6.3. Autonomic Nerves: Overview
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Sympathetic nerve fibres are widely distributed throughout the body. They contribute greatly to the “fight or flight” response. They initially travel in the spinal cord. Despite reaching most areas of the body, the nerve fibres only leave the spinal cord at the levels of T1 – L2/L3. Because of this, the sympathetic division is also called ‘thoracolumbar outflow’. Actually, when they leave the spinal cord, they tend to run beside the somatic intercostal nerves for some of their course. Because we are focusing on the thorax, only T1 – T12 will be discussed, and L1, L2 and L3 will be ignored here.
Parasympathetic nerve fibres have a limited distribution in the body. Some of these nerve fibres travel in the spinal cord, other fibres just run within other larger nerves (i.e. they hitch-hike). Parasympathetic fibres run within particular cranial nerves of the head and neck: cranial nerves 3, 7, 9, 10. Parasympathetic fibres also leave the spinal cord at the levels of S2 – S4. Because of this, the parasympathetic division is also called ‘craniosacral outflow’. Again, because we are focusing on the thorax, we will only look at the relevant nerves. There is in fact only one relevant nerve: cranial nerve 10 (CN10), which is also called the vagus nerve. Although it is classified as a cranial nerve, it leaves the brainstem and descends through the head and neck, thorax and abdomen. It actually ends at the transverse colon! As it descends, the vagus nerve gives off lots of cardiac, pulmonary and gut branches.