NEURONS
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.
At birth, the human brain consists of an estimated 100 billion neurons . Unlike other cells, neurons don’t reproduce or regenerate. They aren’t replaced once they die.
The creation of new nerve cells is called neurogenesis. While this process isn’t well understood, it may occur in some parts of the brain after birth.
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.
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 neuron’s core. The cell body carries 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 which both protects it and allows it to interact with its immediate surroundings.
Axon
An axon is a long, tail-like structure which 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 generally 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 the cerebellum. These cells have highly developed dendritic trees which allow them to receive thousands of signals.
Neurons send signals using action potentials. An action potential is a shift in the neuron’s electric potential caused by the flow of ions in and out of the neural membrane.
Action potentials can trigger both chemical and electrical synapses.
Chemical synapses
In a chemical synapse, action potentials affect other neurons via a gap between neurons called a synapse. Synapses consist of a presynaptic ending, a synaptic cleft, and a postsynaptic ending.
When an action potential is generated, it’s carried along the axon to a presynaptic ending. This triggers the release of chemical messengers called neurotransmitters. These molecules cross the synaptic cleft and bind to receptors in the postsynaptic ending of a dendrite.
Neurotransmitters can excite the postsynaptic neuron, causing it to generate an action potential of its own. Alternatively, they can inhibit the postsynaptic neuron, in which case it doesn’t generate an action potential.
Electrical synapses
Electrical synapses can only excite. They occur when two neurons are connected via a gap junction. This gap is much smaller than a synapse, and includes ion channels which facilitate the direct transmission of a positive electrical signal. As a result, electrical synapses are much faster than chemical synapses. However, the signal diminishes from one neuron to the next, making them less effective at transmitting.
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.
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 the 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 hot, 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.
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