6.2.1. Somatic Nerves: Overview
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Somatic Nerve Fibres are Present in Spinal Nerves
The thoracic spinal nerves emerge from the spinal cord at the levels T1 – T12. A spinal nerve is formed by both sensory and motor nerves that enter and leave the spinal cord. This is shown in the accompanying diagram. Although a spinal nerve can contain somatic and autonomic fibres, we will be looking at the somatic nerves only here.
Consider the general somatic sensory nerves for a moment. These nerves are travelling from areas like the skin covering the anterior and posterior aspects of the thorax. The nerves coming from the anterior aspect eventually travel together in a bundle called the ventral (or anterior) 'ramus'. The nerves coming from the posterior aspect eventually travel together in the dorsal (or posterior) ramus. These two 'rami' eventually join together and are then called a 'spinal nerve'. The spinal nerve and rami will also contain somatic motor nerves running in the opposite direction. Actually, the spinal nerve divides again before it enters the spinal cord (into a ventral root and a dorsal root), but we’ll worry about that another time!
Two Main Groups of Thoracic Spinal Nerves
Note: these contain motor and sensory fibres:
- The intercostal nerves. These bilateral nerves provide motor innervation to the intercostal and abdominal muscles (see the next section). They also provide sensory innervation to the skin over the ribs.
- The phrenic nerve has a root of C3 – C5 in the spinal cord. This means that motor nerves leave the spinal cord at C3, C4 and C5 and travel in their ventral rami. They combine together to form the phrenic nerve, which provides motor innervation to the diaphragm. As you might expect, there is a phrenic nerve on both the left and right sides. The phrenic nerve also carries sensory fibres from the mediastinal and diaphragmatic pleurae covering the central part of the diaphragm. It also provides sensory innervation to the pericardium (see Section 7). REMEMBER: “C3, 4, 5, keeps the diaphragm alive”.