Neuron Structure and Function

Neuron Structure and Function: Psychology

Neurons are specialised cells found in the nervous system, and they number in the billions. The brain itself contains around 86 billion neurons alone, and they form a dense network of communication, hence why we refer to them as nerve cells or brain cells.

Neurons relay information not only in the brain but throughout the body. The neurons take in information from the outside world through the senses and relay information from the brain to muscles and effector organs, which makes all movement and communication with the outside world possible.

What is the Structure and Function of a Neuron?

All cells begin as embryonic stem cells. Later they begin to differentiate, i.e., they develop different forms according to their function in the body. The neuron has a membrane, nucleus, and cytoplasm like other animal cells. However, the cell structure of a neuron differs in that its shape is specialised for the transmission of information – it generally has an input (the dendrites) and an output (the axon and nerve terminals, also known as terminal buttons).

Fig. 2 - A neuron is composed of a cell body, axon, and dendrites.

Neuronal Structure and Function: Dendrites

Dendrites are branched structures that grow from the cell body. They have synapses that receive information from nearby nerve cells, and they too can be specialised in their function.

Neuronal Structure and Function: Axon

The axon is the long part of a neuron along which impulses travel from the cell body to other cells. An axon can be between a few micrometres and a metre long in humans.

The nerve impulse always travels away from the cell body across the axon to the thick parts at the end of the axon. They are called terminal buttons or nerve terminals. It can never travel from the axon terminal to the cell body – nerve impulses are unidirectional. This is because of the way nerve impulses travel, called the action potential.

Axons can also branch, but not as much as dendrites. These branches are called collaterals. Where the axon terminals or buttons meet another cell is called a synapse. Through the synapse, nerve impulses are transmitted from one neuron to the next through neurotransmitter release.

Fig. 3 - (a) The synaptic cleft is the space between the terminal button of one neuron and the dendrite of another neuron. (b) In this pseudo-coloured image from a scanning electron microscope, a terminal button (green) has been opened to reveal the synaptic vesicles (orange and blue) inside.

Neuronal Structure and Function: Myelin Sheath

Axons are often coated with a fatty compound called myelin.

Myelin also speeds up the transmission of nerve impulses. The more myelin wrapped around an axon, the faster the electrical impulse is sent to the next cell. Myelin is made up of glial cells that wrap around the axon. The parts of the axon where there are gaps in the myelin sheaths are called nodes of Ranvier. Action potentials jump from node to node in myelinated neurons, and conduction is faster than in an unmyelinated neuron as a result.

The Structural and Functional Classification of Neurons

Different types of neurons exist in the body, which can be classified either by their appearance (i.e. their structure) or by their function. We will first look at the structural classification of neurons and then the functional classification.

The structural classification sorts neurons into types based on how many axons and dendrites a neuron has. Some of these neurons are only found in certain organisms or in certain parts of the body.

The functional classification of neurons categorises them according to how they work in the body. The main function of all neurons is to transmit information, either into, within, or out of the body. The function of a particular neuron depends on its type and location.

Motor, Relay and Sensory Neuron Structure and Function

As we discussed above, neurons can differ structurally, and thus, their function can also differ. There are three classifications of neurons: motor neurons, relay neurons or interneurons and sensory neurons.

Fig. 4. Morphology of the different types of neurons.

• Sensory neurons gather information from receptors and send it to the brain and spinal cord (the central nervous system, CNS).
• Relay neurons (interneurons) connect sensory neurons to motor neurons, allowing impulses to travel between the two. They are characterised by having short axons.
• Motor neurons send the information back from the brain and spinal cord (CNS) to effectors (muscle and effector organs).

Fig. 5 - Sensory neurons register heat and relay neurons pass this information to motor neurons to cause muscles to contract and pull away, the reflex arc.

Ideally, the three types of neurons work together smoothly. If any one of them is disturbed, it would lead to serious diseases in the organism. The transmission of nerve impulses through these three types of neurons is the process involved in all actions, including reflexes.

What is the Function of Sensory Neurons?

Sensory neurons in the body take physical information such as light, and pressure in the form of sound, touch, temperature, or chemical information and convert it into information that the brain can process.

Damage to sensory neurons can be problematic to the body, as in the case of individuals with congenital insensitivity to pain with anhidrosis (CIPA), a disease in which sufferers cannot feel pain.

Loss of sensory receptors is also the cause of many types of disabilities – blindness, deafness, and anosmia (in which people cannot smell). People with these disabilities have to navigate their environment differently than most.

What is the Function of Relay Neurons?

Relay neurons are located in the brain and spinal cord. Their function is to connect sensory neurons to motor neurons. Their dendrites and axons are usually relatively short because they do not have to span long distances, and they are not myelinated.

In the central nervous system, different ways of relaying nerve impulses can be present simultaneously.

• When you are frightened, there may be one relay that makes you pull back quickly and unconsciously (that's a reflex), but another that consciously tells you that you are not in immediate danger when your dog licks your face.

Most diseases that affect relay neurons, such as Alzheimer's, Parkinson's, and Huntington's disease, are undergoing extensive research and are incurable at the moment. Relay neurons are so vital that damage to them can irreversibly alter a person's personality (as in a stroke or brain haemorrhage) or even lead to death.

What is the Function of Motor Neurons?

Motor neurons set the body in motion, just as a motor moves a machine.

• They transmit nerve impulses from the brain or spinal cord to a muscle or gland.
• They send impulses to the muscles, causing them to contract or relax.

The brain constantly sends nerve impulses to the body via motor neurons. These neurons have some of the longest axons in the human body, extending from the spine to the foot.

When motor neurons are damaged, people have trouble moving or controlling vital functions such as breathing, chewing, and swallowing. Muscle movement and coordination may be impaired, and sufferers may have twitching limbs or be paralysed. This is the case with ALS and other motor neuron diseases such as multiple sclerosis.