The Central Nervous System
Neurons have four major components: (1) cell body, (2) axons, (3) dendrites, and (4) synapses (Figure 2.1). The cell body contains the nucleus and receives inputs, providing the machinery for the generation of neurotransmitters and action potentials. An action potential occurs when a neuron's membrane is depolarized beyond its threshold. This depolarization is propagated along the axon. The axon is the "sending" part of the neuron, and it conducts these action potentials to the synapse to release neurotransmitters. The synapse is a specialized space or contact zone between neurons that allows interneuronal communication. One or more dendrites comprise the "receiving" part of the neuron, providing a massive receptive area for the neuronal surface (Figure 2.2).
Neurons act on other neurons to exert three major functions: inhibition, excitation, and neuromodulation. Inhibition means that one neuron inhibits another neuron, often through the release of an inhibitory neurotransmitter at the synapse. Excitation means that one neuron activates another neuron through the release of an excitatory neurotransmitter at the synapse. Neuromodulation means that a neuron influences neurotransmission, often at a long distance.