All you need to know about neurons

Neurons are responsible for carrying information throughout the human body. Using electrical and chemical signals, they help coordinate all of the necessary functions of life. In this article, we explain what neurons are and how they work.

In short, our nervous systems detect what is going on around us and inside of us; they decide how we should act, alter the state of internal organs (heart rate changes, for instance), and allows us to think about and remember what is going on. To do this, it relies on a sophisticated network — neurons.

It has been estimated that there are around 86 billion neurons in the brain; to reach this huge target, a developing fetus must create around 250,000 neurons per minute.

Each neuron is connected to another 1,000 neurons, creating an incredibly complex network of communication. Neurons are considered the basic units of the nervous system.

Because they are

Neurons, sometimes called nerve cells, make up around 10 percent of the brain; the rest consists of glial cells and astrocytes that support and nourish neurons.

What do neurons look like?

Neurons can only be seen using a microscope and can be split into three parts:

Soma (cell body) — this portion of the neuron receives information. It contains the cell's nucleus.

Dendrites — these thin filaments carry information from other neurons to the soma. They are the "input" part of the cell.

Axon — this long projection carries information from the soma and sends it off to other cells. This is the "output" part of the cell. It normally ends with a number of synapses connecting to the dendrites of other neurons.

Both dendrites and axons are sometimes referred to as nerve fibers.

Axons vary in length a great deal. Some can be tiny, whereas others can be over 1 meter long. The longest axon is called the dorsal root ganglion (DRG), a cluster of nerve cell bodies that carries information from the skin to the brain. Some of the axons in the DRG travel from the toes to the brain stem — up to 2 meters in a tall person.

Types of neurons

Neurons can be split into types in different ways, for instance, by connection or function.

Connection

Efferent neurons — these take messages from the central nervous system (brain and spinal cord) and deliver them to cells in other parts of the body.

Afferent neurons — take messages from the rest of the body and deliver them to the central nervous system (CNS).

Interneurons — these relay messages between neurons in the CNS.

Function

Sensory — carry signals from the senses to the CNS.

Relay — carry signals from one place to another within the CNS.

Motor — carry signals from the CNS to muscles.

How do neurons carry a message?

If a neuron receives a large number of inputs from other neurons, these signals add up until they exceed a particular threshold.

Once this threshold is exceeded, the neuron is triggered to send an impulse along its axon — this is called an action potential.

An action potential is created by the movement of electrically charged atoms (ions) across the axon's membrane.

Neurons at rest are more negatively charged than the fluid that surrounds them; this is referred to as the membrane potential. It is usually -70 millivolts (mV).

 Source: Medical News Today