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Neurons are cells of the central nervous system, located within the grey matter, and responsible for all neurological functions of the brain.
Neurons vary in morphology and size substantially, but all share a number of features 1:
- a cell body
- perikaryon: cytoplasm surrounding the nucleus
- a single process of variable length (even over a meter)
- generally the 'output' of the neuron
- terminates in terminal buttons which release neurotransmitters
- usually multiple branching processes
- generally the 'input' of the neuron
Generally, input from other neurons or sensory receptors received by dendrites may lead to an action potential within the cell body which then propagates down the axon, eventually resulting in the release of neurotransmitters from the terminal buttons. This, in turn, can affect other neurons or end organs 1.
A number of markers are useful to identify neurons, typically marking the body and dendrites 3:
- synaptophysin: a presynaptic vesicle protein
- NeuN: a neuronal nuclear protein
- microtubule-associated protein-2 (MAP2)
- neurofilament subunits
Types of neurons
Three main morphological patterns of neurons are described, depending on the relationship of the cell body to the axon and dendrites 1.
Additional variation in morphology and size results in a variety of specific neuron types, depending on the location within the nervous system. For example within the cerebral cortex five types of neurons are present 1:
- pyramidal cells
- fusiform cells
- cells of Martinotti
- horizontal cells of Cajal
- stellate cells
Pyramidal cells are by far the most common neuron type found in human neocortex, and are located in layers II, III, V and VI (see below). They are, as their name suggests, pyramidal in shape with the apex pointing outwards, towards the pial surface of the brain. From the apex, a thick single dendrite extends towards more superficial layers and branches into numerous dendrites. From the base of the cell body, an axon extends into deeper layers and/or into the subcortical white matter. Additional dendrites extend from the base of the cell body to connect with other neurons in the same layer 1.
The largest examples of pyramidal cells are the Betz cells found in the motor cortex 1.
Fusiform cells extend orthogonally to their cell layer with dendrites extending from either end into deeper and more superficial layers. From the cell body, an axon arises which passes superficially 1.
Cells of Martinotti
Cells of Martinotti are small cells with a limited number of short dendrites and an axon which extends superficially 1.
Horizontal cells of Cajal
Horizontal cells of Cajal are only found in layer I and are the least common cell type. They are also fusiform in shape but are aligned parallel to their cell layer with their axon extending laterally within the layer to synapse with nearby pyramidal cells 1.
Stellate cells, also known as granule cells, are rounded with short dendrites radiating circumferentially out from the body. They are predominantly found in layers II, IV and VI 1.
Etymology and spelling
You may have, as many have before you 2, been confused about the spelling of neuron vs neuron. Generally, the UK spelling is with a terminal 'e' (neuron) rather than the US spelling without the 'e' (neuron).