From the intumescences in the spinal cord arise the nerves that supply the limbs. These nerves interweave to form a network or plexus, giving rise to nerves whose course is more difficult to follow. In the neck there is the cervical plexus formed by the upper four cervical nerves. These supply the skin on the back of the head and the neck. An important branch, the phrenic nerve, descends through the neck and chest to supply the diaphragm, a sheet of muscle which contracts when we breathe in.
The lower four cervical nerves and the first thoracic nerve form the brachial plexus. They divide and intertwine to form nerves that pass to the whole of each arm. This complex interweaving in the brachial plexus enables fibres from the original spinal segments of the spinal cord to pass to many thousands of muscle fibres on the front and the back of the arm. Complicated movements can thus be co-ordinated so that they are smooth and easy.
The lumbar plexus is formed by the upper four lumbar spinal nerves. The nerves arising from the lumbar plexus innervate the muscles of the pelvic girdle and carry sensory impulses from the genitals and thighs. The sacral plexus derives from the fifth lumbar and the upper three sacral spinal nerves. They intertwine and form the largest nerve in our body: the sciatic nerve. Practically all leg muscles are innervated by it. If this nerve is damaged or irritated high up along its course, pain can be felt shooting down the leg. In the lumbosacral region there are also autonomic nerves passing to bladder and rectum, to control the muscular movements involved when they empty.