In biopsychology, the idea is that if we better understand biological structures and functions, we may be able to unravel the mystery of how the mind and soul (in Greek, 'psyche') work. Although many used to think that the mind resided in the heart or liver, we now know that the brain controls the body. Let us take a closer look at the cells that make up the brain. There are two main types of cellular components in the nervous system, neurons and glial cells. Neuron structure and function establish how neurons operate within the body.
- First, we will discuss the structural and functional classification of neurons, delving into the world of neurons and how they differ.
- We will cover neuron structure and function in psychology.
- Paying particular attention to motor, relay, and sensory neurons' structure and function, we will explore their differences.
- Neuron cell structure and function will be touched upon throughout the explanation to illustrate our points, alongside diagrams.
Fig. 1 - Neuron structure and function can differ.
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 transmit nerve impulses. They are the specialised cells of the nervous system, and neurons send, integrate, and receive information from and to other neurons or non-neuronal structures (effectors).
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.
Without your brain controlling the movement of your eye muscles, for example, you would not be able to read this text.
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).
All neurons have a cell body, axon, and dendrites.
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.
The word comes from the Greek 'dendron', meaning tree.
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 sheaths insulate the electrical activity of the axon to prevent electrical interference with other nerve impulses in the densely packed neuron network of the central nervous system.
It is similar to the rubber insulation that wraps around the wires of your phone charging cord.
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.
Unipolar neurons have only one process extending from the cell body. They are found in invertebrates.
Bipolar neurons have a single axon and dendrite, and the cell body is placed centrally.
Multipolar neurons have multiple dendrites, a cell body and an axon. They are the most common neurons in 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.
Let us consider a scenario:
- You feel something wet on your cheek.
- Sensory neurons send a nerve impulse into your body, also called an afferent nerve impulse.
- Your brain decides how to respond.
- Relay neurons pass nerve impulses in the central nervous system from one neuron to another. This is where conscious and automatic decisions are made about what action to take. A command is sent to your muscles. They contract to pull your head away from your bad/friendly dog.
- Motor neurons send nerve impulses to the muscles to move away from the surprising stimulus, which is also called an efferent nerve impulse.
Afferent nerve impulses travel to the CNS, and efferent nerve impulses travel away from the CNS.
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.
This conversion of physical or chemical information into electrochemical information of the brain is called transmutation in biology. You can think of it as a kind of energy conversion.
For example, human skin has different receptors for heat, cold, pain, hard pressure, and gentle pressure. When the skin is exposed to a sudden hot temperature (e.g. fire on a stove), the information from a thermoreceptor ('hot') is converted into an electrical signal that travels along the sensory neuron to the brain.
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.
Individuals with CIPA cannot avoid dangerous stimuli in the environment, as they cannot sense if something is causing them pain. Most of the time, these people have to lead a conscientious life.
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.
'Receptors' is a confusing term in biopsychology because it's used to describe sensory neuron cells and smaller structures on the cell membrane that react to certain types of neurotransmitter molecules.
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.
Decisions made consciously and unconsciously are relayed to the motor neurons for execution. Think of it as a command to the muscles that is transmitted through the relay neurons.
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.
All of life is movement – heartbeat, the diaphragm moving up and down to create breathing in the lungs, the conscious muscle movement muscles in your legs when you walk to the kitchen in the morning.
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.
Neuron structure and function - Key takeaways
- Neurons transmit nerve impulses. They are the specialised cells of the nervous system, and neurons send, integrate, and receive information from and to other neurons or non-neuronal structures (effectors).
- Like other cells, neurons have a cell membrane, a cell body, and a nucleus.
- Unlike other cells, neurons have dendrites and an axon. Dendrites are branched structures that act as the input of the neuron. The axon is the long, thin output of the neuron.
- There are three types of neurons: sensory neurons, relay neurons, and motor neurons. Sensory neurons relay information to the central nervous system. Relay neurons connect sensory and motor neuron cells in the central nervous system. Motor neurons relay information from the central nervous system to the periphery.
- Overall, neurons can differ both in structure and function.
References
- Fig. 5: Explicative diagram of reflex arc by MartaAguayo, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
- Fig. 3: Diagram of synaptic transmission by Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020). 3.2 Cells of the Nervous System. In Psychology 2e. OpenStax. https://openstax.org/books/psychology-2e/pages/3-2-cells-of-the-nervous-system is licensed by https://creativecommons.org/licenses/by/4.0/ (CC By 4.0) (credit b: modification of work by Tina Carvalho, NIH-NIGMS; scale-bar data from Matt Russell)
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