Peripheral Nervous System.docx

PERIPHERAL NERVOUS SYSTEM The peripheral nervous system (PNS) consists of:\ a. Nerves b\. Scattered ganglia (groups of neuronal cell bodies found outside the CNS). STRUCTURE OF THE NERVE As noted earlier in this chapter, a nerve is a bundle of neuron fibers found outside the CNS. Within a nerve,...

PERIPHERAL NERVOUS SYSTEM The peripheral nervous system (PNS) consists of:\ a. Nerves b\. Scattered ganglia (groups of neuronal cell bodies found outside the CNS). STRUCTURE OF THE NERVE As noted earlier in this chapter, a nerve is a bundle of neuron fibers found outside the CNS. Within a nerve, neuron fibers, or processes, are wrapped in protective connective tissue coverings. Each fiber is surrounded by a delicate connective tissue sheath, an endoneurium (en″do-nu′re-um). Groups of fibers are bound by a coarser connective tissue wrapping, the perineurium (per″˘ınu′re-um), to form fiber bundles, or fascicles. Finally, all the fascicles are bound together by a tough fibrous sheath, the epineurium, to form the cordlike nerve. Like neurons, nerves are classified according to the direction in which they transmit impulses. Nerves that carry impulses only toward the CNS are called sensory (afferent) nerves, whereas those that carry only motor fibers are motor (efferent) nerves. Nerves carrying both sensory and motor fibers are called mixed nerves; all spinal nerves are mixed nerves. CRANIAL NERVES The 12 pairs of cranial nerves primarily serve the head and neck. Only one pair (the vagus nerves) extends to the thoracic and abdominal cavities. The cranial nerves are numbered in sequence, and in most cases their names reveal the most important structures they control. Most cranial nerves are mixed nerves; however, three pairs---the optic, olfactory, and vestibulocochlear---are purely sensory in function. (The older name for the vestibulocochlear nerve is acoustic nerve, a name that reveals its role in hearing but not in equilibrium.) This study tool may help you remember the cranial nerves in order. The first letter of each word in the saying is the first letter of the cranial nerve to be remembered: "Oh, oh, oh, to touch and feel very good velvet at home." SPINAL NERVES AND NERVE PERPLUXES The 31 pairs of human spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord. Although each of the cranial nerves issuing from the brain is named specifically, the spinal nerves are named for the region of the cord from which they arise. Almost immediately after being formed, each spinal nerve divides into the dorsal ramus and ventral ramus (plural rami), making each spinal nerve only about ½ inch long. The rami, like the spinal nerves, contain both motor and sensory fibers. Thus, damage to a spinal nerve or either of its rami results both in loss of sensation and in flaccid paralysis of the area of the body served. The smaller dorsal rami serve the skin and muscles of the posterior body trunk. The ventral rami of spinal nerves T1 through T12 form the intercostal nerves, which supply the muscles between the ribs and the skin and muscles of the anterior and lateral trunk. The ventral rami of all other spinal nerves form complex networks of nerves called plexuses, which serve the motor and sensory needs of the limbs. DIVISIONS OF PERIPHERAL NERVOUS SYSTEM The autonomic nervous system (ANS) or Motor Subdivision of the PNS -- controls body activities automatically. It is composed of a specialized group of neurons that regulate cardiac muscle (the heart), smooth muscles (found in the walls of the visceral organs and blood vessels), and glands. Although all body systems contribute to homeostasis, the relative stability of our internal environment depends largely on the workings of the ANS. At every moment, signals flood from the visceral organs into the CNS, and the autonomic nervous system makes adjustments as necessary to best support body activities. For example, blood flow may be shunted to more "needy" areas, heart and breathing rate may be sped up or slowed down, blood pressure may be adjusted, and stomach secretions may be increased or decreased. Most of this fine-tuning occurs without our awareness or attention---few of us realize when our pupils dilate or our arteries constrict--- hence the ANS is also called the involuntary nervous system, as noted at the beginning of this chapter. IMPORTANT DIFFERENCES BETWEEN THE SOMATIC AND AUTONOMIC SUBDIVISIONS OF THE PNS 1. 2. - - TWO DIVISIONS OF THE AUTONOMIC NERVOUS SYSTEM 1. 2. Both serve the same organs but cause essentially opposite effects, counterbalancing each other's activities to keep body systems running smoothly. ANATOMY OF PARASYMPATHETIC DIVISION The preganglionic neurons of the parasympathetic division are located in brain nuclei of several cranial nerves---III, VII, IX, and X (the vagus being the most important of these) and in the S2 through S4 levels of the spinal cord. For this reason, the parasympathetic division is also called the craniosacral division. The neurons of the cranial region send their axons out in cranial nerves to serve the head and neck organs. There they synapse with the ganglionic motor neuron in a terminal ganglion. From the terminal ganglion, the postganglionic axon extends a short distance to the organ it serves. In the sacral region, the preganglionic axons leave the spinal cord and form the pelvic splanchnic nerves, also called the pelvic nerves, which travel to the pelvic cavity. In the pelvic cavity, the preganglionic axons synapse with the second motor neurons in terminal ganglia on, or close to, the organs they serve. ANATOMY OF SYMPATHETIC DIVISION The sympathetic division is also called the thoracolumbar division because its preganglionic neurons are in the gray matter of the spinal cord from T1 through L2. The preganglionic axons leave the cord in the ventral root, enter the spinal nerve, and then pass through a ramus communicans, or small communicating branch, to enter a sympathetic trunk ganglion. The sympathetic trunk, or sympathetic chain lies alongside the vertebral column on each side. After it reaches the ganglion, the axon may synapse with the second (ganglionic) neuron in the sympathetic chain at the same level or a different level, and the postganglionic axon then reenters the spinal nerve to travel to the skin. Or, the preganglionic axon may pass through the ganglion without synapsing and form part of the splanchnic nerves. The splanchnic nerves travel to the viscera to synapse with the ganglionic neuron, found in a collateral ganglion anterior to the vertebral column. The major collateral ganglia---the celiac and the superior and inferior mesenteric ganglia---supply the abdominal and pelvic organs. The postganglionic axon then leaves the collateral ganglion and travels to serve a nearby visceral organ. FUNCTIONS OF AUTONOMIC NERVOUS SYSTEM - Body organs served by the autonomic nervous system receive fibers from both divisions. Exceptions are most blood vessels and most structures of the skin, some glands, and the adrenal medulla, all of which receive only sympathetic fibers. When both divisions serve the same organ, they cause antagonistic effects, mainly because their postganglionic axons release different neurotransmitters. The parasympathetic fibers, called cholinergic fibers, release acetylcholine. The sympathetic postganglionic fibers, called adrenergic fibers, release norepinephrine. The preganglionic axons of both divisions release acetylcholine. To emphasize the relative roles of the two arms of the ANS, we will focus briefly on situations in which each division is "in control." - -