A sensory receptor detects a stimulus and changes it into an electrical impulse. The impulse travels along a sensory neurone to the central nervous system (CNS), which is the brain and spinal cord. Inside the CNS, relay neurones connect sensory neurones to motor neurones. A motor neurone carries the impulse to an effector, which is a muscle or a gland.
A neurone is long and thin. The dendron carries impulses towards the cell body and the axon carries them away from it. Many axons are wrapped in a fatty myelin sheath. The myelin sheath insulates the axon, so impulses travel faster.
Neurones do not touch. There is a tiny gap between them called a synapse. An electrical impulse cannot cross the gap. Instead, when an impulse reaches the end of a neurone, chemicals called neurotransmitters are released. They diffuse across the gap and bind to receptors on the next neurone, which starts a new electrical impulse. This only works in one direction, because only the first neurone releases neurotransmitters.
