The neuron is the basic unit in the nervous system, acting as a specialized conductor cell that receives and transmits electrochemical nerve impulses, explains Dr. Ananya Mandal. A typical neuron has a cell body and long processes that transmit impulses from one part of the body to another part.
A neuronal cell body has several high-branched, thick extensions called dendrites: They carry nerve impulses into the cell body. Also extending from the cell body is an axon, which is a long, thin process that carries impulses away from the cell body to another neuron or tissue. There is often only one axon per neuron, explains Dr. Mandal.
According to Ka Kiong Charand of Georgia State University, the axon is a transmission line of successively excitable segments. The segments are wrapped with sheaths of myelin. However, between each segment is a gap that does not contain myelin. These gaps are known as nodes of Ranvier. The nodes of Ranvier are sites for the exchange of ions, which act to propel the transmission further down the line.
At the end of the axon is a synaptic knob, which upon excitement releases chemical neurotransmitter molecules to receptors on a neighboring neuron or a targeted tissue. Categorized according to structure and function, neurons are separated into three distinct groups: multi-polar, bi-polar and uni-polar neurons. Dr. Mandal explains that multi-polar neurons have one axon and several dendrites. These are common in the brain and spinal cord. Bi-polar neurons have one axon and one dendrite, and are found in the retina of the eye, the inner ear and the olfactory area for smell. Uni-polar neurons have one divided process extending from the cell body: half the process acts as an axon and the other half acts as a dendrite. This division appears in the spinal cord.