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14.2G: Sympathetic Nervous System

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    Sympathetic ganglia are the ganglia of the sympathetic nervous system that initiate fight-or-flight, stress-mediated responses.

     

    LEARNING OBJECTIVES

     

    Describe the features of the sympathetic division of the autonomic nervous system

     

    KEY TAKEAWAYS

    Key Points

     

    • The bilaterally symmetric sympathetic chain ganglia, also called the paravertebral ganglia, extend from the upper neck down to the coccyx, lateral and slightly ventral to the vertebral column.
    • Most sympathetic ganglia are bilaterally symmetric, but an example of an unpaired ganglion, called the ganglion impar is present in front of the coccyx.
    • The sympathetic nervous system is said to have thoracolumbar outflow based on the proximity of the ganglia to the corresponding thoracic and lumbar vertebrae.

     

    Key Terms

     

    • paravertebral ganglia: Located along the length of the sympathetic trunk, these ganglia are designated as cervical, thoracic, lumbar, and sacral and, except in the neck, closely correspond in number to the vertebrae.
    • fight or flight: All the coordinated physiological responses that the sympathetic nervous system initiates in response to stress or other emergency situations.
    • sympathetic chain ganglia: Also called the paravertebral ganglia, these are located just ventral and lateral to the spinal cord. The chain extends from the upper neck down to the coccyx, forming the unpaired coccygeal ganglion.

    This is a diagram of how the nerves innervate the autonomic nervous system. The spinal cord is shown with the ganglia of the sympathetic nervous system (the red lines in the diagram) linked to their vertebral position and the organs they innervate. The parasympathetic nervous system, shown as blue lines, is a division of the autonomic nervous system, and is also linked to its vertebral positions and the organs it innervates.

    Nerve innervation of the autonomic nervous system: The parasympathetic nervous system, shown in blue, is a division of the autonomic nervous system.

    The sympathetic ganglia are the ganglia of the sympathetic nervous system (the red lines in the diagram below). They deliver information to the body about stress and impending danger, and are responsible for the familiar fight-or-flight response.

    This response is also known as the sympathetico-adrenal response because the pre-ganglionic sympathetic fibers that end in the adrenal medulla—like all sympathetic fibers—secrete acetylcholine. This secretion activates the secretion of adrenaline (epinephrine) and to a lesser extent noradrenaline (norepinephrine) from the adrenal medulla.

    Therefore, this response is mediated directly via impulses transmitted through the sympathetic nervous system, and indirectly via catecholamines secreted from the adrenal medulla, and acts primarily on the cardiovascular system.

    An example of a sympathetic ganglion in a thoracic nerve is shown in. ganglia contain approximately 20000–30000 nerve cell bodies and are located close to and on either side of the spinal cord in long chains. Sympathetic ganglia are the tissue from which neuroblastoma tumours arise.

    The bilaterally symmetric sympathetic chain ganglia, also called the paravertebral ganglia, are located just ventral and lateral to the spinal cord. The chain extends from the upper neck down to the coccyx, forming the unpaired coccygeal ganglion.

    Preganglionic nerves from the spinal cord create a synapse at one end of the chain ganglia and the postganglionic fiber extends to an effector, typically a visceral organ, in the thoracic cavity. There are usually 21 or 23 pairs of these ganglia: 3 in the cervical region, 12 in the thoracic region, 4 in the lumbar region, 4 in the sacral region and a single, unpaired ganglion lying in front of the coccyx called the ganglion impar.

    This is a drawing of an intercostal nerve with the sympathetic ganglion identified near the nerve's posterior division.

    Sympathetic ganglion: This intercostal nerve shows the sympathetic ganglion at the top left.

    Neurons of the collateral ganglia, also called the prevertebral ganglia, receive input from the splanchnic nerves and innervate organs of the abdominal and pelvic region. These include the celiac ganglia, the superior mesenteric ganglia, and the inferior mesenteric ganglia.

    The sympathetic nervous system is said to have thoracolumbar outflow based on its location.

     

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