Neuron

A neuron (American English), neurone (British English), or nerve cell, is a special kind of cell that helps our bodies send and receive messages. These cells are very important because they are the building blocks of the nervous system, which controls many things we do, like moving and thinking. Neurons can send electrical signals called action potentials to talk to other cells. They do this through tiny connections called synapses, using tiny amounts of special chemicals called neurotransmitters.

Neurons are found in all animals except for simple creatures like sponges and placozoans. They are the main parts of nervous tissue and help us feel things like touch, sound, and light. There are three main types of neurons: sensory neurons that bring information from our senses to the brain, motor neurons that tell our muscles to move, and interneurons that connect neurons inside the brain and spinal cord.

Each neuron has parts like a nucleus and mitochondria, which are found in other cells too, but neurons also have special parts like dendrites that receive signals and an axon that sends signals out. These cells work together in groups called neural circuits to help our brains and bodies function properly.

Nervous system

Neurons are the main parts of the nervous system. They work with special support cells called glial cells. The nervous system has two big parts: the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, which has the autonomic, enteric, and somatic nervous systems. Most neurons are in the central nervous system, but some are in areas outside it, like in sense organs such as the retina and cochlea.

Neurons have long parts called axons. In the peripheral nervous system, these axons can group together to form nerves, like wires in a cable. Inside the central nervous system, groups of axons are called nerve tracts.

Anatomy and histology

Neurons are special cells that help our bodies send signals. They come in many shapes and sizes, but they all have parts that help them do their job.

• The soma is the main part of the neuron. It contains the nucleus, which helps make important materials for the cell.
• The dendrites are branches that receive signals from other cells.
• The axon is a long, thin part that sends signals away from the soma. It can be very long, even stretching from the spine to the toes in humans. The end of the axon, called the axon terminal, helps send signals to other cells using special chemicals.

Neurons are wrapped in a thin layer called the plasma membrane, which helps control what goes in and out of the cell. Inside the neuron, there are tiny structures that help make proteins and support the cell's shape. These include things like neurofilaments and neurotubules.

Classification

Neurons come in many shapes and sizes, and scientists group them in different ways. One way looks at how they are built. Some neurons have long arms called axons that carry signals far away, while others have shorter ones. These long-armed neurons can be further divided depending on where their main body, called the soma, is located. They all have a soma and a long axon wrapped in a special covering called myelin. Around the soma are branches called dendrites that pick up signals from other neurons. The axon ends in tiny branches that release chemicals called neurotransmitters into a tiny gap between cells, called the synaptic cleft, to talk to the next neuron.

Structural classification

Most neurons can also be grouped by their shape. Some have just one branch and are called unipolar, and they are usually involved in sensing things like touch. Others have one axon and one dendrite and are called bipolar, found in places like the nose and eyes. Multipolar neurons have one axon and several dendrites and are the most common type. There are also neurons called anaxonic where the axon and dendrites look the same, and pseudounipolar neurons which have one branch that works as both an axon and a dendrite.

Functional classification

Neurons can also be grouped by what they do. Some carry information from the body to the brain and are called afferent or sensory neurons. Others carry commands from the brain to muscles or glands and are called efferent or motor neurons. Interneurons connect neurons to each other inside the brain.

Neurons talk to each other by releasing neurotransmitters, which can either excitement or calm down the next neuron, depending on the type of receiver on that neuron. The most common neurotransmitters in the brain are glutamate, which usually excites, and GABA, which usually calms down.

Neurons can also be grouped by how they fire their signals — some fire steadily, while others burst in groups. They can also be grouped by which neurotransmitters they use, such as acetylcholine, noradrenaline, GABA, glutamate, dopamine, serotonin, ATP, or histamine, each playing different roles in the brain and body.

Connectivity

Main articles: Synapse and Chemical synapse

Neurons talk to each other using special points called synapses. These points can be where one neuron's ending touches another neuron's branch, body, or, less often, another ending. Some neurons, like Purkinje cells in the cerebellum, can have over 1000 branches and connect with many thousands of other cells. Others, like magnocellular neurons in the supraoptic nucleus, have just one or two branches but still connect with thousands of other cells.

When a signal reaches the end of a neuron's branch, it can open special gates that let in tiny particles called calcium ions. This causes small bags filled with message carriers to release their contents. These carriers travel across a tiny space and tell the next neuron what to do.

The human brain has about eighty six billion neurons. Each neuron connects to about 7,000 other neurons on average. A young child's brain has about 1 quadrillion of these connections, which change as the child grows up.

Nonelectrochemical signaling

Besides electrical and chemical signals, neurons might also talk through other ways, like pushing and pulling on their branches or sharing special proteins. They can also be influenced by signals from the body and hormones. The brain's immune cells, called microglia, also talk to neurons using special touch points, helping to keep the neurons healthy.

Mechanisms for propagating action potentials

Main article: Action potential

Scientists use special cells from squids to study how neurons work because they are bigger and easier to examine. These cells help us understand how neurons send electrical signals.

Neurons have special parts in their membranes that let them create and send these electrical signals. Different things like touch, chemicals, or changes around the cell can make a neuron active. These signals travel through ions such as sodium, potassium, chloride, and calcium. Some neurons have a protective coating called myelin, made by other supporting cells, which helps the signals travel faster with less energy.

Neural coding

Neural coding is about how neurons in the brain represent information from our senses and other sources. Scientists study how a specific thing we see, hear, or feel connects to how neurons respond. They also look at how groups of neurons work together. Neurons might carry information in two ways: like on/off signals or in varying levels, similar to volume controls.

All-or-none principle

Main article: All-or-none law

Nerve impulses follow an all-or-none rule. This means that when a neuron sends a signal, it sends it fully. Stronger stimuli, like brighter lights or louder sounds, don't make the signal stronger. Instead, they can make the neuron fire more often.

There are different types of receptors that respond to stimuli in various ways. Some receptors, called tonic receptors, keep firing as long as the stimulus is present. They often fire more frequently when the stimulus gets stronger. Other receptors, called phasic receptors, stop firing when the stimulus stays the same. For example, when you touch your skin, neurons fire, but if the pressure stays the same, they stop firing.

The pacinian corpuscle is a special structure in the skin that helps detect pressure and vibration. It has layers like an onion around the nerve ending. When pressure is applied, it deforms and sends a signal. If the pressure stays the same, the signal stops. These neurons send a signal only when the pressure starts or stops.

Recent research suggests that while neurons either fire a signal or don’t, the strength and length of the signal can vary. This may allow neurons to carry more information than previously thought.

Etymology and spelling

A scientist named Heinrich Wilhelm Waldeyer first used the word neuron in 1891. He took the word from an old Greek word meaning "sinew, cord, nerve."

In French, the word was spelled neurone. Some writers in English also used this spelling, but today it is mostly used in America and is less common in British English. Before this, some people used the phrase "nerve cell" to describe these tiny parts of the body.

History

Further information: History of neuroscience

The neuron's role as the main part of the nervous system was first understood in the late 1800s thanks to the work of the Spanish scientist Santiago Ramón y Cajal.

To see the shape of individual neurons, Ramón y Cajal improved a silver staining process that was created by Camillo Golgi. This improved process is still used today.

In 1888, Ramón y Cajal wrote about the bird brain and said that he could not find links between the parts of neurons that send and receive signals. He called each part of the nervous system its own small area. This idea became known as the neuron doctrine, a key part of modern neuroscience.

In 1891, a German scientist named Heinrich Wilhelm Waldeyer wrote an important review of the neuron doctrine and used the word neuron to describe the working unit of the nervous system.

These silver stains help scientists study the brain because they show the full shape of some neurons without mixing them with others.

Neuron doctrine

The neuron doctrine is the important idea that neurons are the main building blocks of the nervous system. This idea was started by Santiago Ramón y Cajal in the late 1800s. It says that neurons are separate cells, not connected like a net, and each one works on its own.

Later discoveries added more details to this idea. For example, glial cells, which are not neurons, help with processing information. Also, some neurons connect directly, and some can even join together very closely.

Ramón y Cajal also suggested that neurons receive signals at one end and send them out from another end. But there are exceptions to this, as some parts can both receive and send signals.

Compartmental modelling of neurons

Even though neurons are called the basic parts of the brain, they do complex work inside. Neurons collect information in their branches, and this detail is often missed in simple models. These branches can be studied as separate areas, which helps us understand how very small neurons work, like those in tiny creatures such as Drosophila melanogaster.

Compartmental modelling of dendrites can be useful for learning about neurons that are too small to measure with tools.

Neurons in the brain

The number of neurons in the brain changes a lot between different animals. In humans, there are about 10 to 20 billion neurons in the cerebral cortex and 55 to 70 billion in the cerebellum. In comparison, a tiny worm called the nematode Caenorhabditis elegans has only 302 neurons, which makes it easy for scientists to study. Another common subject in experiments, the fruit fly Drosophila melanogaster, has around 100,000 neurons and can show many complicated behaviors. Scientists can learn about more complex animals by studying these simpler ones because many features of neurons are the same across different species.

Neurological disorders

Main article: Neurological disorders

Some health problems affect the way our nerves work. Charcot–Marie–Tooth disease is a common inherited condition that causes loss of muscle and touch feeling, especially in the feet and legs. Alzheimer's disease is another condition where memory and thinking skills slowly get worse over time. Parkinson's disease affects movement, making it hard to control muscles and speak clearly. Myasthenia gravis causes muscles to become weak and tired easily.

Demyelination is when the protective covering around nerves breaks down, which can disrupt signals between parts of the body. This can happen in diseases like multiple sclerosis. When nerves get injured, they can also break apart and stop working properly, a process called axonal degeneration. These conditions show how important healthy nerves are for our bodies to function well.

Development

Main article: Neurogenesis

Neurons grow and form through a process called neurogenesis. Special cells called neural stem cells split to become different types of neurons. Once they become neurons, they stop dividing.

These neurons start forming from a part of the embryo called the neural tube. The neural tube has layers that help create different parts of the brain. Big cells that help control muscles, called motor neurons, form first. Smaller cells that help sense things, along with other supporting cells called glial cell, form around the time of birth.

Some new neurons can still form in adults, but most of the brain’s neurons are made before we are born and stay for our whole lives. Scientists can also turn skin cells into neurons in a lab by giving them new roles.

Nerve regeneration

Main article: Neuroregeneration

If a part of the nerve called an axon is cut, it can grow back. However, a neuron cannot be replaced by a different kind of neuron. Each type of neuron is special and cannot be swapped for another.