All parts of the nervous system (brain, spinal cord and nerves) are made of nervous tissue. There are only two main types of nerve cells in nervous tissue.
The neuron is the structural unit of the nervous system that transmits electrical signals.
The glial cells that provide a support system for the neurons.
Structure of a Neuron : Neurons are typically made up of a cell body (or soma), dendrites, and an axon, as shown in Figure
The cell body contains the nucleus and other organelles similar to other body cells.
The dendrites extend from the cell body and receive a nerve impulse from another cell. Some neurons can have over 1000 dendrite branches other neurons have only 1 or 2 dendrites
The axon is a long, membrane-bound extension of the cell body that passes the nerve impulse onto the next cell. In general, neurons have only one axon and the longest axon of a human neuron can be over a meter long.
The end of the axon is called the axon terminal. The axon terminal is the point at which the neuron communicates with the next cell.
The axons of many neurons are covered with an electrically insulating phospholipid layer called a myelin sheath. The myelin speeds up the transmission of a nerve impulse along the axon. It acts like a layer of insulation, like the plastic you would see around an electrical cord.
Types of Neurons Neurons are highly specialized for the processing and transmission of cellular signals and can be classified by their structure or function. Structural classification is based on the number of dendrites and axons that a cell has. Functional classification groups neurons according to the direction in which the nerve impulse is moving in relation to the CNS.
Thre are three functional groups:
• Sensory neurons carry signals from tissues and organs to the central nervous system and are sometimes also called afferent neurons.
• Motor neurons carry signals from the central nervous system to muscles and glands and are sometimes called efferent neurons.
• Interneurons connect sensory and motor neurons in neural pathways that go through the central nervous system. Interneurons are found only in the central nervous system where they connect neuron to neuron.