Neurons, also known as nerve cells, send and receive signals from your brain. While neurons have a lot in common with other types of cells, they’re structurally and functionally unique.

Neurons, also known as nerve cells, send and receive signals from your brain. While neurons have a lot in common with other types of cells, they’re structurally and functionally unique.

Specialized projections called axons allow neurons to transmit electrical and chemical signals to other cells. Neurons can also receive these signals via rootlike extensions known as dendrites.

A 2009 study estimated that the human brain houses about 86 billion neurons. The creation of new nerve cells is called neurogenesis. While this process isn’t well understood, we know that it’s much more active when you’re an embryo. However, 2013 evidence suggests that some neurogenesis occurs in adult brains throughout our lives.

As researchers gain insight into both neurons and neurogenesis, many are also working to uncover links to neurodegenerative diseases such as Alzheimer’s and Parkinson’s.

Illustrated diagram of the parts of a neuron. Share on Pinterest
Illustration by Sophia Smith

Neurons vary in size, shape, and structure depending on their role and location. However, nearly all neurons have three essential parts: a cell body, an axon, and dendrites.

Cell body

Also known as a soma, the cell body is the core section of the neuron. The cell body contains genetic information, maintains the neuron’s structure, and provides energy to drive activities.

Like other cell bodies, a neuron’s soma contains a nucleus and specialized organelles. It’s enclosed by a membrane that both protects it and allows it to interact with its immediate surroundings.

Axon

An axon is a long, tail-like structure. It joins the cell body at a specialized junction called the axon hillock. Many axons are insulated with a fatty substance called myelin. Myelin helps axons to conduct an electrical signal.

Neurons usually have one main axon.

Dendrites

Dendrites are fibrous roots that branch out from the cell body. Like antennae, dendrites receive and process signals from the axons of other neurons. Neurons can have more than one set of dendrites, known as dendritic trees.

How many they have generally depends on their role. For instance, Purkinje cells are a special type of neuron found in a part of the brain called the cerebellum. These cells have highly developed dendritic trees which allow them to receive thousands of signals.

Illustrated diagram of the different types of neurons and their shapes.Share on Pinterest
Illustration by Sophia Smith

Neurons vary in structure, function, and genetic makeup. Given the sheer number of neurons, there are thousands of different types, much like there are thousands of species of living organisms on Earth.

However, there are five major neuron forms. Each combines several elements of the basic neuron shape.

  • Multipolar neurons. These neurons have a single axon and symmetrical dendrites that extend from it. This is the most common form of neuron in the central nervous system.
  • Unipolar neurons. Usually only found in invertebrate species, these neurons have a single axon.
  • Bipolar neurons. Bipolar neurons have two extensions extending from the cell body. At the end of one side is the axon, and the dendrites are on the other side. These types of neurons are mostly found in the retina of the eye. But they can also be found in parts of the nervous system that help the nose and ear function.
  • Pyramidal neurons. These neurons have one axon but several dendrites to form a pyramid type shape. These are the largest neuron cells and are mostly found in the cortex. The cortex is the part of the brain responsible for conscious thoughts.
  • Purkinje neurons. Purkinje neurons have multiple dendrites that fan out from the cell body. These neurons are inhibitory neurons, meaning they release neurotransmitters that keep other neurons from firing.

In terms of function, scientists classify neurons into three broad types: sensory, motor, and interneurons.

Sensory neurons

Sensory neurons help you:

  • taste
  • smell
  • hear
  • see
  • feel things around you

Sensory neurons are triggered by physical and chemical inputs from your environment. Sound, touch, heat, and light are physical inputs. Smell and taste are chemical inputs.

For example, stepping on hot sand activates sensory neurons in the soles of your feet. Those neurons send a message to your brain, which makes you aware of the heat.

Motor neurons

Motor neurons play a role in movement, including voluntary and involuntary movements. These neurons allow the brain and spinal cord to communicate with muscles, organs, and glands all over the body.

There are two types of motor neurons: lower and upper. Lower motor neurons carry signals from the spinal cord to the smooth muscles and skeletal muscles. Upper motor neurons carry signals between your brain and spinal cord.

When you eat, for instance, lower motor neurons in your spinal cord send signals to the smooth muscles in your esophagus, stomach, and intestines. These muscles contract, which allows food to move through your digestive tract.

Interneurons

Interneurons are neural intermediaries found in your brain and spinal cord. They’re the most common type of neuron. They pass signals from sensory neurons and other interneurons to motor neurons and other interneurons. Often, they form complex circuits that help you to react to external stimuli.

For instance, when you touch something sharp like a cactus, sensory neurons in your fingertips send a signal to interneurons in your spinal cord. Some interneurons pass the signal on to motor neurons in your hand, which allows you to move your hand away. Other interneurons send a signal to the pain center in your brain, and you experience pain.

Neurons send signals using action potentials. An action potential is a shift in the neuron’s potential electric energy caused by the flow of charged particles in and out of the membrane of the neuron. When an action potential is generated, it’s carried along the axon to a presynaptic ending.

Action potentials can trigger both chemical and electrical synapses. Synapses are locations where neurons can pass these electrical and chemical messages between them. Synapses are made up of a presynaptic ending, a synaptic cleft, and a postsynaptic ending.

Chemical synapses

In a chemical synapse, the neuron releases of chemical messengers called neurotransmitters. These molecules cross the synaptic cleft and bind to receptors in the postsynaptic ending of a dendrite.

Neurotransmitters can trigger a response in the postsynaptic neuron, causing it to generate an action potential of its own. Alternatively, they can prevent activity in the postsynaptic neuron. In that case, the postsynaptic neuron doesn’t generate an action potential.

Electrical synapses

Electrical synapses can only excite. These synapses form when two neurons are connected by a gap junction. This gap is much smaller than a chemical synapse and is made up of ion channels that help transmit a positive electrical signal.

Because of the way these signals travel, signals move much faster across electrical synapses than chemical synapses. However, these signals can diminish from one neuron to the next. This makes them less effective at transmitting repeated signals.

While research has advanced our understanding of neurons in the last century, there’s still much we don’t understand.

For instance, until recently, researchers believed that neuron creation occurred in adults in a region of the brain called the hippocampus. The hippocampus is involved in memory and learning.

But a 2018 study found that the production of neurons in the hippocampus actually drops after birth. This means that virtually no new neurons are created there by adulthood.

Experts considered this discovery a setback in terms of using neurogenesis to treat diseases like Alzheimer’s and Parkinson’s. These conditions are both the result of neuron damage and death.

There is still hope, though, that neural stem cells can be used to make new neurons. Neural stem cells can produce new neurons, according to the National Institute of Neurological Disorders and Stroke. But researchers are still trying to figure out the best way to use these stem cells to produce specific types of neurons in the laboratory setting.

If this can be done, these nerve cells could be created to replace those that are lost through age, damage, and disease.

Ongoing clinical trials

Many clinical trials are currently underway to test the use of newly created neural cells. For example, this one targets people who have had an ischemic stroke.

Also, a 2019 study used fluorescent probes to observe real-time activity in the nerve cells of mice. This technology could be used to help map brain activity, uncover the problems that lead to neurological disorders, and advance the field of artificial intelligence.

Educational resources

Want to see how much you learned today? Use the resources below to quiz yourself (or your students) on the anatomy and different types of neurons.

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Quiz yourself on the anatomy of a neuron!

Nervous system cells are called neurons. They have three distinct parts, including a cell body, axon, and dendrites. These parts help them to send and receive chemical and electrical signals.

While there are billions of neurons and thousands of varieties of neurons, they can be classified into three basic groups based on function. These are motor neurons, sensory neurons, and interneurons.

There’s still a lot we don’t know about neurons and the role they play in the development of certain brain conditions. But many research projects and clinical trials are underway to try and find those answers.