• Document: Chapter 15. Sympathetic Nervous System
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Chapter 15 Sympathetic Nervous System Somatic versus Autonomic Pathways Somatic efferent innervation ACh Myelinated fiber Somatic effectors (skeletal muscles) Autonomic efferent innervation ACh ACh or NE Myelinated Unmyelinated preganglionic fiber postganglionic fiber Visceral effectors (cardiac muscle, smooth muscle, glands) Autonomic ganglion ANS – two neurons from CNS to effectors • preganglionic neuron whose cell body is in CNS (myelinated) • postganglionic neuron cell body in peripheral ganglion (unmyelinated) Sympathetic Nervous System • also called the thoracolumbar division because it arises from the thoracic and lumbar regions of the spinal cord • relatively short preganglionic and long postganglionic fibers • preganglionic neurosomas in lateral horns and nearby regions of the gray matter of spinal cord – fibers exit spinal cord by way of spinal nerves T1 to L2 – lead to nearby sympathetic chain of ganglia (paravertebral ganglia) • series of longitudinal ganglia adjacent to both sides of the vertebral column from cervical to coccygeal levels • usually 3 cervical, 11 thoracic, 4 lumbar, 4 sacral, and 1 coccygeal ganglion • sympathetic nerve fibers are distributed to every level of the body Sympathetic Nervous System – each paravertebral ganglion is connected to a spinal nerve by two branches – communicating rami – preganglionic fibers are small myelinated fibers that travel form spinal nerve to the ganglion by way of the white communicating ramus (myelinated) – postganglionic fibers leave the ganglion by way of the gray communicating ramus (unmyelinated) • forms a bridge back to the spinal nerve – postganglionic fibers extend the rest of the way to the target organ Sympathetic Chain Ganglia Cardiac n. Thoracic ganglion Communicating Bronchi ramus Superior Sympathetic vena cava chain Rib Splanchnic n. Phrenic n. Heart Vagus n. Diaphragm © From A Stereoscopic Atlas of Anatomy by David L. Basett. Courtesy of Dr. Robert A. Chase, MD Sympathetic Nervous System • after entering the sympathetic chain, the postganglionic fibers may follow any of three possible courses – some end in ganglia which they enter and synapse immediately with a postganglionic neuron – some travel up or down the chain and synapse in ganglia at other levels • these fibers link the paravertebral ganglia into a chain • only route by which ganglia at the cervical, sacral, and coccygeal levels receive input – some pass through the chain without synapsing and continue as splanchnic nerves Sympathetic Nervous System • nerve fibers leave the sympathetic chain by spinal, sympathetic, and splanchnic nerves – spinal nerve route • some postganglionic fibers exit a ganglion by way of the gray ramus • returns to the spinal nerve and travels the rest of the way to the target organ • most sweat glands, piloerector muscles, and blood vessels of the skin and skeletal muscles – sympathetic nerve route • other nerves leave by way of sympathetic nerves that extend to th

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