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1.1. The Neuron
Introducing the 'Neuron'
A neuron is the standard communicating and processing cell of the nervous system. To fulfil its role, it can conduct electrical impulses. A neuron can carry sensory information, motor information or other information. Neurons exist throughout the brain, spinal cord and all the nerves in the rest of the body. Neurons frequently communicate and work together. Take the following example. Imagine you are leaving for school or work one morning but as soon as you step outside you realise it's freezing cold. The sensory neurons in the skin of your body (particularly your hands) detect the temperature of the cold air and they carry this information to the sensory area of the brain. From here, interneurons carry information to other parts of the brain that allow you to remember times in the past when you were cold and help you to realise the importance of wrapping up warm. Neurons in the advanced brain centres enable you to make a decision to return indoors to put on a thicker coat and gloves. Motor neurons are then activated, and these carry electrical impulses to the muscles in your legs that will stimulate the muscles to contract and enable you to walk back inside to wrap up warm.
Features of a Neuron
Most neurons have a standard set of features, and these are described below (see Figure 1.1.1.).
Cell body (aka cell soma)
The cell body contains the all-important, DNA-containing nucleus of the neuron. The cell body communicates with the rest of the cell by using transport proteins that run along the axon.
Axon
This is the long thin tube-like part of the neuron (in fact, it can be up to 1m long, as is seen in the sciatic nerve in the leg!) The membrane of the axon has lots of ion pumps and ion channels that allow it to conduct electrical impulses (see Section 1.3. Action Potentials). As mentioned already, the axon contains transport proteins too.
Ion pumps and channels
These are embedded in the axonal membrane and allow the movement of particular ions into or out of the axon. Ion pumps are described as active because they require energy (via ATP) to function. Ion channels tend to be passive and not require energy because they allow diffusion down their concentration gradient. The main ions that are transported are sodium and potassium ions.
Dendrites
These are spiky projections with many branches that receive electrical impulses from other neurons and carry them into the cell body or axon.
Presynaptic terminal
After travelling along the axon, the electrical impulse arrives at the presynaptic terminal. Here, it will be converted into a chemical message and released from the neuron. This chemical message will then be transmitted into an electrical message in another neuron (see Section 1.4. Synapses).
Myelin Sheaths
These structures (not shown in Figure 1.1.1.) are present on most but not all nerves. Myelin is an insulating substance that wraps around most of the axon, leaving a few gaps. This layout forces the electrical impulse to skip some ion channels and 'jump' to the next gap – this speeds up the conduction of an electrical impulse along the axon. Myelin sheaths are formed by Schwann cells.
Cell body (aka cell soma)
The cell body contains the all-important, DNA-containing nucleus of the neuron. The cell body communicates with the rest of the cell by using transport proteins that run along the axon.
Axon
This is the long thin tube-like part of the neuron (in fact, it can be up to 1m long, as is seen in the sciatic nerve in the leg!) The membrane of the axon has lots of ion pumps and ion channels that allow it to conduct electrical impulses (see Section 1.3. Action Potentials). As mentioned already, the axon contains transport proteins too.
Ion pumps and channels
These are embedded in the axonal membrane and allow the movement of particular ions into or out of the axon. Ion pumps are described as active because they require energy (via ATP) to function. Ion channels tend to be passive and not require energy because they allow diffusion down their concentration gradient. The main ions that are transported are sodium and potassium ions.
Dendrites
These are spiky projections with many branches that receive electrical impulses from other neurons and carry them into the cell body or axon.
Presynaptic terminal
After travelling along the axon, the electrical impulse arrives at the presynaptic terminal. Here, it will be converted into a chemical message and released from the neuron. This chemical message will then be transmitted into an electrical message in another neuron (see Section 1.4. Synapses).
Myelin Sheaths
These structures (not shown in Figure 1.1.1.) are present on most but not all nerves. Myelin is an insulating substance that wraps around most of the axon, leaving a few gaps. This layout forces the electrical impulse to skip some ion channels and 'jump' to the next gap – this speeds up the conduction of an electrical impulse along the axon. Myelin sheaths are formed by Schwann cells.
Types of Neurons
Most neurons are multipolar neurons. This means they have lots of dendrites feeding into the cell body, and an axon leaving the cell body. One of the benefits of having a multipolar neuron is its ability to integrate impulses from lots of other neurons. Think about it - if ten neurons each synapse onto a single dendrite of one multipolar cell, they will massively increase the signal reaching the cell body. this is particularly important in scenarios such as auditory processing. The other two types of neurons are described below.
Unipolar neurons (see Figure 1.1.2, top)
In these neurons, the cell body is attached to the rest of the neuron by only one short axon. A good example is a sensory neuron in a spinal nerve (see Section 3.1. Spinal Nerves). Side note: technically, the neuron shown here as 'unipolar' is actually classified as a 'pseudounipolar' neuron. This is because it has one very short axon that links to two other axons - it is kind of like a bipolar neuron in disguise.
Bipolar neurons (see Figure 1.1.2., bottom)
These neurons have two axons, one entering the cell body and another leaving the cell body. A good example is a neuron found in the retina of the eye.
Unipolar neurons (see Figure 1.1.2, top)
In these neurons, the cell body is attached to the rest of the neuron by only one short axon. A good example is a sensory neuron in a spinal nerve (see Section 3.1. Spinal Nerves). Side note: technically, the neuron shown here as 'unipolar' is actually classified as a 'pseudounipolar' neuron. This is because it has one very short axon that links to two other axons - it is kind of like a bipolar neuron in disguise.
Bipolar neurons (see Figure 1.1.2., bottom)
These neurons have two axons, one entering the cell body and another leaving the cell body. A good example is a neuron found in the retina of the eye.
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