Sympathetic nervous system

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The sympathetic nervous system (SNS) is the division of the autonomic nervous system responsible for fight or flight, a response or adaptation to stress resulting in widespread activation through postganglionic innervation of visceral structures of the body.

It consists of both afferent and efferent neurons. Though the cell bodies of preganglionic fibres are located in the intermedio-lateral grey horns, present only in the T1 to L2 segments of the spinal cord, the sympathetic ganglia in which they synapse are found in association with all spinal nerves arranged in two longitudinal rows, one along each side of the vertebral column.

Sympathetic trunks are lateral to the vertebral bodies of C1-S5 bilaterally within the thoracic, abdominal, and pelvic cavities.

Efferent sympathetic neuron pathways

  • When a pre-ganglionic fibre enters a sympathetic ganglion (via a white ramus communicans), it may do one of the following:


  • True from T1 - L2
  1. Preganglionic fibre enters Ventral Ramus
  2. Preganglionic sympathetic fibre enters ganglion via White ramus communicans

SYNAPSE POINT (orange circle)

  1. Postganglionic sympathetic fibre leaves ganglion via Grey ramus communicans
  2. Postganglionic sympathetic fibres enter ventral (---) & Dorsal (...) rami


  1. Preganglionic sympathetic fibre enters ventral ramus
  2. Preganglionic sympathetic fibre enters ganglion via white R. C.

SYNAPSE POINT (orange circle)

  1. Postganglionic sympathetic fibre leaves ganglion independently
  2. Postganglionic sympathetic fibre reaches visceral plexus (no synapse), then to viscera

C pass up or down the sympathetic trunk to a ganglion at a different level, where it synapses with one or more post-ganglionic neurons which enter the spinal nerve at that level, via a grey ramus communicans (as in (a) above) or leaves the ganglion by a separate route to join plexuses (as in (b) above). Branches of such plexuses often reach their targets by passing along the surfaces of blood vessels, e.g., internal carotid plexus to structures in the head.

  • Life has become too hard. One has to choose either to kill oneself or skip pulling in the graphics for the other two pathways.


  • Pass through the ganglion without synapsing and enter another ganglion, closer to the target, where it synapses. Pre-ganglionic fibres which behave in this way form splanchnic nerves (splanchnon = organ). In this case, the post-ganglionic fibres are comparatively short and often travel along blood vessels to reach their targets. Many abdominal and pelvic viscera are supplied in this way.