The intercostal nerves are part of the somatic nervous system, and arise from anterior divisions (rami anteriores; ventral divisions) of the thoracic spinal nerves from T1 to T11 (shown in Figure 1). The intercostal nerves are distributed chiefly to the thoracic pleura and abdominal peritoneum and differ from the anterior divisions of the other spinal nerves in that each pursues an independent course without plexus formation.
The first two nerves supply fibers to the upper limb in addition to their thoracic branches. The anterior division of the first thoracic nerve divides into two branches: one, the larger, leaves the thorax in front of the neck of the first rib, and enters the brachial plexus; the other smaller branch, the first intercostal nerve, runs along the first intercostal space, and ends on the front of the chest as the first anterior cutaneous branch of the thorax.
The next four are limited in their distribution to the parietes of the thorax; the lower five supply the parietes of the thorax and abdomen. The seventh intercostal nerve terminates at the xyphoid process, at the lower end of the sternum. The tenth intercostal nerve terminates at the umbilicus. The twelfth (subcostal) thoracic is distributed to the abdominal wall and groin.
Unlike the nerves from the autonomic nervous system that innervate the visceral pleura of the thoracic cavity, the intercostal nerves arise from the somatic nervous system. This enables them to control the contraction of muscles, as well as provide specific sensory information regarding the skin and parietal pleura. This explains why damage to the internal wall of the thoracic cavity can be felt as a sharp pain localized in the injured region. Damage to the visceral pleura is experienced as an un-localized ache.