6.4.1. Sympathetic Nerve Fibres, Part 1
The Course of a Sympathetic Nerve Fibre
The detailed course of a sympathetic nerve fibre is as follows:
- The sympathetic nerve fibre leaves the brain and travels down as part of the spinal cord.
- The nerve fibre can exit the spinal cord between the levels of T1 – L2/L3. This occurs bilaterally.
- The nerve fibre exits the spinal cord and travels a very short distance with the spinal nerve of that level. The nerve fibre then leaves the spinal nerve and enters the white ramus communicans. It travels in this structure for a short distance, then enters the ‘sympathetic trunk’ (see accompanying diagram).
The sympathetic trunk is a nervous system structure which is made up of a series of swollen bumps lying parallel to the vertebral column. The swollen bumps are ganglia – this is where preganglionic sympathetic fibres synapse with postganglionic sympathetic fibres. In the thorax, there are 11 ganglia. These lie at the vertebral levels of T2 – T12 (the ganglion that should be at T1 is actually joined to the inferior cervical ganglion in the neck, and together they are called the stellate ganglion). The accompanying table shows the distribution of postganglionic sympathetic fibres in the thorax.
As you can see in the table, there are various nerve plexuses in the thorax. These will be discussed in more depth after we have looked at the parasympathetic nervous system – the reason for this is the plexuses contain both sympathetic and parasympathetic fibres.