Neuron is a highly differentiated cell, which has the function of sensing stimulus and conducting excitation, and is the most basic structural and functional unit of the nervous system. Although neurons are diverse in form and function, they can be broadly divided into two parts: 1) neuronal cell body: composed of three parts: cell membrane, cytoplasm and nucleus, which is the metabolic center of neurons and is mainly used to store nutrients, integrate and issue nerve impulses; 2) neuronal peripheral protrusions: divided into dendrites and axons. The role of the dendrites is to receive external stimuli and transmit this excitation into the cell body; the role of the axons is to transmit excitation from the cell body to other neurons, or to other tissues. In order to avoid interference between axons and to speed up the transmission of electrical signals, axons are usually covered with myelin sheaths. Neurons can be classified according to their function as sensory (afferent neurons), motor (efferent neurons), and liaison (interneurons). Generally neurons are in an inhibited state, but when the organism is stimulated by some excitation, the neuron will be in an excited state.