Neurons and Hormonal Coordination

Multicellular organisms coordinate their activities using two communication systems: the fast, electrical nervous system and the slower, chemical endocrine system. Together they allow the body to respond to changes in its internal and external environment and to maintain a stable internal state.

Neurons and Nerve Impulses

The nervous system is built from specialized cells called neurons that carry electrical signals. A typical neuron has a cell body, branching dendrites that receive signals, and a long axon that carries the impulse. Many axons are wrapped in an insulating myelin sheath, which speeds up the transmission of impulses. Sensory neurons carry impulses from receptors to the central nervous system, motor neurons carry impulses to effectors such as muscles and glands, and relay neu