The Human Brain : From Neurone to Nervous System

THE AUTONOMIC NERVOUS SYSTEM (ANS)


  1. The autonomic nervous system is a system of efferent (motor) nerves that connect the CNS with internal organs, blood vessels and glands, but unlike the motoneurones that connect directly with skeletal muscles, the pathway between the spinal cord and a target organ contains a synapse within an autonomic ganglion.

  2. Autonomic nerves are a motor system that regulates the activity of internal organs, including the heart, stomach and intestines. These nerves differ in structure from the somatic nerves innervating skeletal muscle. The autonomic nerves arise in the lateral horns of the spinal cord and are finely myelinated; they synapse on other, mainly unmyelinated, neurones in autonomic ganglia.

  3. The sympathetic nerves originate from segments T1 to L2 of the human spinal cord ('the thoraco-lumbar outflow') and synapse within sympathetic ganglia; the sympathetic chain occupies a para-vertebral position, alongside the vertebral column.

  4. The parasympathetic nerves arise for the lateral horns of the sacral spinal cord (S2-4) and synapse in parasympathetic ganglia situated close to the organs they innervate (the lower third of the intestine and the pelvic viscera). In addition parasympathetic nerves travel in the cranial nerves, so the parasympathetic is sometimes called the 'cranio-sacral outflow'.

  5. In some internal organs the effects of the sympathetic and parasypathetic are opposed, e.e.in the intesting the parasympathetic causes smooth muscle to contract amd the sympatheitc nerves caus it to relax. Blood vessels do not receive a parasympathetic nerve supply.

  6. Key Words: sympathetic and parasympathetic nerves; thoraco-lumbar and cranio-sacral outflows; autonomic ganglia; innervation of internal organs, glands and blood vessels; homeostasis





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