Hello, dear reader God created for us the nervous system, a system that controls the actions we do The nervous system consists of the neuron that will be our conversation today
The nerve cell
Neurons are the cells that make up the brain and the nervous system. They are the fundamental units that send and receive signals which allow us to move our muscles, feel the external world, think, form memories and much more
The function of a neuron is to transmit nerve impulses along the length of an individual neuron and across the synapse into the next neuron.
Anatomy of a Neuron
The neuron contains the soma (cell body) from which extend the axon (a nerve fiber conducting electrical impulses away from the soma) and dendrites (tree-like structures that receive signals from other neurons). The myelin sheath is an insulating layer that forms around the axon and allows nerve impulses to transmit more rapidly along the axon.
Neurons do not touch each other, and there is a gap, called the synapse, between the axon of one neuron the dendrite of the next.
Neuron (Nerve Cells) Structure
The unique structure of neurons permits it to receive and
carry messages to other neurons and throughout the body.
Dendrites
Dendrites are the tree-root-shaped part of the neuron which are usually shorter and more numerous than axons. Their purpose is to receive information from other neurons and to transmit electrical signals towards the cell body.
Dendrites are covered in synapses, which allows them to receive signals from other neurons. Some neurons have short dendrites, whilst others have longer ones.
In the central nervous system, neurons are long and have complex branches that can allow them to receive signals from many other neurons.
For instance, cells called Purkinje cells which are found in the cerebellum have highly developed dendrites to receive signals from thousands of other cells.
Soma (Cell Body)
The soma, or cell body, is essentially the core of the neuron. The soma’s function is to maintain the cell and to keep the neuron functioning efficiently (Luengo-Sanchez et al., 2015).
The soma is enclosed by a membrane which protects it, but also allows it to interact with its immediate surroundings.
The soma contains a cell nucleus which produces genetic information and directs the synthesis of proteins. These proteins are vital for other parts of the neuron to function.
Axon
The axon, also called a nerve fiber, is a tail-like structure of the neuron which joins the cell body at a junction called the axon hillock.
The function of the axon is to carry signals away from the cell body to the terminal buttons, in order to transmit electrical signals to other neurons.
Most neurons just have one axon which can range in size from 0.1 millimeters to over 3 feet (Miller & Zachary, 2017). Some axons are covered in a fatty substance called myelin which insulates the axon and aids in transmitting signals quicker.
Axons are long nerve processes that may branch off to transfer signals to many areas, before ending at junctions called synapses.
Myelin Sheath
The myelin sheath is a layer of fatty material that covers the axons of neurons. Its purpose is to insulate one nerve cell from another and so to prevent the impulse from one neuron from interfering with the impulse from another. The second function of the myelin sheath is to speed up the conduction of nerve impulses along the axon.
The axons which are wrapped in cells known as glial cells (also known as oligodendrocytes and Schwann cells) form the myelin sheath.
The myelin sheath which surrounds these neurons has a purpose to insulate and protect the axon. Due to this protection, the speed of transmission to other neurons is a lot faster than the neurons that are unmyelinated.
The myelin sheath is made up of broken up gaps called nodes of Ranvier. Electrical signals are able to jump between the nodes of Ranvier which helps in speeding up the transmission of signals.
Axon Terminals
Located at the end of the neuron, the axon terminals (terminal buttons) are responsible for transmitting signals to other neurons.
At the end of the terminal button is a gap, which is known as a synapse. Terminal buttons hold vessels which contain neurotransmitters.
Neurotransmitters are released from the terminal buttons into the synapse and are used to carry signals across the synapse to other neurons. The electrical signals convert to chemical signals during this process.
It is then the responsibility of the terminal buttons to reuptake the excess neurotransmitters which did not get passed onto the next neuron.
Types of neuron
Sensory neuron
Sensory neurons are the nerve cells that are activated by sensory input from the environment – for example, when you touch a hot surface with your fingertips, the sensory neurons will be the ones firing and sending off signals to the rest of the nervous system about the information they have received.
The inputs that activate sensory neurons can be physical or chemical, corresponding to all five of our senses. Thus, a physical input can be things like sound, touch, heat, or light. A chemical input comes from taste or smell, which neurons then send to the brain.
Most sensory neurons are pseudounipolar, which means they only have one axon which is split into two branches.
Motor neurons
Motor neurons of the spinal cord are part of the central nervous system (CNS) and connect to muscles, glands and organs throughout the body. These neurons transmit impulses from the spinal cord to skeletal and smooth muscles (such as those in your stomach), and so directly control all of our muscle movements. There are in fact two types of motor neurons: those that travel from spinal cord to muscle are called lower motor neurons, whereas those that travel between the brain and spinal cord are called upper motor neurons.
Motor neurons have the most common type of ‘body plan’ for a nerve cell – they are multipolar, each with one axon and several dendrites.
Interneurons
As the name suggests, interneurons are the ones in between – they connect spinal motor and sensory neurons. As well as transferring signals between sensory and motor neurons, interneurons can also communicate with each other, forming circuits of various complexity. They are multipolar, just like motor neurons.
Neurons in the brain
In the brain, the distinction between types of neurons is much more complex. Whereas in the spinal cord we could easily distinguish neurons based on their function, that isn’t the case in the brain. Certainly, there are brain neurons involved in sensory processing – like those in visual or auditory cortex – and others involved in motor processing – like those in the cerebellum or motor cortex.
However, within any of these sensory or motor regions, there are tens or even hundreds of different types of neurons. In fact, researchers are still trying to devise a way to neatly classify the huge variety of neurons that exist in the brain.
Looking at which neurotransmitter a neuron uses is one way that could be a useful for classifying neurons.
However, within categories we can find further distinctions. Some GABA neurons, for example, send their axon mostly to the cell bodies of other neurons; others prefer to target the dendrites. Furthermore, these different neurons have different electrical properties, different shapes, different genes expressed, different projection patterns and receive different inputs. In other words, a particular combination of features is one way of defining a neuron type.
The thought is that a single neuron type should perform the same function, or suite of functions, within the brain. Scientists would consider where the neuron projects to, what it connects with and what input it receives.
This is really the purpose of trying to classify neurons: in the same way as we can say that spinal cord sensory neurons bring sensory input from the periphery to the central nervous system, we would like to be able to say that the role of ‘neuron X’ in the hippocampus is to (for example) let you distinguish between similar but slightly different memories.
So the answer to the question
‘What types of neurons are there?’
isn’t something we can fully answer yet. In the spinal cord, it is pretty simple. But part of what gives the brain its complexity is the huge number of specialised neuron types. Researchers are still trying to agree on what these are, and how they should be classified. Once we can do that, we’ll be in a good position to delve even deeper into how the brain operates.
What does a neuron look like?
A useful analogy is to think of a neuron as a
tree. A neuron has three main parts: dendrites, an axon, and a
cell body or soma (see image below), which can be represented as the branches, roots and trunk of a tree, respectively. A dendrite (tree branch) is where a neuron receives input from other cells. Dendrites branch as they move towards their tips, just like tree branches do, and they even have leaf-like structures on them called spines.
The axon (tree roots) is the output structure of the neuron; when a neuron wants to talk to another neuron, it sends an electrical message called an action potential throughout the entire axon. The soma (tree trunk) is where the nucleus lies, where the neuron’s DNA is housed, and where proteins are made to be transported throughout the axon and dendrites.
Glia are non-neuronal cells (i.e. not nerves) of the brain and nervous system. There are a variety of subtypes of glial cells, including astrocytes, oligodendrocytes, and microglia, each of which is specialised for a particular function.
Glia do not fire action potentials, and because of this, were previously thought to be little more than housekeepers that ensured neurons could function properly. This view is now shifting, and astrocytes in particular are recognised as key components of synapses that can influence how we process information.
What’s the difference between neurons and glia?
Neurons have axons and dendrites. However, glia, unlike neurons, cannot generate action potentials (also known as spikes, or nerve impulses).
Types of glia
Macroglia
Astrocytes
Astrocytes are star-shaped cells that maintain a neuron’s working environment. They do this by controlling the levels of neurotransmitter around synapses, controlling the concentrations of important ions like potassium, and providing metabolic support.
But astrocytes don’t just maintain the environment around synapses. An active area of research addresses how astrocytes modulate how neurons communicate. Because astrocytes have the ability to sense neurotransmitter levels in synapses, and can respond by releasing molecules that directly influence neuronal activity, astrocytes are increasingly seen as important for modifying synapses.
Oligodendrocytes
Oligodendrocytes provide support to axons of neurons in the central nervous system, particularly those that travel long distances within the brain. They produce a fatty substance called myelin, which is wrapped around axons as a layer of insulation. Similar in function to insulation layers around power cables, the myelin sheath allows electrical messages to travel faster, and gives white matter its name—the white is the myelin wrapped around axons. Multiple sclerosis is caused by a loss of the myelin sheath around neurons.
Major glial cells in the brain include oligodendrocytes (blue), astrocytes (green) and microglia (maroon). Neurons are shown in yellow, with the blue of oligodendrocytes forming the myelin sheath around the axon.
Microglia
Microglia are the brain’s immune cells, serving to protect it against injury and disease. Microglia identify when something has gone wrong and initiate a response that removes the toxic agent and/or clears away the dead cells. Thus microglia are the brain’s protectors. However, the situation may be different in neurodegenerative disorders such as Alzheimer’s disease—there is evidence that microglia may become hyperactivated, promoting neuroinflammation that can lead to the characteristic toxic protein deposits seen in Alzheimer’s (amyloid plaques and neurofibrillary tangles). Finally, recent work shows that microglia play a role in the developing brain. Normally, far more synapses are created than are needed, with only the strongest, most important ones surviving. Microglia directly contribute to this synaptic ‘pruning’ process by eating up the synapses tagged as unnecessary.
In the end, after I showed you what a neuron is, I hope this article has helped you
Reference
1-Herculano-Houzel, S. (2009). The human brain in numbers: a linearly scaled-up primate brain. Frontiers in human neuroscience, 3, 31.
2-Luengo-Sanchez, S., Bielza, C., Benavides-Piccione, R., Fernaud-Espinosa, I., DeFelipe, J., & Larrañaga, P. (2015). A univocal definition of the neuronal soma morphology using Gaussian mixture models. Frontiers in neuroanatomy, 9, 137.
3-Miller, M. A., & Zachary, J. F. (2017). Mechanisms and morphology of cellular injury, adaptation, and death. Pathologic basis of veterinary disease, 2.
4-Nicholls, J. G., Martin, A. R., Wallace, B. G., & Fuchs, P. A. (2001). From neuron to brain (Vol. 271). Sunderland, MA: Sinauer
.Associates
5-Pereda, A. E. (2014). Electrical synapses and their functional interactions with chemical synapses. Nature Reviews Neuroscience, 15(4), 250-263.
Written by Reem Tariq
Designed by Samah Ahmed
