Types of Neurons and Their Functions

When parts of the central nervous system (CNS) are critically injured, it cannot generate new neurons nor regenerate new axons for the damaged neurons. No treatment can help recover nerve function after an injury to the CNS.

The central nervous system comprises the brain and spinal cord, while the peripheral nervous system (PNS) comprises nerves which help connect the CNS to each and every part of the body. A neuron is a specialized cell that is capable of transferring electrochemical impulses called nerve impulses.

The brain contains about 100 billion neurons and glial cells (cells that provide nutrition to neurons and hold them in their places). Neurons communicate with other cells via synapses.

A synapse is a junction where the plasma membrane of the neuron comes in close contact with the membrane of the target cell. The molecular machinery present there allows rapid and precise transmission of electrical as well as chemical signals.

Like other body cells, neurons also have a cell body (soma) which contains the nucleus, cytoplasm, and other cell parts. Most neurons have axons (a protoplasmic protrusion), the finger-like projections, with the help of which they can reach out to distant parts of the body and make thousands of synaptic contacts.

Bundles of neurons that are present throughout the body are called nerves. Glial and Schwaan cells form the coating for some axons. The coating is known as a myelin sheath. The branched projections of a neuron which look like a crown are called dendrites.

Dendrites have numerous synapses that help receive signals. They transmit electrochemical signals received from other neural cells to their cell body. They are shorter and more branched than axons. The impulses are conveyed to the terminal branches through the axons. 

Thus, a cell body, an axon, and dendrites are the main parts of a neuron. The nerves send and receive signals, and act as messengers for the brain, spinal cord, and other body parts.

Considering their shapes, neurons are classified as multipolar, unipolar, and bipolar neurons.

This is the most common type of neuron in the vertebrate nervous system. It consists of a cell body, from which emerges a single long axon, and relatively shorter and more branched dendrites. Multipolar neurons have two or more dendrites, separate from the axon, and they serve principally as motor neurons.

Neurons with very long axons are called Golgi Type I (for example, pyramidal cells, Purkinje cells), while those with short, local circuit axons (form smaller circuits between neighboring cells) are called Golgi Type II (for example, granule cell).

This is the most common type of neurons in insects. It has only one protoplasmic projection, which works as both axon and dendrites (dendrite and axon emerging from same extension). Unipolar neurons serve as sensory neurons.

This is a variant of a bipolar neuron. A neuron that begins as a bipolar neuron and becomes a unipolar one as it develops is known as pseudounipolar neuron.

Bipolar neurons have two extensions―axon and a single dendrite―on opposite ends of the soma. They are present in sensory organs like the eyes and nose. They are specialized sensory neurons for the transmission of senses.

They help convey information regarding smell, sight, taste, and hearing. They are also present within the vestibular nerve. They help maintain an equilibrium and play an important role in motion detection.

Neurons are named as anaxonic when the axon cannot be distinguished from the dendrites. Such neurons are found in the brain and the retina.

Neurons are classified according to their functions too. Their functions vary depending upon the type of tissues with which they form connections.

Neurons that establish connections with sensory surfaces like the skin are known as primary sensory neurons. In all species, many types of primary sensory neurons are unipolar or pseudounipolar. They have special structures, with the help of which they convert the physical stimuli (light, sound, temperature, etc.) into an electrical activity.

The single axon that they have conveys the signals from the sense organs to the spinal cord or brain (central nervous system). They collect information from internal organs or from external stimuli (from sensory organs) and send it to the central nervous system. Sensory neurons thus form the afferent neural pathways that send signals to the CNS.

Neurons which form synapses with the muscles are called motor neurons. They carry information from the central nervous system to the muscles, and regulate the function of muscles, glands, and organs (voluntary movement). They are described as efferent, as they conduct impulses away from the central nervous system.

Neurons which form connections with other neurons are the interneurons or connecting neurons. Neurons present in the brain that connect the sensory and the motor neurons are connecting neurons. They help exchange the signals between motor and sensory neurons.

In many species, most neurons are interneurons, as they receive all the information from other neurons and pass on their output to other neurons.

Neurons are also classified according to the shapes of dendrites; for example, stellate cells (neurons with several dendrites radiating from the cell body, look like a star). If dendrites have spines, the cells are called spiny, otherwise they are called aspinous. Medium-sized neurons with large and extensive dendritic trees are called medium spiny neurons.

Neurons are further classified as per the types of neurotransmitters released by them.

► Those which release acetylcholine are called cholinergic neurons.
► GABAergic neurons release gamma aminobutyric acid (GABA)
► Glutamatergic neurons produce glutamate.
► Dopaminergic neurons produce dopamine.
► Serotonergic neurons release serotonin.

Taking into consideration different electrophysiological characteristics, neurons are classified as

• Fast Spiking: These are known for their high firing rates; for example, cells in globus pallidus (a sub-cortical structure of the brain) and retinal ganglion cells (RGC), located near the inner surface of the retina.

• Phasic or Bursting: These neurons fire in bursts.
• Tonic or Regular Spiking: These neurons are constantly active. For example, interneurons in neurostriatum.

Taking into consideration their location and unique shapes, neurons are further classified as:

➽ Anterior horn cells, motoneurons located in the spinal cord.

➽ Basket cells, interneurons that are found in the cortex and cerebellum.

➽ Betz cells, large motor neurons.

➽ Granule cells, a type of Golgi II neuron.

➽ Lugaro cells, interneurons of the cerebellum.

➽ Medium spiny neurons, most neurons present in the corpus striatum (a subcortical part of the forebrain).

➽ Purkinje cells, huge neurons in the cerebellum, a type of Golgi I multipolar neuron.

➽ Pyramidal cells, neurons with triangular soma, a type of Golgi I.

➽ Renshaw cells, neurons with both ends linked to alpha motor neurons (large lower motor neurons of the brainstem and spinal cord).

➽ Spindle cells, interneurons that connect distant areas of the brain.

➽ Unipolar brush cells, interneurons with unique dendrite ending in a brush-like tuft.

Thus, 100 billion neurons that are present in the human brain can be divided into hundreds of types, taking into consideration their size, the brain parts where they originate or are situated, their structures, functions, their electrophysiological characteristics, and other genetic features.