A map of the nervous system

Twelve pairs of nerves leave the brain and 31 pairs leave the spinal cord. Together they reach almost every part of the body.

Two parts: central and peripheral

Touch a hot pan and your hand pulls back before you have time to think about it. A moment later you feel the pain, and then you decide to run cold water over your fingers. All three of those events are the work of one system. The nervous system collects information, makes sense of it and sends out commands, and it does all of this through cells that pass electrical and chemical signals to one another.

Anatomists divide the system into two regions. The central nervous system (CNS) is the brain and the spinal cord. The brain sits in the cranial cavity of the skull, and the spinal cord runs down the vertebral cavity inside the backbone. The peripheral nervous system (PNS) is everything else: the nerves that branch out to the skin, muscles and organs, and small clusters of nerve cells along the way. The OpenStax anatomy textbook points out that this line is a convenient start rather than a perfect rule, because a few peripheral structures sit inside the skull or the spine.

Gray matter, white matter and the names of things

Cut fresh brain or spinal cord and you see two shades. Gray matter is where the cell bodies and dendrites of nerve cells crowd together. White matter is mostly axons, the long fibres that carry signals away from cell bodies. It looks white because many axons are wrapped in myelin, a fatty insulation, and fat looks pale. In the brain the gray matter forms the outer layer, the cortex, with white matter underneath.

The same kind of structure gets a different name depending on where it is. A cluster of cell bodies is a nucleus in the CNS and a ganglion in the PNS. A bundle of axons is a tract in the CNS and a nerve in the PNS. The optic nerve is an exception to this naming rule: it is part of the CNS, as is the optic tract beyond the optic chiasm.

Sense, decide, respond

Seen by what it does, the nervous system has three basic jobs. Sensation is receiving information about a change, called a stimulus, either outside the body or inside it, such as the stretch of an organ wall. Integration is weighing that input against other input, memories and the state you are in. Response is the output: muscles contract or glands release something. A batter who sees a pitch does not swing automatically. The speed of the ball, the count and the score all go into the decision, and that weighing is integration.

Who is in control: somatic and autonomic

The somatic nervous system handles conscious perception and the skeletal muscles that move your bones. Not every somatic response is a choice, though. If a friend jumps out and shouts, you may flinch or yell before you decide anything, and skills you practise, like typing or riding a bike, slowly become automatic.

The autonomic nervous system works without your attention. It controls heart muscle, the smooth muscle in blood vessels and organs, and glands such as sweat glands, usually to keep conditions inside the body steady. It has two divisions that often pull in opposite directions. The sympathetic division is linked with the fight-or-flight response and speeds the heart up. The parasympathetic division, nicknamed rest and digest, slows it down. The digestive tract also has its own network, the enteric nervous system, which can run the gut's smooth muscle and glands even without the CNS.

The cables: cranial and spinal nerves

Nerves are named for where they join the CNS. Twelve pairs of cranial nerves connect to the brain, mostly through the brain stem, and serve the head and neck, including most of the special senses. They are numbered with Roman numerals from I to XII. Three of them, the olfactory (I), optic (II) and vestibulocochlear (VIII) nerves, carry only sensory information. The vagus nerve (X) is different from the rest because it reaches down to organs in the chest and abdomen, including the heart.

Thirty-one pairs of spinal nerves leave the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Every spinal nerve is mixed, carrying sensory fibres in and motor fibres out. Near the cord each one splits into two roots. Sensory axons enter through the dorsal (back) root, and motor axons leave through the ventral (front) root. You will meet those roots again in the lesson on reflexes.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.