AUTONOMIC NERVOUS SYSTEM (SYMPATHETIC PART) LEARNING OBJECTIVES • • • • • • •
Define Autonomic nervous system. Define sympathetic part of ANS. Compare somatic and autonomic systems. Recognize the components of sympathetic part of nervous system (thoracolumbar outflow: lateral gray horn, paravertebral sympathetic chain, prevertebral ganglia and plexuses). Differentiate between white and gray rami communicans. Describe the different fates (destination) of white and gray rami (preganglionic and post ganglionic fibres). List the functions.
AUTONOMIC NERVOUS SYSTEM •
The portion of the nervous system that controls most of visceral functions of the body and is called the autonomic nervous system. • Helps to control arterial pressure, gastrointestinal motility, gastrointestinal secretions, urinary bladder emptying, sweating, body temperature, and many other activities • Some of the above functions are controlled almost entirely and some only partially by the autonomic nervous system.
SYMPATHETIC NERVOUS SYSTEM • Sympathetic part of autonomic nervous system prepares the body for emergency. • Sympathetic system consists of efferent outflow from spinal cord, to ganglionated sympathetic trunks, important branches, plexus and regional ganglia
COMPARISON OF AUTONOMIC AND SOMATIC MOTOR SYSTEMS •
Somatic motor system
-One motor neuron extends from the CNS to skeletal muscle
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-Axons are well myelinated, conduct impulses rapidly Autonomic nervous system – Chain of two motor neurons • Preganglionic neuron • Postganglionic neuron
– Conduction is slower due to thinly or unmyelinated axons
COMPARISON OF SYMPATHETIC AND PARASYMPATHETIC DIVISIONS
ANATOMICAL DIFFERENCES IN SYMPATHETIC AND PARASYMPATHETIC DIVISIONS
• Arise from different regions of the CNS – Sympathetic also called the thoracolumbar division
– Parasympathetic also called the craniosacral division
ANATOMICAL DIFFERENCES IN SYMPATHETIC AND PARASYMPATHETIC DIVISIONS •
Length of postganglionic fibers – Sympathetic – long postganglionic fibers – Parasympathetic – short postganglionic fibers.
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Branching of axons – Sympathetic axons – highly branched • Influences many organs – Parasympathetic axons – few branches • Localized effect.
NEUROTRANSMITTERS OF AUTONOMIC NERVOUS SYSTEM •
Neurotransmitter released by preganglionic axons: – Acetylcholine for both branches (cholinergic)
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Neurotransmitter released by postganglionic axons: – Sympathetic – most release NOREPINEPHRINE (adrenergic). – Parasympathetic – release ACETYLCHOLINE
GENERAL ORGANIZATION OF THE PERIPHERAL PORTIONS OF SYMPATHETIC NERVOUS SYSTEM Shown specifically in the figure are (1) One of the two paravertebral sympathetic chains of ganglia that are interconnected with the spinal nerves on the side of the vertebral column, (2) Prevertebral ganglia (the celiac and hypogastric), and (3) Nerves extending from the ganglia to the different internal organs.
ORGANIZATION AND ANATOMY OF THE SYMPATHETIC DIVISION • Much more complex than the parasympathetic division, both anatomically and functionally. • Sympathetic preganglionic neuron cell bodies are housed in the lateral horn of the T1–L2 regions of the spinal cord. • Preganglionic sympathetic axons travel with somatic motor neuron axons to exit the spinal cord and enter first the anterior roots and then the T1–L2 spinal nerves. • Preganglionic sympathetic axons remain with the spinal nerve for merely a short distance before they branch off and leave the spinal nerve.
LEFT AND RIGHTSYMPATHETIC TRUNKS OR PARAVERTEBRAL GANGLIA • • •
Immediately anterior to the paired spinal nerves are the left and right sympathetic trunks. Each is located immediately lateral to the vertebral column. sympathetic trunk (or paravertebral) ganglia, house sympathetic ganglionic neuron cell bodies.
LEFT AND RIGHT SYMPATHETIC TRUNKS OR PARAVERTEBRAL GANGLIA • Joined to ventral rami by white and gray rami communicantes • One sympathetic trunk ganglion is approximately associated with each spinal nerve. • Fusion of ganglia fewer ganglia than spinal nerves • The cervical portion of each sympathetic trunk is partitioned into only three sympathetic trunk ganglia —the superior, middle, and inferior cervical ganglia—as opposed to the eight cervical spinal nerves.
WHITE RAMI • • • • •
Connecting the spinal nerves to each sympathetic trunk are rami communicantes. Carry preganglionic sympathetic axons from the T1–L2 spinal nerves to the sympathetic trunk. Associated only with the T1–L2 spinal nerves. Preganglionic axons are myelinated. The white ramus has a whitish appearance.
GRAY RAMI • • • •
Carry postganglionic sympathetic axons from the sympathetic trunk to the spinal nerve. Axons are unmyelinated gray rami have a grayish appearance. Connect to all spinal nerves, including the cervical, sacral, and coccygeal spinal nerves. Sympathetic information that started out in the thoracolumbar region can be dispersed to all parts of the body.
SYMPATHETIC DIVISION OF THE ANS
SPLANCHNIC NERVES SPLANCHNIC NERVES • Composed of preganglionic sympathetic axons. • Run anteriorly from the sympathetic trunk to most of the viscera. • Larger splanchnic nerves have specific names: 1. Greater thoracic splanchnic nerves 2. Lesser thoracic splanchnic nerves 3. Least thoracic splanchnic nerves 4. Lumbar splanchnic nerves 5. Sacral splanchnic nerves
PREVERTEBRAL (OR COLLATERAL) GANGLIA
• • • • • • •
Splanchnic nerves terminate in prevertebral (or collateral) ganglia. Called ―prevertebral‖ because they are immediately anterior to the vertebral column Unpaired, not segmentally arranged Occur only in abdomen and pelvis Typically cluster around the major abdominal arteries and are named for these arteries. Example: Celiac, superior mesenteric, inferior mesenteric, inferior hypogastric ganglia Sympathetic postganglionic axons extend away from the ganglionic neuron cell bodies in these ganglia and innervate many of the abdominal organs.
SUMMARY ; SYMPATHETIC SYSTEM
SUMMARY (continued) •
Output from preganglionic neurons in lateral horn of thoracic and upper lumbar spinal cord (thoracolumbar outflow). • Short preganglionic axons travel to sympathetic chain ganglia via white ramus communicantes. Postsynaptic neuron exits via gray ramus communicantes. • Long postganglionic axons travel to their targets via the gray ramus communicantes. • Some preganglionic axons travel to prevertebral ganglia via the splanchnic nerves--are not paired a) Celiac ganglion b) Superior mesenteric ganglion c) Inferior mesenteric ganglion d) Inferior hypogastric ganglion
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Adrenal medulla Acts like a sympathetic postganglionic neuron, but secretes epinephrine as opposed to norepinephrine. • Effects increase alertness, increased energy, cardiovascular & respiratory activity, elevation of muscle tone, mobilization of energy reserves
FUNCTIONS Sympathetic part of autonomic nervous system prepares the body for emergency. Heart rate is increased. Arterioles of skin and intestine are constricted but the arterioles of skeletal muscles are dilated. Blood pressure is raised. Pupils are dilated. It inhibits the smooth muscles of bronchi, intestine and bladder and closes the sphincters. Hair is made to stand and sweating occurs.
REFERENCES •
Clinical Neuroanatomy by Richard S. Snell
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