With over 100
billion neurons in the
nervous system it becomes necessary to have a scheme for classifying them into distinguishable types. Several general methods have arisen in order to group neurons and make understanding their role within the nervous system more manageable. Since these
classifications are based on differing criteria such as
function or physical properties, it is possible for a particular neuron to be included into more than one group.
This distinction involves neurons whose axon form part of peripheral nerves. The two types are motor neurons and sensory neurons.
Sensory: This particular type of neuron carries information from peripheral structures (eyes, ears, etc.) back to the Central Nervous System.
Motor: Neurons which carry a signal sent from the brain to effectors (muscles or glands).
2. Location of Target Structure
This dichotomy is based on the location of the receiving cell to which the axon is sending its signal. The two types are projection neurons and local circuit neurons.
Projection Neurons: These cell types innervate remote structures. Projection neurons are also known as principal neurons.
Local Circuit Neurons: As the name suggests these neurons supply targets in the same local structure as the originating cell. They are also referred to as intrinsic neurons or interneurons.
Golgi Type I: These cells are large with long axons, which are thick and myelinated. Type I cells are commonly presynaptic to type II cells and have spinous dendrites with many synapses. They are generally projection neurons.
Golgi Type II: These cells are smaller with thin unmyelinated axons and thin smooth dendrites. They terminate on type I cells in several ways other than the typical axodendritic synapse including axosomatic and dendrodendritic synapses. Type II cells are generally local circuit nerons.
4. Number of Cellular Processes
This classification system distinguishes neurons based on the number of processes that extend from the cell body.
Multipolar: these are the most common type of neuron and are cells which possess multiple processes, including an axon and many dendrites. Examples of multipolar neurons are as follows:
i) Pyramidal cells- They get their name from the pyramid shaped cell body. They are Golgi type I cells with apical and basal (extending from the base) dendrites. These cells are found within the cortex.
ii) Purkinje cells- These are projection neurons found within the cerebellum. They are immediately distinguishable by their elaborate and extensive dendritic branching.
iii) Granual cells- These cells have roundish cell bodies that are largely occupied by the nucleus and are found in compact clusters.
Bipolar: These cells have two processes attatched to the cell body and are often sensory neurons associated with the auditory and visual systems.
Unipolar: A single process extends from the cell body and bifurcates into a dendritic and axonal region. These are sensory neurons and their cell bodies are only found in the periheral nervous system.