrain (mesencephalon) contains the cerebral peduncles, the corpora quadrigemina, and the cerebral aqueduct; and (3) the hindbrain (rhombencephalon) comprises the medulla oblongata, the pons, the cerebellum, and the fourth ventricle.
The cerebrum consists of two hemispheres interconnected by a large band of neurons known as the corpus callosum. Each hemisphere is further subdivided into four lobes that correspond in name to the overlying bones of the cranium. These lobes are the frontal, parietal, temporal, and occipital lobes (Fig. 10-1). Both hemispheres consist of an external cortex of gray matter, the underlying white matter tracts, and the basal ganglia (cerebral nuclei). Each hemisphere also contains a lateral ventricle, which is an elongated cavity concerned with the formation and circulation of cerebrospinal fluid (CSF).
The cerebral cortex has an elaborate mantle of gray matter and is the most highly integrated area in the nervous system. It is arranged in a series of folds that dip down into the underlying regions. These folds greatly expand the surface area of the gray matter within the limited confines of the skull. Each fold is known as a gyrus. Grooves exist between these gyri. A shallow groove is known as a sulcus, whereas a deeper grove is known as a fissure.
Each hemisphere has three sulci between the lobes. The central sulcus (also known as the fissure of Rolando) separates the frontal and parietal lobes. The lateral sulcus (the fissure of Sylvius) lies between the frontal and parietal lobes above and the temporal lobe below. The small parietooccipital sulcus is located between its corresponding lobes (see Fig. 10-1).
The white matter of the cerebrum is situated below the cortex and is composed of three main groups of myelinated nerve fibers arranged in related bundles or tracts. The commissural fibers transmit impulses between the left and right hemispheres. The largest of these fibers is the corpus callosum. The projection fibers are afferent and efferent nerve fibers that transmit impulses between the cortex, lower parts of the brain, and the spinal cord. A notable example is the internal capsule that surrounds most of the basal ganglia and, in part, connects the thalamus and the cerebral cortex. Finally, the association fibers transmit impulses from one part of the cortex to another within the same hemisphere.1
Nearly every portion of the cerebral cortex is connected with underlying structures of the diencephalon, midbrain, and hindbrain, and no areas in the cortex are exclusively motor or exclusively sensory in nature. However, some regions are primarily concerned with the control of motor movement, whereas others are primarily involved in the perception of sensory information. The activities of these areas are integrated by association fibers that compose the remainder of the cerebral cortex. Association fibers play important roles in complex intellectual and emotional processes.
No single area of motor control exists within the brain because the integration and control of muscle activity depends on the harmonious activities of several areas, including the cerebral cortex, the basal ganglia, and the cerebellum.
The primary motor area of the cerebral cortex is located in the precentral gyrus of the frontal lobe, just in front of the central sulcus, and is concerned mainly with the voluntary initiation of finely controlled movements, such as those of the hands, fingers, lips, tongue, and vocal cords. The amount of area in the primary motor cortex devoted to a particular muscle or muscle group is a reflection of the degree of fine motor control required for the proper functioning of these muscles. For example, the muscles that control speech or the use of the fingers are represented by many more neurons within the primary motor cortex then are the larger muscles of the legs or trunk. This disproportionate representation within the primary motor cortex is a reflection of the relative importance the brain places on the proper control of different muscles.
Axons from the primary motor cortex descend through the internal capsule, midbrain, and the pons to the medulla. These axons are called pyramidal because of the shape of the structure they form with the medulla. Within the medulla, most of these axons decussate and continue down into the spinal cord via the lateral corticospinal tracts. Fibers that do not decussate in the medulla descend down the spinal cord via the ventral corticospinal tracts. Most of these fibers eventually decussate at lower levels within the cord. Pyramidal cell axons also connect within the brain with the basal ganglia, the brainstem, and the cerebellum. Generally, these pyramidal motor nerves constitute a direct pathway from the primary motor area to the muscles and are concerned mostly with control of discrete, detailed body movements.
The premotor area of each hemisphere is located in the cortex immediately in front of the primary motor cortex in the frontal lobe. On the whole, this area is concerned with movement of the opposite side of the body, especially with control and coordination of skilled movements of a complex nature, such as throwing or kicking a ball. In addition to its subcortical connections with the primary motor area, its neurons also have direct connections with the basal ganglia and related nuclei in the brainstem, for example, the reticular formation. Many of the axons from these subcortical centers cross to the opposite side before descending as extrapyramidal tracts in the spinal cord. Collectively, the connections from the premotor area to these related nuclei compose the extrapyramidal system, which coordinates gross skeletal muscle activities that are largely automatic in nature. Examples are postural adjustments, chewing, swallowing, gesticulating during speech, and associated movements that accompany voluntary activities. Certain portions of the extrapyramidal tract also have an inhibitory effect on spontaneous movements initiated by the cerebral cortex and serve to prevent tremors and rigidity. Complete structural and functional separation of the pyramidal and extrapyramidal systems is impossible because they are so closely connected in the harmonious work of executing complex coordinated movements.
Of interest to the PACU nurse is that drugs used during the perioperative period can cause extrapyramidal reactions. More specifically, the neuroleptics, such as the phenothiazines (of which chlorpromazine is the prototypal drug), the butyrophenones, as typified by droperidol (Inapsine) and haloperidol (Haldol), and the antiemetic metoclopramide (Reglan) are known to produce extrapyramidal reactions. The following four types of extrapyramidal reaction exist: drug-induced parkinsonism, akathisia, acute dystonic reactions, and tardive dyskinesia.
Drug-induced parkinsonism, which can occur 1 to 5 days after the administration of the neuroleptic drug, is typified by a generalized slowing of automatic and spontaneous movements (bradykinesia), with a masklike facial expression and a reduction in arm movements. The most noticeable signs of the drug-induced parkinsonism syndrome are rigidity and oscillatory tremor at rest. The treatment is an antiparkinsonian agent, such as levodopa, trihexyphenidyl, and benztropine.
Akathisia, which can occur 5 to 60 days after the administration of a neuroleptic drug, refers to a subjective feeling of restlessness accompanied by a need on the part of the patient to move about and pace back and forth, acute anxiety, and the feeling impending of doom. Treatment requires a reduction in the dosage of the responsible drug and the administration of a benzodiazepine if encountered during the perioperative period.
Acute dystonic reactions may occur after the administration of some psychotropic drugs and are characterized by torsion spasms, such as facial grimacing and torticollis. These reactions are occasionally seen when a phenothiazine is first administered, and they are associated with oculogyric crises. Acute dystonic reactions may be mistaken for hysterical reactions or seizures and can usually be reversed with anticholinergic antiparkinsonian drugs, such as benztropine or trihexyphenidyl.
Tardive dyskinesia is a late-appearing neurologic syndrome that is characterized by stereotypic, involuntary, rapid, and rhythmically repetitive movements, such as continual chewing movements and darting movements of the tongue. Treatment is not always satisfactory because antiparkinsonian drugs sometimes exacerbate tardive dyskinesia. Tardive dyskinesia often persists despite discontinuation of the responsible drug.2,3
Two important structural aspects of the premotor area are worth noting for those who care for neurosurgical patients. First, the fibers from both the primary motor and the premotor areas are funneled through the narrow internal capsule as they descend to lower areas of the CNS. This action is significant because the internal capsule is a common site of cerebrovascular accidents that can result in a variety of motor deficits. Second, lesions within one side of the internal capsule result in paralysis of the skeletal muscles on the opposite side of the body because of the crossing of fibers within the medulla.4
This area is only one point in the complicated network needed to form spoken and written words. The motor speech area lies at the base of the motor area and slightly in front of it in the inferior frontal gyrus and is also known as the Broca area. In right-handed people (most of the population), the language and speech areas are usually located in the left hemisphere. In those who are left-handed, these areas may lie within the right or the left hemisphere.
This area of the frontal lobe lies anterior to the premotor area, has extensive connections with other cortical areas, and is believed to have an important role in complex intellectual activities, such as mathematic and philosophic reasoning; abstract and creative thinking; learning; judgment and volition; and social, moral, and ethical values. The prefrontal area also influences certain autonomic functions of the body with the conduction of impulses directly or indirectly through the thalamus to the hypothalamus, which makes possible certain physiologic responses to feelings such as anger, fear, and lust.
Sensory information from one side of the body is received by the somatic sensory area of the opposite hemisphere, which is located in the parietal lobe in the area of the postcentral gyrus. Crude sensations of pain, temperature, and touch can be experienced at the level of the thalamus, but true localization and discrimination of these sensations is a function of the parietal cortex. The activities of the somatic sensory area allow for proprioception, for the recognition of the size, shape, and texture of objects, and for the comparison of stimuli as to intensity and location.
The auditory area lies in the cortex of the superior temporal lobe. Each hemisphere receives impulses from both ears. The visual area is located in the posterior occipital lobe, where extremely complex transformations in the signals conveyed by the optic nerve occur. The right occipital cortex receives impulses from the right half of each eye, and the left occipital cortex receives impulses from the left half of each eye. The olfactory area is believed to be located in the medial temporal lobe, and the gustatory area is located nearby at the base of the postcentral gyrus.
Large areas of the cortex remain for which no discrete function is known. These areas are called association areas. They play a major role in the integration of the sensory and motor phases of cortical function by providing complex connections between them.
The principal structural and functional units of the limbic system are the two rings of limbic cortex and a number of related subcortical nuclei, the anterior thalamic nuclei, and portions of the basal nuclei (Fig. 10-2). In general, the limbic system is concerned with a wide variety of autonomic somatosensory and somatomotor responses, especially those involved with emotional states and other behavioral responses. Within the limbic system, the benzodiazepine and opiate receptors have been identified (see Chapters 19, 21, and 22).
(From Standring S: Gray’s anatomy: the anatomical basis of clinical practice, ed 40, London, 2009, Churchill Livingstone.)
The limbic system, which acts in close concert with the hypothalamus, can evoke a variety of autonomic responses, including changes in heart rate, blood pressure, and respiratory rate. This system plays an intimate role in the creation of emotional states, particularly anxiety, fear, and aggression. Stimulation of the limbic system also evokes complex motor responses directly related to feeding behavior. The limbic system has been shown to have major relationships with the reticular formation of the brainstem and is presumed to have a role in the alerting or arousal process. The system is also implicated in the hypothalamic regulation of pituitary activity and may be associated somehow with the memory process for recent events as well. In addition, it is intimately concerned with complex phenomena, such as the control of various biologic rhythms, sexual behavior, and motivation.
A cerebral nucleus is a group of neuron cell bodies within the CNS. Five of these deep-lying masses of gray matter are located within the white matter of each hemisphere and are collectively known as the basal ganglia. These masses are the caudate nucleus, the putamen, the globus pallidus, the substantia nigra, and the subthalamic nucleus. Together, they exert a steadying influence on muscle activity. The basal ganglia are an important part of the extrapyramidal motor pathway that connects nuclei with each other, with the cortex, and with the spinal cord. The ganglia also connect with areas in the hindbrain (the red nucleus and the substantia nigra) to assist in the role of smoothing and coordinating muscle movements. Disturbances in these ganglia result in tremor, rigidity, and loss of expressive and walking movements, as seen in Parkinson syndrome.5
The second major division of the forebrain is the diencephalon (Fig. 10-3), which consists of the thalamus and the hypothalamus. The diencephalon also contains the third ventricle and is almost completely covered by the cerebral hemispheres. This portion of the brain has a primary role in sleep, emotion, thermoregulation, autonomic activity, and endocrine control of ongoing behavioral patterns.
(From Fitzgerald MJT, et al: Clinical neuroanatomy and neuroscience, ed 6, St. Louis, 2011, Saunders.)
The thalamus consists of right and left egg-shaped masses, which compose the greatest bulk of the diencephalon and form the lateral wall of the third ventricle. Each thalamus serves as a relay center for all incoming sensory stimuli, except for taste. These impulses are then grouped and transmitted to the appropriate area of the cerebral cortex. Because of its interconnections with the hypothalamus, the limbic system, and the frontal, temporal, and parietal lobes, this structure is also integrally involved with emotional activities, instinctive responses, and attentive processes.
The hypothalamus is a group of bilateral nuclei that forms the floor and part of the lateral walls of the third ventricle. Extremely complex in function, the hypothalamus has extensive connections with the autonomic nervous system and with other parts of the CNS. It also influences the endocrine system by virtue of direct and indirect connections with the pituitary gland and the release of its own hormones. In association with these other structures, the hypothalamus participates in the regulation of appetite, water balance, carbohydrate and fat metabolism, growth, sexual maturity, body temperature, pulse rate, blood pressure, sleep, and aspects of emotional behavior. Because of the connection of the hypothalamus with the thalamus and cerebral cortex, emotions can influence visceral responses on certain occasions.6
The midbrain, or mesencephalon, is a short narrow segment of nervous tissue that connects the forebrain with the hindbrain. The midbrain is vital as a conduction pathway and as a reflex control center. Passing through the center of the midbrain is the cerebral aqueduct, a narrow canal that serves to connect the third ventricle of the diencephalon with the fourth ventricle of the hindbrain for the circulation of CSF. In addition, cranial nerves III (oculomotor) and IV (trochlear) originate in the ventral aspect of the midbrain.7
FIG. 10-4 Ventral view of the brainstem in situ.
(From Fitzgerald MJT, et al: Clinical neuroanatomy and neuroscience, ed 6, St. Louis, 2011, Saunders.)
The pons is literally the bridge between the midbrain and the medulla oblongata as it lies in front of the fourth ventricle and separates it from the cerebellum. It receives many ascending and descending fibers en route to other points in the CNS. The pons also contains the motor and sensory nuclei of cranial nerves V (trigeminal), VI (abducens), VII (facial), and VIII (acoustic). The roof of the pons contains a portion of the reticular formation, and the lower pons assists in the regulation of respiration.
The medulla oblongata is an expanded continuation of the spinal cord and is located between the base of the skull, the foramen magnum, and the pons. It is anatomically complex and not usually amenable to surgery. Many of the white fiber tracts between the brain and spinal cord decussate as they pass through the medulla. Centers for many complex reflexes are located in the medulla oblongata and include those for swallowing, vomiting, coughing, and sneezing. The originating nuclei of cranial nerves IX (glossopharyngeal), X (vagus), XI (accessory), and XII (hypoglossal) are found in the medulla oblongata (Table 10-1). Because of these originating nuclei, the medulla has an essential role in the regulation of cardiac, respiratory, and vasomotor reflexes. Injuries to the medulla, such as those that accompany basal skull fracture, often prove fatal.