The central nervous system, the brain and spinal cord, are involved in the processing of information. The peripheral nervous system has the functions of carrying sensory information into and motor commands out of the central nervous system. Anatomically, the peripheral nervous system may be thought of as having two components, the cranial nerves and the spinal nerves.

There are twelve pairs of cranial nerves originating in the brain and upper cervical spinal cord. Their names and some of their functions are listed below. The cranial nerves are numbered, usually with Roman numerals, from top to bottm in the order that they enter the brain (figure 1). Because the functions of these nerves are often quite specific, and because these nerves have precise origins within the central nervous system, changes in the functions of these nerves are useful clinical indicators of the locations of problems in the brain and spinal cord. You will see that most of the cranial nerves have their origins (nuclei) in the brain stem (figure 2).

The cranial and spinal nerves contain sensory fibres which provide us with information from all of our tissues, and also information conveyed from the external environment, for example sight and hearing. The peripheral nervous system also contains motor fibres which carry conscious commands to our muscles, and unconscious or involuntary commands to organs such as the viscera. Those motor neurons which carry unconscious commands to visceral effectors (organs) constitute the peripheral components of the autonomic nervous system. The autonomic nervous system has two complementary divisions, the sympathetic nervous system and the parasympathetic nervous system.

The Cranial Nerves

There are 31 pairs of spinal nerves. The first pair of cervical spinal nerves emerges between the occiput and the atlas. Each successive spinal nerve then emerges from the intervertebral foramen between success pairs of vertebrae. Hence, there are eight (pairs of) cervical nerves, twelve thoracic nerves, five lumbar nerves, five sacral nerves, and usually a single coccygeal nerve.

Each spinal nerve contains both sensory and motor fibres (figure 5). The sensory fibres enter the dorsal aspect of the spinal column via the dorsal root. The cell bodies of the peripheral sensory neurons form a mass in the dorsal root called the dorsal root ganglion. The motor fibers to the spinal nerves originate from the ventral aspect of the spinal cord and form the ventral root. The cell bodies of the peripheral motor neurons are located within the spinal cord.

The dorsal and ventral nerve roots unite to form the spinal nerve which emerges from the intervertebral foramen. The spinal nerve then divides into an anterior (ventral) ramus and a posterior (dorsal) ramus. The sensory fibres from the posterior ramus enter the skin of the back and then run almost in a circle around to the front of the body (figure 6). Hence a problem with sensation in a particular region of the skin can sometimes be traced to a specific level of the spine or spinal cord. The distribution of motor innervation is also relatively specific, so that a loss of strength in a particular muscle or set of muscles may point to a disorder at a particular spinal level. In the thoracolumbar region, the ventral rami contain preganglionic sympathetic neurons which give rise to the sympathetic trunk. The peripheral parasympathetic fibres are contained within the the cranial nerves and lumbosacral spinal nerves.

Self-assessment