Nervous system: peripheral nerves

The brain and spinal cord cannot do much alone. They need a network of nerves that reaches every part of the body. This network is the peripheral nervous system, and it connects the central nervous system to your skin, muscles, and organs.

A nerve is a bundle of axons wrapped in connective tissue, like a cable of many wires. You have 12 pairs of cranial nerves that come directly out of the brain, serving the eyes, ears, face, and more, and 31 pairs of spinal nerves that leave the spinal cord and branch to the rest of the body. Messages travel two ways. Sensory neurons carry information toward the central nervous system, and motor neurons carry commands away from it.

The peripheral system has two parts. The somatic nervous system controls voluntary movement of your skeletal muscles and carries sensations from your skin and joints. The autonomic nervous system controls involuntary actions: heart rate, digestion, breathing rate, sweating, and the size of your pupils. The autonomic system has two sides that balance each other. The sympathetic side prepares the body for action, the "fight or flight" response, raising heart rate and widening airways. The parasympathetic side calms the body down, the "rest and digest" response, slowing heart rate and helping digestion.

Some actions are too urgent to wait for the brain. A reflex is a rapid, automatic response to a stimulus. In a reflex arc, a sensory neuron detects the stimulus and sends a signal to the spinal cord. An interneuron in the cord passes it to a motor neuron, which commands a muscle to respond. You pull your hand from a hot pan this way, and only afterward does the signal reach your brain so you feel the pain. The knee jerk when a doctor taps just below your kneecap is another reflex arc.

Common mistake: thinking the brain controls every action directly. Reflexes bypass the brain to save precious fractions of a second. A second mistake is thinking the autonomic system is separate from everything else; it is constantly adjusting your body in response to what the brain senses. Next lesson: the endocrine system, a different way the body sends messages.