The nervous system and reflex arcs

The nervous system carries fast electrical impulses. Sensory neurones carry impulses from receptors to the central nervous system (brain and spinal cord). Relay neurones in the CNS connect to motor neurones, which carry impulses to effectors. Between neurones there is a tiny gap called a synapse. When an impulse arrives, chemicals called neurotransmitters diffuse across the gap and start an impulse in the next neurone.

A reflex arc is a fast, automatic response that does not involve the conscious part of the brain: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response. Reflexes protect the body, for example the pupil narrowing in bright light. Reaction time can be measured with a ruler-drop test; caffeine tends to shorten it.