6.4.2. Sympathetic Nerve Fibres, Part 2

The Course of a Sympathetic Nerve Fibre (Continued)

The sympathetic nerve fibre, after entering the sympathetic trunk, will now do one of 4 things:

(1) It will synapse where it is, i.e. at its current level.
(2) It will travel upwards, through the sympathetic trunk, to a higher level and synapse there.
(3) It will travel downwards, through the sympathetic trunk, to a lower level and synapse there.
(4) It will not synapse at all and continue to travel out of the sympathetic trunk. This type of sympathetic nerve fibre is called a splanchnic nerve (pronounced as ‘splank-nick’).

The Final Stage

If the nerve fibre synapses, the post-synaptic fibre (technically called the postganglionic fibre) will travel to the target organ to affect it in some way. If the nerve fibre doesn’t synapse here, it will leave and synapse in another ganglia (e.g. coeliac ganglia in the abdomen - don't worry about this at the minute). Regardless of whether the nerve fibre synapses or not, it will have to leave the sympathetic trunk. It can leave in one of two ways:

  • It can leave via the grey ramus communicans and re-enter the spinal cord. The nerve fibres that do this will head towards general structures (e.g. the sympathetic nerve fibres will travel to the skin and blood vessels to innervate arrector pili muscles, sweat glands and vasoconstrict peripheral arterioles).
  • The sympathetic fibre can also leave the sympathetic trunk via another route to innervate specific nearby viscera/organs, e.g. the cardiac muscle of the heart, oesophageal smooth muscle, tracheal and bronchial smooth muscle.