Neuron: Definition, Structure, Function & Types

The human nervous system has one basic but incredibly vital function: to communicate with and receive information from different parts of the body and generate situation-specific responses to this information.

Unlike other systems in the body, the function of most of the components of the nervous system can only be appreciated using microscopy. While the brain and the spinal cord can be visualized easily enough on gross examination, this fails to provide even a fraction of the extent of the elegance and complexity of the nervous system and its tasks.

Nervous tissue is one of the four major tissues of the body, the others being muscle, epithelial and connective tissue. The functional unit of the nervous system is the neuron, or nerve cell.

Although neurons, like almost all eukaryotic cells, contain nuclei, cytoplasm and organelles, they are highly specialized and diverse, not only in relation to cells in different systems but also when compared to different kinds of nerve cells.

The human nervous system can be separated into two categories: the _central nervous system (CNS), which includes the human brain and spinal cord, and the peripheral nervous system_ (PNS), which includes all other nervous system components.

The nervous system is made up of two major cell types: neurons, which are the "thinking" cells, and **glia**, which are supporting cells.

Apart from the anatomic division of the nervous system into the CNS and the PNS, the nervous system can also be divided into functional divisions: the somatic and the autonomic. "Somatic" in this context translates to "voluntary," while "autonomic" essentially means "automatic," or involuntary.

The autonomic nervous system (ANS) can be further divided on the basis of function into the sympathetic and parasympathetic nervous systems.

The former is dedicated chiefly to "up-tempo" activities, and its revving into gear is often referred to as the "fight-or-flight" response. The parasympathetic nervous system, on the other hand, deals in "down-tempo" activities such as digestion and secretion.

Neurons differ widely in their structure, but all of them feature four essential elements: the cell body itself, dendrites, an axon, and the axon terminals.

"Dendrite" comes from the Latin word for "tree," and on inspection the reason is obvious. Dendrites are tiny out branches of the nerve cell that receive signals from one or more (often many more) other neurons.

The dendrites converge on the cell body, which, isolated from the specialized components of the nerve cell, closely resembles a "typical" cell.

Running from the cell body is a single axon, which carries integrated signals toward the target neuron or tissue. Axons usually have a number of branches of their own, though these are fewer in number than the dendrites; these are referred to as axon terminals, which function more or less as signal splitters.

While as a rule dendrites carry signals toward the cell body and axons carry signals away from it, the situation in sensory neurons is different.

In this case, the dendrites running from the skin or other organ with sensory innervation merge directly into a peripheral axon, which travels to the cell body; a central axon then leaves the cell body in the direction of the spinal cord or brain.

In addition to their four major anatomical features, neurons have a number of specialized elements that facilitate their job of transmitting electrical signals along their length.

The myelin sheath plays the same role in neurons as insulating material does in electrical wires. (Most of what human engineers have figured out was developed by nature a very long time ago, often with still-superior results.) Myelin is a waxy substance made chiefly of lipids (fats) that surrounds axons.

The myelin sheath is interrupted by a number of gaps as it runs along the axon. These nodes of Ranvier allow something called the action potential to be propagated along the axon at high speed. Loss of myelin is responsible for a variety of degenerative diseases of the nervous system, including multiple sclerosis.

The junctions between nerve cells and other nerve cells, plus target tissues, that allow transmission of electrical signals are called synapses. Like the hole in a doughnut, these represent an important physical absence rather than a presence.

Under the direction of the action potential, the axonal end of a neuron releases one of a variety of types neurotransmitter chemicals that convey the signal across the small synaptic cleft and to the waiting dendrite or other element on the far side.

Action potentials, the means by which nerves communicate with one another and with non-neural target tissues such as muscles and glands, represent one of the more fascinating developments in evolutionary neurobiology. A full description of the action potential requires a lengthier description than can be presented here, but to summarize:

Sodium ions (Na+) are maintained by an ATPase pump in the neuronal membrane at a higher concentration outside the neuron than within it, while the concentration of potassium ions (K+) is kept higher inside the neuron than outside of it by the same mechanism.

This means that sodium ions always "want" to flow into the neuron, down their concentration gradient, while potassium ions "want" to flow outward. (_Ions_ are atoms or molecules bearing a net electrical charge.)

Different stimuli, such as neurotransmitters or mechanical distortion, can open substance-specific ion channels in the cell membrane at the beginning of the axon. When this occurs, Na+ ions rush in, disrupting the cell's resting membrane potential of -70 mV (millivolts) and making it more positive.

In response, K+ ions rush outward to restore the membrane potential to its resting value.

As a result, the depolarization propagates, or spreads, very quickly down the axon, Imagine two people holding rope taut between them and one of them flicking the end upward.

You would see a "wave" move quickly toward the other end of the rope. In neurons, this wave consists of electrochemical energy, and it stimulates the release of neurotransmitter from the axon terminal(s) at the synapse.

The major types of neurons include:

• Motor neurons (or motoneurons) control movement (usually voluntary, but sometimes autonomic).
• Sensory neurons detect sensory information (e.g., the sense of smell in the in olfactory system).
• Interneurons act as "speed bumps" in the chain of signal transmission to modulate information sent between neurons.
• Various specialized neurons in different areas of the brain, such as Purkinje fibers and pyramidal cells.  

In myelinated neurons, the action potential moves smoothly between the nodes of Ranvier because the myelin sheath prevents depolarization of the membrane between the nodes. The reason the nodes are spaced as they are is that a closer spacing would slow the transmission down to prohibitive speeds, while a greater spacing would risk the "dying out" action potential before it reaches the next node.

Multiple sclerosis (MS) is a disease that affects between 2 and 3 million people worldwide. Despite being known since the mid-1800s, MS is without a cure as of 2019, largely because it is unknown just what causes the pathology seen in the disease. As loss of myelin in CNS neurons progresses over time, loss of neuron function predominates.

The disease can be managed with steroids and other medications; it is not fatal per se, but is extremely debilitating, and intensive medical research is underway to seek a cure for MS.