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. 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. 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. Scientists group them in different ways. Some neurons have long arms called axons that carry signals far away. Others have shorter arms. All neurons have a main body called the soma. 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. Others have one axon and one dendrite and are called bipolar. Multipolar neurons have one axon and several dendrites and are the most common type. There are also neurons called anaxonic and pseudounipolar.

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. 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 or by which neurotransmitters they use, such as acetylcholine, noradrenaline, GABA, glutamate, dopamine, serotonin, ATP, or histamine.

Connectivity

Main articles: Synapse and Chemical synapse

Neurons talk to each other using special points called synapses. These points can touch where one neuron's ending meets another neuron's branch or body. Some neurons have many branches and connect with thousands of other cells. Others have just a few branches but still connect with many cells.

When a signal reaches the end of a neuron's branch, it can open special gates. This lets in tiny particles called calcium ions. This causes small bags to release message carriers. 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 many connections that change as the child grows up.

Nonelectrochemical signaling

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

Mechanisms for propagating action potentials

Main article: Action potential

Scientists study neurons using special cells from squids. These cells are bigger and easier to examine. They help us learn how neurons send electrical signals.

Neurons have special parts in their membranes that let them create and send these signals. Things like touch or chemicals 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. This coating helps the signals travel faster.

Neural coding

Neural coding is how neurons in the brain show us information from our senses and other places. Scientists study how something we see, hear, or feel links to how neurons react. They also watch how groups of neurons work as a team. Neurons can share information in two ways: like simple on/off signals or in changing levels, like turning a volume knob.

All-or-none principle

Main article: All-or-none law

Nerve impulses follow a simple rule: when a neuron sends a signal, it sends it fully. Stronger things 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 things in different ways. Some receptors, called tonic receptors, keep firing as long as the thing is there. They often fire more often when the thing gets stronger. Other receptors, called phasic receptors, stop firing when the thing 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 we 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 is the main part of the nervous system. We learned about this in the late 1800s because of a Spanish scientist named Santiago Ramón y Cajal.

To see the shape of neurons, Ramón y Cajal improved a silver staining process that was created by Camillo Golgi. We still use this process today.

In 1888, Ramón y Cajal studied bird brains. He said he could not find links between the parts of neurons that send and receive signals. He thought each part of the nervous system was its own small area. This idea is called the neuron doctrine. It is very important in modern neuroscience.

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

These silver stains help scientists study the brain. They show the full shape of some neurons.

Neuron doctrine

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

Later discoveries added more details. For example, glial cells, which are not neurons, help with processing information. Some neurons connect directly, and some can come very close together.

Ramón y Cajal suggested that neurons receive signals at one end and send them out from another end. But there are exceptions. 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. These branches can be studied as separate areas. This helps us understand how very small neurons work, like those in tiny creatures such as Drosophila melanogaster.

Compartmental modelling of dendrites can help us learn 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. This 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 can change how our nerves work. Charcot–Marie–Tooth disease is a condition that can make it hard to feel and move, especially in the feet and legs. Alzheimer's disease is a condition where memory and thinking skills slowly get worse. Parkinson's disease makes it hard to control movements and speak clearly. Myasthenia gravis can make muscles feel weak and tired.

Demyelination happens when the protective layer around nerves breaks down. This can happen in diseases like multiple sclerosis. If nerves get hurt, they might stop working well. These conditions show how important healthy nerves are for our bodies.

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 helps 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 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. But a neuron cannot be replaced by a different kind of neuron. Each type of neuron is special and cannot be swapped for another.