Neuron Classification

The Nerve cells can be categorized by their connections, structure, and neurotransmitters. The Structural classification is depends on shape and size of the cell body, its dendritic tree, axon length, and the nature of the connections it make. The Neurons with one, two, or more than two neurites, are unipolar, bipolar, and multipolar respectively as shown in figure. Most of neurons in vertebrate nervous systems are multipolar, but there are significant exceptions. For illustration, bipolar neurons in the retina synapse with sensory neurons, and photoreceptors in the dorsal root ganglion are explained as pseudounipolar as they begin life as bipolar cells but their two neurites fuse. The Unipolar neurons dominate in invertebrates.

The shape of the dendritic tree, and the presence or absence of dendritic spines, helps to find out the efficacy of its synaptic connections and therefore the functionality of the cell. The Pyramidal cells constitute some 60% of neurons in the cerebral cortex and have pyramidal-shaped cell bodies and dendritic trees. The other cortical cells are known as stellate cells as of the star-like looks of their dendritic trees. The Purkinje cells of the cerebellar cortex have the exclusive feature that their dendrites form a 2-dimensional array.

The Neurons can also be categorized by axon length. The Projection (relay, principal, or Golgi type I) neurons have long axons that extend into other regions of the nervous system. The Pyramidal and Purkinje cells fall into this group. In contrast, the interneurons (local circuit or Golgi type II) have small axons and produce direct effects only in their instant neighborhood (example, stellate cells).

The Neurons can be categorized by the connections they made, and this reflects their function. Any given region of the nervous system receives inputs from afferent neurons and projects by efferent neurons to the other regions of the nervous system or an effector organ (like a muscle or gland). The Afferent neurons capable of responding directly to physiological stimuli are sensory neurons. The Efferent projection neurons in motor pathways are known as motor neurons.

Lastly, neurons can be classified according to the neurotransmitters which they secrete. There is frequently a clear correlation between the neuron structure and neurotransmitter. For illustration, pyramidal cells release glutamate while stellate and Purkinje cells secrete γ-aminobutyrate. This gives circumstantial evidence for function as generally glutamate excites, whereas γ -aminobutyrate inhibits, the other neurons.