The Nervous System

The nervous system as a whole is divided into two subdivisions: the central nervous system (CNS) and the peripheral nervous system (PNS).

The Central Nervous System

The brain and spinal cord make up the central nervous system. They are in the dorsal body cavity, encased in bone for protection. The brain is in the cranial vault, and the spinal cord is in the vertebral canal of the vertebral column. Although considered to be two separate organs, the brain and spinal cord are continuous at the foramen magnum.

The Peripheral Nervous System

The organs of the peripheral nervous system are the nerves and ganglia. Nerves consists of nerve fibers, similar to muscles. Cranial nerves and spinal nerves extend from the CNS to peripheral organs such as muscles and glands. Ganglia are collections, or small knots, of nerve cell bodies outside the CNS, often serving as a processing station for nerve signals.

The peripheral nervous system is further subdivided into an afferent (sensory) division and an efferent (motor) division. The afferent or sensory division transmits impulses from peripheral organs to the CNS. The efferent or motor division transmits impulses from the CNS out to the peripheral organs to cause an effect or action.

Finally, the efferent or motor division is again subdivided into the somatic nervous system and the autonomic nervous system. The somatic nervous system, also called the somatomotor or somatic efferent nervous system, supplies motor impulses to the skeletal muscles. It is sometimes called the voluntary nervous system because these nerves permit conscious control of the skeletal muscles.

Autonomic Nervous System

The autonomic nervous system is a visceral efferent system, sending motor impulses to the visceral organs. It functions automatically and continuously, to innervate smooth muscle, cardiac muscle, and glands. It controls heart rate, breathing rate, blood pressure, body temperature, and other visceral activities to maintain homeostasis.

The autonomic nervous system consists of two parts, the sympathetic division and the parasympathetic division. Many visceral organs are supplied with fibers from both divisions. In this case, one stimulates (sympathetic) and the other inhibits (parasympathetic). This antagonistic functional relationship serves as a balance to help maintain homeostasis.

Note: In this revised module, all aspects of the nervous system will be covered, which includes the peripheral nerves (C47), which were incorporated into the Malignant CNS and Peripheral Nerves Site group of the Solid Tumor Rules.

Updated: September 23, 2026