The Nervous System

The nervous system has two parts: the central nervous system (CNS) — the brain and spinal cord, which process information — and the peripheral nervous system (PNS) — the nerves carrying messages in and out.

Three kinds of neuron

• Sensory neurons carry impulses from receptors to the CNS.
• Interneurons (relay) connect neurons within the CNS.
• Motor neurons carry impulses from the CNS to effectors (muscles/glands).

Between neurons is a tiny gap, the synapse, crossed by chemical neurotransmitters.

The reflex arc

A reflex is a fast, automatic, protective response that routes through the spinal cord without waiting for the brain:

Reflex arc: receptor → sensory neuron → spinal cord → motor neuron → effector.