All neurons are not identical. These basic units of the brain are highly diverse, even within a specific brain area. They are distinguishable according to:
- The shape of the cell body (or soma).
- Their organization and morphology of the extensions (i.e. dendrites and axon) that arise from the cell body.
Four major types of neurons found in the central nervous system (composed of the brain and spinal cord) are:
- The multipolar neuron: which possesses a single (usually long) axon and many dendrites, allowing for the integration of a great deal of information from other neurons. These dendritic branches can also emerge from the cell body. Multipolar neurons constitute the majority of neurons in the brain.
- The bipolar neuron: whose short axon and the limited dendritic branching arise from the opposite sides of the cell body.
- The pseudo-unipolar neuron: possesses a branch that splits close to the cell body to form dendrites (D) and an axon (A).
- The unipolar neuron: displays a single large extension that arises from the cell body (C).
The multipolar neuron’s (1) morphology is diverse and is distinguishable not only from the shape of the cell body, but also from the shape of the axon or dendrites.
- The axon of a multipolar neuron can be long, ranging from a few centimetres to a meter or more. These are known as Golgi type I neurons. Some are known as pyramidal cells, whose cell body, which is located in the cerebral cortex, sends extensions to the spinal cord.
- The neuron can also have short axons of a few hundredths of a millimetre allowing them to be closely packed together. This type of neuron remains within a structure, and is known as a Golgi type II neuron. Examples of Golgi type II neurons include stellate or basket neurons in the cerebellum.
- Some neurons have an axon as well as collaterals that display varicosities. Others possess dendrites with spines known as dendritic spines.
In summary, neurons exist in many forms. This diversity has an impact on the capacity of the neuron to receive and to treat the information it receives from other neurons.