Peripheral Nervous System

The peripheral nervous system (PNS) comprises the cranial and spinal nerves (Fig. 1), with their associated roots and ganglia. Through these nerves, sensory impulses come to the CNS and motor impulses go to muscles and glands. Like the cranial structures they

Nervous System Peripheral Neuropathy
Figure 1 Basic subdivisions of adult nervous system: central nervous system (CNS) and peripheral nervous system (PNS). From Han, A. W., Histology, 5th ed., J. B. Lippincott, 1961 (illustration by Louise Miller).

innervate, most of the 12 cranial nerves are specialized. Some, like the optic nerve, are purely sensory, whereas others, like the abducens, are purely motor. Still others, like the vagus, are mixed sensory and motor (Fig. 5, Table II).

By contrast, the spinal nerves are much alike, providing a basic segmental pattern of sensory and motor innervation for the rest of the body, including the limbs. In the cervical, brachial, lumbar, and sacral regions, nerve plexuses interpose between the spinal cord and the nerves themselves. In these complex webs, nerve fibers leaving the spinal cord segmentally sort out and recombine into specific nerves. This involves a process of diverging and converging roots, trunks, divisions, cords, and branches, eventually combining in nerves, such as the phrenic, radial, femoral, and sciatic, to cite prominent nerves derived from each plexus in turn. These and other nerves continue to ramify.

C. Autonomic Nervous System

The autonomic nervous system (ANS) is an involuntary division for maintaining homeostasis (Fig. 6). It sends motor fibers to the viscera, blood vessels, sweat glands, arrector pili, pupillary smooth muscles, etc. and regulates heart rate. It features two small visceral motor neurons in tandem: a preganglionic neuron with its cell body in the CNS and a postganglionic neuron in an autonomic ganglion. Both have meager dendritic trees and thin, lightly myelinated, slow-conducting axons that travel in peripheral or autonomic nerves to smooth muscle in a viscus, blood vessel, or gland.

The ANS has three subdivisions: sympathetic (thoracolumbar), parasympathetic (craniosacral), and enteric. In the first, preganglionic nerve cell bodies lie in thoracic and upper lumbar segments of the spinal cord and postganglionic ones in the paravertebral sympathetic chain ganglia or the prevertebral ganglia (celiac, superior mesenteric, and inferior mesenteric). In the second subdivision, preganglionic nerve cell bodies lie in the brain stem (in cranial nerve nuclei III, VII, IX, and X; see Table II) and in sacral spinal cord segments 2-4, with postganglionic ones in or near the organs innervated. In the enteric component, postganglionic neurons lie in the alimentary wall in ganglia and intramural plexuses. These nets contain 100 million neurons. They function almost autonomously, subject to control and override by preganglionic parasympathetic and sympathetic neurons.

The CNS receives visceral sensory axons not included in the ANS as originally defined. Visceral afferents, from mechanoreceptors, chemoreceptors, and nociceptors, are poorly understood but important to homeostasis and behavior. The sympathetic part of the ANS forms a distinct pair of cords in the PNS: the sympathetic chain ganglia alongside the spinal cord. The parasympathetic part, by contrast, is less obvious as its fibers are components of cranial and sacral nerves.

The sympathetic subsystem is the ''fight or flight'' component. When energy must be burned, it acts

Figure 2 Basic subdivisions of embryonic nervous system: at 28 days, prosencephalon (1), mesencephalon (2), rhombencephalon (3). At 42 and 49 days, telencephalon (1a), diencephalon (1b), mesencephalon (2), metencephalon (3a), myelencephalon (3b). From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Steven J. Harrison).

Figure 2 Basic subdivisions of embryonic nervous system: at 28 days, prosencephalon (1), mesencephalon (2), rhombencephalon (3). At 42 and 49 days, telencephalon (1a), diencephalon (1b), mesencephalon (2), metencephalon (3a), myelencephalon (3b). From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Steven J. Harrison).

almost instantly in a concerted manner and over some duration. Heart rate goes up, blood vessels in skeletal muscles dilate, as do respiratory bronchioles and pupils, suprarenal glands kick in massive secretions of epinephrine and norepinephrine, hepatic glycogen-olysis provides glucose for energy, hairs stand on end, and the mouth is suddenly very dry. The parasympathetic subsystem functions less dramatically. It is the

Table I

The Main Subdivisions of the Embryonic CNS and Their Adult Fates

Three-vesicle stage

Five-vesicle stage

Adult derivatives l. Prosencephalon (forebrain)

2. Mesencephalon (midbrain)

la. Telencephalon (endbrain) lb. Diencephalon (tweenbrain) 2. Mesencephalon (midbrain)

S. Rhombencephalon (hindbrain) Sa. Metencephalon (afterbrain)

3b. Myelencephalon (cordbrain)

4. Remaining caudal part of neural tube

4. Remaining caudal part of neural tube

Cerebral hemispheres, lateral ventricles, basal ganglia, corpus callosum

Thalamus, hypothalamus, third ventricle, optic nerves and tracts, retinae, pineal gland

Superior and inferior colliculi, cerebral aqueduct, cerebral peduncles, midbrain tegmentum

Cerebellum, rostral part of fourth ventricle, pons, pontine tegmentum

Medulla oblongata, caudal part of fourth ventricle, medullary tegmentum

Spinal cord (myel, from Greek, ''marrow'') and central canal

Lateral ventricle

Superior Inferior Posterior horn horn horn

Superior Inferior Posterior horn horn horn

Figure 3 Brain ventricular system: the cerebral hemispheres, diencephalon, midbrain, pons, and medulla oblongata have central cavities, interconnected as a four-chambered ventricular system with a connecting aqueduct. Three apertures in the fourth ventricle allow cerebrospinal fluid (CSF) to exit the system into the subarachnoid space. The lumen of the embryonic spinal cord is no longer patent. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Maureen Killackey).

Figure 3 Brain ventricular system: the cerebral hemispheres, diencephalon, midbrain, pons, and medulla oblongata have central cavities, interconnected as a four-chambered ventricular system with a connecting aqueduct. Three apertures in the fourth ventricle allow cerebrospinal fluid (CSF) to exit the system into the subarachnoid space. The lumen of the embryonic spinal cord is no longer patent. From Principles of Neuroanatomy by J. B. Angevine, Jr., and C. W. Cotman, copyright 1981 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc. (illustration by Maureen Killackey).

Embryonic Brain Formation

Figure 4 Embryonic and fetal brain development: flexures and differential expansion of its chambers, along with cerebral fissuration, bring about the definitive appearance of the brain. From J. B. Angevine, Jr., Morphogenesis of the Central Nervous System, BNI Quarterly, Vol. 5, No. 4, 1989 (illustration by Janice H. Angevine, modified from Carol Donner).

Figure 4 Embryonic and fetal brain development: flexures and differential expansion of its chambers, along with cerebral fissuration, bring about the definitive appearance of the brain. From J. B. Angevine, Jr., Morphogenesis of the Central Nervous System, BNI Quarterly, Vol. 5, No. 4, 1989 (illustration by Janice H. Angevine, modified from Carol Donner).

''rest and replenishment'' component. When activity brings fatigue and energy must be restored, it goes about its business slowly, quietly, selectively. Heart rate goes down, blood vessels stay the same, bronchioles and pupils constrict to normal size, and digestive, urinary, and reproductive systems proceed with normal function.

Usually, sympathetic and parasympathetic divisions act separately and at times cooperatively. Sweat glands and blood vessels in the limbs have only sympathetic innervation; the pupil and bladder are dominated by parasympathetic fibers. Certain disparate domains see both divisions working together: in never-ending modulation (with neurohormonal assistance) of cardiac rhythmicity and intermittent assistance in male sexual function (erection is mediated mainly by parasympathetic fibers but ejaculation by sympathetic fibers). Visceral and somatic functions may also be performed cooperatively. This requires the interplay of many parts of the CNS: spinal cord, brain stem, limbic system, hypothalamus, basal ganglia, and others. Performances of respiratory, digestive, and sexual functions offer eloquent illustrations of concerted viscerosomatic activity.

Visceral sensations may intrude deeply upon thoughts and feelings. Often difficult to describe and localize, they are usually unpleasant and at times uncompromising. As biofeedback studies show, the ANS can be classically conditioned. Once activated, it plays a pivotal role in human decision making. It now receives major attention in studies of normal, abnormal, and criminal behavior, with benefit to informed family counseling, crisis mediation, law enforcement, corrections, and society at large.

Figure 5 Base of forebrain and inferior view of brain stem showing cranial nerves and major arteries; olfactory bulbs and tracts are visible anterior to optic nerves. From J. Nolteand J. B. Angevine, Jr., The Human Brain. In Photography and Diagrams, 2nded., Mosby, St. Louis, 2000 (photograph by Biomedical Communications, The University of Arizona College of Medicine).

Figure 5 Base of forebrain and inferior view of brain stem showing cranial nerves and major arteries; olfactory bulbs and tracts are visible anterior to optic nerves. From J. Nolteand J. B. Angevine, Jr., The Human Brain. In Photography and Diagrams, 2nded., Mosby, St. Louis, 2000 (photograph by Biomedical Communications, The University of Arizona College of Medicine).

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