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THE NERVOUS SYSTEM: AUTONOMIC DIVISIONS

INTRODUCTION

1. The autonomic nervous system carries the visceral motor output of the CNS.

2. ANS centers within the brain stem regulate visceral activities by monitoring and adjusting visceral reflexes.

3. The most important ANS centers mediate multisystem responses to stimuli. These nuclei are located in the hypothalamus.

4. The output of the autonomic nervous system can be influenced by higher centers.

5. The central motor neuron communicates with ganglionic neurons that may be close to the spinal cord or within the tissues of peripheral organs.

6. Visceral efferents from the thoracic and lumbar segments form the thoracolumbar, or sympathetic, division of the ANS.

7. The visceral efferents leaving the brain and sacral segments of the spinal cord form the craniosacral, or parasympathetic, division of the ANS.

GROSS ANATOMY AND ORGANIZATION

The Sympathetic Division

1. The sympathetic output emerges between the first thoracic and second lumbar vertebrae.

2. The preganglionic fibers may synapse within the nearest sympathetic ganglion, ascend or descend to reach another ganglion along the sympathetic chain, or continue to a collateral ganglion.

3. Considerable divergence exists, with the ratio of preganglionic to postganglionic neurons approximating 1:32.

4. Sympathetic ganglia contribute agray ramus to every spinal nerve and to regions innervated by cranial nerves.

5. The viscera are innervated by the sympathetic neurons of collateral ganglia. The preganglionic fibers form the splanchnic nerves that communicate with the celiac, superior mesenteric, and inferior mesenteric ganglia.

6. Splanchnic nerves also innervate the adrenal gland, reaching the modified neurons of the adrenal medulla. The postganglionic neurons secrete their neurotransmitters into the bloodstream.

7. The sympathetic division has a segmentally arranged chain of sympathetic ganglia, regionally organized collateral ganglia, short preganglionic and long postganglionic fibers, and shows considerable divergence when activated.

The Parasympathetic Division

1. The parasympathetic division has intramural ganglia within the innervated organs, long preganglionic and short postganglionic fibers, and produces localized effects upon stimulation.

2. Cranial nerves III, VII, IX, and X carry parasympathetic outflow to peripheral ganglia.The vagus nerve contains roughly three-quarters of the parasympathetic efferents and reaches organs as distant as the initial segments of the large intestine.

3. The pelvic nerves innervate the terminal segments of the large intestine, the kidney, bladder, and sex organs.

ANATOMICAL AND FUNCTIONAL RELATIONSHIPS

1. The sympathetic division reaches all regions serviced by spinal and cranial nerves. Sympathetic activation has widespread effects.

2. The parasympathetic system reaches visceral organs and structures innervated by the cranial nerves. Its effects are more localized.

3. Several organs show dual innervation, and the two divisions often have opposing effects.

4. In the body cavities the parasympathetic and sympathetic nerves intermingle to form a series of characteristic nerve plexuses. These include the cardiac, pulmonary, celiac, and hypogastric plexuses.

PHYSIOLOGY AND FUNCTION

1. All preganglionic fibers release ACh at their synaptic terminals. The sympathetic and parasympathetic divisions differ in the characteristic neurotransmitters released at the neuroeffector junctions.

2. Postganglionic parasympathetic fibers are also cholinergic, releasing ACh.The effects are determined by the nature and properties of the receptor molecules on the effector surface.

3. Postganglionic sympathetic fibers are called adrenergic, and they release norepinephrine.

The Sympathetic Division

1. Sympathetic postganglionic fibers end in enlarged terminal knobs.

2. Following stimulation and norepinephrine release, the transmitter remains active for several seconds. It may then be reabsorbed by the terminal knob, diffuse out of the area, or be inactivated by special enzymes.

3. The adrenal medulla secretes 75 percent epinephrine and 25 percent norepinephrine. The peripheral effects last much longer than do those produced by stimulation of the terminal knobs.

4. Two membrane receptors have been identified. Alpha receptors respond to norepinephrine or epinephrine by depolarizing the membrane. They are common on the surfaces of smooth muscles, and stimulation leads to muscular contraction.

5. Beta receptors are particularly sensitive to epinephrine. They are found on the surface of cardiac muscle cells, liver cells, and skeletal muscle fibers.

6. Special sympathetic fibers distributed through the spinal nerves release ACh. They trigger sweat gland secretion and vascular dilation at skeletal muscles.

The Parasympathetic Division

1. All of the preganglionic and postganglionic fibers of the parasympathetic division release ACh.

2. The effects are short-lived because acetylcholinesterase in the synapse and tissue cholinesterases in the surrounding extracellular fluids rapidly destroy the ACh molecules.

3. Two different receptors have been identified. Muscarinic receptors are found on the surfaces of the peripheral effectors. Exposure to ACh may produce excitation or inhibition, depending on the effector considered.

4.Nicotinic receptors are found on the surfaces of ganglionic neurons. Exposure to ACh always leads to depolarization and neural excitation.

5. Sympathetic/parasympathetic opposition can be seen along the digestive tract, at the heart, in the lungs, and in most places showing dual innervation. In some cases the two divisions control separate aspects of a complex secretory or sexual function.

6. The parasympathetic system can be considered as a "rest and repose" command center. The sympathetic system initiates "fight or flight" responses.

CONTROL AND INTEGRATION

1. Visceral reflexes form the basis for the autonomic nervous system.

2. The autonomic motor neurons are always active, establishing the individual's autonomic tone .

3. Biofeedback techniques can be used to treat psychosomatic disorders resulting from abnormal CNS/autonomic function.

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