The myelin sheath is a fatty covering around the axon that facilitates the jumping of the impulse. Thus, saltatory conduction increases the speed of transmission. Synaptic transmission occurs when the impulse causes the release of the neurotransmitter which crosses a synaptic cleft from one neuron to another neuron, a muscle, or a gland. The neurotransmitter is destroyed or recycled. The neuron evaluates the excitatory and inhibitory messages.
Nervous systems are highly organized collections of neurons and supporting cells.
Nervous response in the sponge is not defined by a system. Other lower animals have nerve nets. These give way to more highly developed cephalized systems. The complex, well-developed brains of the chordates are all quite similar. The central nervous system and the peripheral nervous system can be divided in several ways.