Learning the basic anatomy of the neuron and how they are connected
The neuron is the primary cell of the nervous system. This cell facilitates communication across the whole body internally and externally (a bit like the internet).
You have probably heard of nerves, as they are the ones that are most cited and used term in our everyday lives. Nerves are actually a bundle of these tiny cells called neurons.
They are fascinating little cells that basically dictate our whole existence: from feelings, to reactions, movement and even breathing.
The structure of these crazy interesting cells is made out of the following:
1. Cell body and Dendrites
The dendrites are responsible with receiving information. With the help of receptors found on their membrane they accept or decline information under the form of chemicals structures called neurotransmitters.
These chemicals produce electrical changes in the neuron which are processed in the cell body (also known as Soma) by the nucleus which holds the DNA information. Subsequent to processing the cell body passes along the information in an area called Axon hillock, which is found between the cell body and the axon.
Please note that a collection of cell bodies are knows as grey matter and clustered together are referred as Nuclei in the CNS (Central Nervous system) or Ganglia in the PNS (Peripheral nervous system)
2. The Axon
Axons are long projections that can be between 1mm to 1m long. They are covered by a membrane called axolemma and as a bundle they are referred as tracts in the CNS and nerves in the PNS.
Their main responsibility is to help the information travel through the neuron. If the signal or stimulus is strong enough it will generate an action potential which will be passed along its axis.
3. The Myelin sheath and nodes of Ranvier
The axon is also covered by a multi-layered lips and protein cover called the Myelin sheath which acts as an isolation cover which prevents the electrical signal from degrading.
The nodes of Ranvier are gaps in the myelin rich in Sodium channels. Positive sodium ions rejuvenates the action potential preventing it from dying out along the axon. The action potential slows down when reaching a node of Ranvier because of the lack of myelin and speeds up again in a process called Saltatory conduction by leaps of depolarisation (more information: Action potential)
Collectively these myelinated axons is what appears and is known as the White matter when observing a region of the brain or spinal cord.
Please note that not all axons are myelinated. Unmyelinated axons are slower at processing action potential in step by step depolarisation and can be found predominantly in the CNS.
4. Axon terminals
The axon terminal or also called synaptic end bulbs. This is where the neurotransmitter are going to be released to travel onwards to other nerve cells via the synapse cleft. These neurotransmitters found in the synaptic bulbs are what pass the information onwards as described below.
Synapses are the connections between two neurons. In most of the cases is the way that the Axon terminal of one cell connects to the Dendrites of another nerve cell. There are two types of synapses: Electrical or Chemical.
In the case of chemical synapses the Axon Terminal of one nerve doesn’t touch the Dendrite of the next neuron. There is gap between the two which constitutes the synaptic cleft.
Zooming in on the axon terminals, as the electrical impulse makes its way through the axon opening up Na+ channels (more information: Action potential) when reaching the end also causes Ca+ channels to open. The positive charged ions brought through the action potential nearing the Ca+ channels allow an inflow of positively charged Ca+ ions to enter the neurone.
In the axon terminal cytosol, also called synaptic end bulbs we have vesicles. Vesicles are protein membrane bound sacs containing neurotransmitters. The Ca+ that has just flooded in the axon terminal cause exocytosis of these vesicles. The calcium binds to the protein of the membrane of these vesicles drawing closer to the nerve membrane where they release their neurotransmitters content in the synaptic cleft.
The neuron that has just undertaken this process is now called the pre-synaptic neuron.
Once in the synaptic cleft these neurotransmitters bond to the receptor of the next neurone (now called post-synaptic neuron) and can have an excitatory, inhibitory or modular effect. In the case of the excitatory neurotransmitters these will open up ion channels creating an inflow of postively charged ions hence resulting in another action potential. The inhibitory neurotransmitter will stop the message from travelling, hyperpolorising the nerve.
Most neurotransmitters are either small amine molecules, amino acids, or neuropeptides. There are about a dozen known small-molecule neurotransmitters and more than 100 different neuropeptides, and neuroscientists are still discovering more about these chemical messengers. These chemicals and their interactions are involved in countless functions of the nervous system as well as controlling bodily functions.
We can categorise these based on their chemical composition:
Amino acids: Glycine, Glutamic acid and GABA (γ-aminobutyric acid)
Biogenic amines: Dopamine, Norephinerine, epinephrine, serotonin and histamine
Purinegric: ATP and adenosine
Neuropeptides: these are numerous in number but a few example: Endorphines, Substance P, Neuropeptide Y
Following a nerve impulse, neurotransmitter need to be inactivated and removed from the process to be able to start again. This happens through either diffusion, enzyme breakdown or through re-absorption.
There are all unique and carry on important roles in our bodies, so they will be analysed individually in other posts.
Electrical synapses are in lower number than chemical synapses and can be found in different parts of our nervous system.
Electrical synapses connect the pre-synaptic nerve to the post-synaptic nerve via a gap junction. These gap junctions contain aligned channels between the two membranes allowing the flow of ions from one neuron to the other to pass easily through diffusion.This allows an action potential to travel very fast between neurons.
This also means that the communication is bidirectional compared to chemical synapses (there are still some gap junctions where the communication is unidirectional). The electrical signal can travel either direction depending where an action potential has originated.
A more general purpose of these electrical synapsed is to synchronise electrical activity among populations of neurons. For example, certain hormone-secreting neurons within the hypothalamus are connected by electrical synapses. This arrangement ensures that all cells fire action potentials at about the same time, thus facilitating a burst of hormone secretion into the circulation. The fact that gap junctions are large enough to allow molecules such as ATP and second messengers to diffuse also permits electrical synapses to coordinate the intracellular signaling and metabolism of coupled neurons.
The below image demonstrate the difference between the two.
1. College of Naturopathic Medicine lecture on the Nervous system https://www.naturopathy-uk.com/
2. Image 1 source: https://training.seer.cancer.gov/anatomy/nervous/tissue.html
3. Khan Academy, "The Synapse" Available at: https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/a/the-synapse [Accessed 16th of May 2020)
4. Khan Academy, "Neurotransmitters and receptors" https://www.khanacademy.org/science/biology/human-biology/neuron-nervous-system/a/neurotransmitters-their-receptors
6. Image 3 source: https://www.ncbi.nlm.nih.gov/books/NBK11164/figure/A319/?report=objectonly
7. Lodish H, Berk A, Zipursky SL, et al., "Molecular Cell Biology" 4th edition. New York: W. H. Freeman; 2000. Section 21.4, Neurotransmitters, Synapses, and Impulse Transmission. Available from:https://www.ncbi.nlm.nih.gov/books/NBK21521/
8. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Electrical Synapses. Available from: https://www.ncbi.nlm.nih.gov/books/NBK11164/
9. Queensland Brain Institute, "What are neurotransmitters?", Available at: https://qbi.uq.edu.au/brain/brain-physiology/what-are-neurotransmitters [Accessed 17th of May 2020]