Neurons and Their Role in the Nervous System

A neuron is a nerve cell that is the basic building block of the central nervous system and peripheral nervous system. Neurons are similar to other cells in the human body in a number of ways, but there is one key difference between neurons and other cells. Neurons are specialized in transmitting information throughout the body so we can walk, talk, and process information.

Overview

These highly specialized nerve cells are responsible for communicating information in both chemical and electrical forms. There are also several different types of neurons responsible for different tasks in the human body including sensory neurons, motor neurons, and interneurons. They come in different shapes and sizes depending on their specific location and purpose.

In this article, you will learn more about the structure and function of a neuron, how they communicate with each other, and how they are similar and different from other cells in our body.

Structure of a Neuron

There are three basic parts of a neuron: the cell body, the dendrites, and the axon. However, all neurons vary somewhat in size, shape, and characteristics depending on the function and role of the neuron.

The cell body (or soma) contains the nucleus and can be likened to a small factory that produces all of the proteins needed to keep the neuron healthy and functioning. The dendrites and axons extend from the cell body.

Dendrites are the extensions that branch off of the cell body and receive signals from other neurons. Some neurons have few dendritic branches, while others are highly branched in order to receive a great deal of information. For example, a single neuron in the brain can create thousands of connections with other neurons with its dendrites.

The axon extends from the cell body and is what we commonly refer to as a nerve fiber. The axon transmits information away from the cell body to the nerve ending. Most neurons have only one axon, and it is often covered in a fatty substance called myelin that insulates the nerve fiber and assists with transmitting the signal. Depending on the area of the body, some neurons have very short axons, while others can be quite long.

Function of a Neuron

The nervous system is comprised of sensory neurons, motor neurons, and interneurons, each having a unique function. They also work together to perform complex functions in the human body.

Sensory neurons (or afferent neurons) carry information from the sensory receptor cells located throughout the body such as the eyes, ears, and skin, to the brain for processing. Sensory neurons help you taste, see, hear, and smell. We can also feel touch, pressure, and temperature.

Motor neurons (or efferent neurons) transmit information from the brain to the muscles and glands of the body to take action. There are two types: upper motor neurons and lower motor neurons. Upper motor neurons originate in the primary motor cortex of the brain and travel down the spinal cord. The lower motor neurons continue the signal by extending from the spinal cord out to the target muscles and glands. For example, by activating the motor neurons of your muscle fibers, you can swat a fly, kick a ball, and chew your food.

Interneurons are responsible for communicating information between sensory and motor neurons via the spinal cord and brain. Complex movements such as walking and talking require the coordination of many muscles. This involves a sensory-motor feedback loop that allows for fine-tuning of gestures in real time. Interneurons also assist with reflexive actions, like pulling your hand off of the hot stove.

How Neurons Communicate

How do neurons transmit and receive information? In order for neurons to communicate, they need to transmit information both within the neuron and from one neuron to the next. This process utilizes both electrical signals as well as chemical messengers.

Electrical Signals

Electrical communication begins when the dendrites of a neuron receive a stimulus from an axon of another neuron. This triggers a change in the electrical charge of the cell membrane called depolarization, which continues to the cell body. Once the signal has arrived at the beginning of the axon, known as the axon hillock, if the impulse is strong enough, it will travel down the full length of the axon in the form of an electrical signal known as an action potential.

Chemical Messengers

Once an electrical impulse has reached the end of an axon (axon terminal or nerve ending), the information must be transmitted across the synaptic gap, the space between the axon terminal of one neuron and the dendrite of the adjoining neuron. The neuron sending the signal is called the presynaptic neuron. The receiving neuron is called the postsynaptic neuron.

In some cases, synapses allow electrical communication by the simple flow of ions between two neurons. However, the vast majority of synapses require chemical messengers (neurotransmitters) to be released into the synaptic gap, to be picked up by the receptors of the next neuron.

To stop communication between neurons, there are three processes that can take place. In a process known as reuptake, the neurotransmitters are reabsorbed by the presynaptic neuron to be reused. In the case of degradation, the neurotransmitters are broken down in the gap by enzymes. Other neurotransmitters will simply diffuse away from the synaptic gap.

Neurotransmitters

Neurotransmitters are an essential part of our everyday functioning. While it is not known exactly how many neurotransmitters exist, scientists have identified more than 100 of these chemical messengers.

Neurotransmitters are chemical messengers that are released from the axon terminals to cross the synaptic gap and reach the receptor sites of other neurons. When the neurotransmitters attach to their specific receptor site, like a lock and key, they either excite, inhibit, or modify the action of the postsynaptic neuron depending on what type of neurotransmitter has been received.

Excitatory neurotransmitters stimulate nervous system activity while inhibitory neurotransmitters do the opposite and dampen down nervous system activity. Modulatory neurotransmitters will coordinate the activity of other chemical messengers.

The following are just a few of the major neurotransmitters, their known effects, and disorders they are associated with.

Acetylcholine: Associated with memory, learning, and muscle contractions. A lack of acetylcholine in the brain is associated with Alzheimer’s disease.

Endorphins: Known as our "natural pain reliever," these chemical messengers are similar to opiate drugs such as morphine but are significantly stronger. The body releases endorphins in response to injury, fear, or trauma. Endorphins are associated with emotions, pain perception, sexual response, and maternal behavior.

Dopamine: Known as the "feel good chemical," dopamine is associated with pleasurable feelings, as well as motivation, mood, attention, and movement. Parkinson’s disease is one illness associated with the death of dopamine-producing cells in certain parts of the brain, whereas researchers have found strong links between schizophrenia and excessive amounts of dopamine in other parts of the brain. 

Serotonin: Plays a role in mood stabilization, learning and memory, blood clotting, digestion, bone health, and sleep. Serotonin has been implicated in several mental disorders such as depression, anxiety, and obsessive-compulsive disorders. Some of the associated physical health problems are gastrointestinal disorders, high blood pressure, cardiac arrhythmias, and a life-threatening condition called serotonin syndrome.

How Do Neurons Compare to Other Cells

Neurons are similar to other cells in the human body in a variety of ways, but there are also some very unique differences.

Similarities

• All cells in the body are surrounded by a membrane that protects the cell.
• The soma (cell body) of all cells in the body contains a nucleus that holds genetic information.
• The soma of all cells in the body contains the same types of organelles that support the life of the cell, including mitochondria, Golgi bodies, and cytoplasm.

Differences

• Neurons have specialized structures known as dendrites and axons, designed to receive and transmit information.
• Neurons release chemical messengers known as neurotransmitters into synapses, or the space between nerve cells, to communicate with each other.
• Generally, when neurons die they are not replaced, as other cells are in the body. Although researchers have discovered neurogenesis, or the formation of new neurons, does occur in one part of the brain (the hippocampus).
• Research has shown that new connections between neurons form throughout life as we experience and learn new things.
• If injured, neurons can reorganize and form new connections, a process known as brain plasticity.