Neurotransmission

Our body is controlled by our brain. Have you ever thought about what controls our brains? Does the brain need some control mechanism? That's what scientists thought a long time ago from where originated the concept of neurotransmission.

Well, it's not just a concept, it was a discovery. Henry Hallett Dale was the man who discovered acetylcholine. Acetylcholine is one of the most abundant neurotransmitters in the brain. That's how we came to know about neurotransmission.

What is Neurotransmission?

So what is neurotransmission? The human brain has billions of neurons and these neurons communicate through each other through chemical signals. This transfer of information from one neuron to another through chemical signals is called neurotransmission.

As the name indicates, Neuro: "related to the brain" Transmission: "transfer, the passage" Neuron is not the only one target in neurotransmission. The target can be another cell, muscle, any other neuron too. (1)

How Neurotransmission Occurs?

For neurotransmission to
occur precisely, you need neurotransmitter and their precision to act on
specific receptors that are meant for them.

Key Elements in neurotransmission

There are three main parts of a neuron

1. Axon: away from the cell body
2. Dendrite: close to the cell body
3. Cell body

Neurotransmission is the interaction between these. From the neuron, the neurotransmitter is released (particularly from its axon) and interacts with the dendrites of another neuron.

They are produced in the ribosomes of the presynaptic neuron. They are stored in vesicles. These vesicle's location is cytoplasm of a neuron. When an action potential arrives at the pre-synaptic terminal, there is the entry of the calcium ions in pre-synaptic neurons.

Now, after calcium ions entry into the cell, synaptic vesicles merge with pre-synaptic membrane and neurotransmitter is released into the synaptic cleft. When neurotransmitter arrives at the cell membrane of a post-synaptic neuron, certain protein molecules are activated. These protein molecules are receptors for neurotransmitters. (2)

After binding with
receptors, neurotransmitters have two effects on the post-synaptic membrane.

Excitation of the
postsynaptic membrane or its inhibition. During excitation, an action potential
is generated. During inhibition, an action potential is inhibited.

Neurotransmitters are
released in small amounts and produce minimal excitatory or inhibitory effects.
This process takes place regardless of the action potential generated or not.
This process is amplified when an action potential arrives and the required
message is sent from neuron to its target through neurotransmission.

How Different Impulses add up?

Neurons are connected with
multiple other neurons. They receive impulses from them. These impulses can sum
up together to cause the desired effects. This is called summation.

Types of Summation

There are two ways by
which summation occurs:

Spatial summation It happens when
multiple action potentials arrive at the presynaptic terminal and cause the
release of the amount of neurotransmitter enough to exceed the trigger point of
the postsynaptic neuron.

Temporal summation occurs when multiple
action potentials are generated in a short period to reach the desired
threshold of the postsynaptic neuron. (3)

Convergence and Divergence

Both convergence and divergence are important aspects of neurotransmission. By taking input from multiple neurons, neurons converge the information. By sending signals to different neurons, they diverge information.

Co-transmission

Sometimes many
neurotransmitters are released from a single neuron (presynaptic terminal).
This phenomenon is called co-transmission. For example, Glycine and GABA are
released together. The same is true for dopamine and glutamate.

Types

There are two types of
neurotransmission

1. Chemical transmission
2. Electrical transmission

The key component of
Neurotransmission is the neurotransmitter

The main component of the
neurotransmission is a neurotransmitter. 

There are several types of
neurotransmitters. Most common that are in abundance and have a great impact on
the body are Acetylcholine, Dopamine, Norepinephrine, Serotonin, GABA,
endorphins, glycine, and glutamate. (4)

Acetylcholine

Acetylcholine is the most abundant neurotransmitter in the brain. It is responsible for producing the effects of the parasympathetic nervous system. Acetylcholine causes contraction of muscles.

It causes activates of receptors for pain. Acetylcholine is involved in the normal functioning of many glands. It controls many stages of sleep especially the REM sleep stage. Acetylcholine acts as a vasodilator.

It increases sweating, lacrimation, and all body secretions. It can be a culprit for causing a decrease in heart rate.

Norepinephrine

Norepinephrine is an important neurotransmitter. Now why you remain attentive has something to do with norepinephrine levels. Our emotional IQ is dependent on nor-epinephrine levels. It regulates sleeping, dreaming, and learning.

The neurotransmitter nor-epinephrine along with epinephrine is involved in fight or flight response. Norepinephrine cause an increase in blood pressure. Similarly, heart rate and blood glucose levels are affected by it.

Dopamine

Dopamine is an important
neurotransmitter. It is the neurotransmitter of the reward center. It has a
role in administrative functions. It has control over movements of
that body that are involved in muscle coordination, arousal, positive and
negative reinforcement, motivation and reward. It is involved in lactation and sexual
gratification.

GABA

GABA is the most important inhibitory neurotransmitter because it inhibits the transmission of nerve impulses which reduces activity in our brain. Alcohol euphoric effects depend on its interaction with GABA receptors.

 Lack of
GABA neurotransmitters results in loss of inhibition. Loss of inhibition cause
hyperactivity in the nervous system leading to seizures and mood disorders. Raised levels of GABA
can lead to sleepiness.

These inhibitory effects
of GABA are significant as this effect can be utilized in treating diseases
that are caused by hyperactivity of neurons like epilepsy and Huntington’s
disease. They help to relieve pain, give a feeling of relaxation and fight with
stress.

Serotonin

A hormone that keeps you high and happy is Serotonin. So, one needs to take good care of his/her serotonin levels.

Your quality of sleep,
your sexual drive and memory depend on circulating levels of serotonin.

Serotonin has a defense
mechanism in our digestive system. In response to toxins or irritants intake,
it increases gut motility and protects the body. A blood clot is formed after
injury due to vasoconstriction that is mediated by serotonin. (5)

Glutamate

Glutamate is an excitatory neurotransmitter that is present in abundance in vertebrate nervous system. It regulates learning and memory. It plays a role in GABA neurotransmitter production. It helps in the development of the brain.

The brain appears to need
glutamate to form memories. Glutamate provides energy to the muscle during
strenuous exercise.

A study reveals that this
neurotransmitter further delays dystrophy of muscles in animals that lack
vitamin D.

The presence of Glutamate
receptors on macrophages, T cells and B cells suggests their role in innate
immunity. (6)

Impairment in Neurotransmission

Our whole-body control depends upon neurotransmission. So any impairment in neurotransmission results in diseases that affects our body.

Diseases due to dysfunction in
Neurotransmission

Myasthenia gravis is a disease that affects muscle functioning. It is caused by decreased levels of the neurotransmitter acetylcholine. In this disease, antibodies have formed that damage the receptor for acetylcholine at the neuromuscular junction.

The patient presents with drooping of eyelids and difficulty in performing daily activities like walking, talking, etc. Nerve conduction study is diagnostic for this disease.

Parkinson's disease is caused by low levels of the neurotransmitter dopamine in the brain. In this disease, there is a loss of neurons in basal ganglia that are producing dopamine. The patient presented to us with tremors (hallmark of this disease) bradykinesia, stooped posture and masked faces. This disease is diagnosed on clinical examination.

Generalized anxiety disorder, stress, and depression are caused by low levels of serotonin. They are diagnosed with a medical history and in severe cases when leading to suicidal tendencies.

In Alzheimer's disease,
there is a loss of memory and the
patient can't take care of himself. In this disease, there are low
levels of acetylcholine in the brain. (7)

Significance of Neurotransmission impairment in Pharmacology

Our medicine industry is dependent on knowledge of neurotransmission to treat treating various diseases. They manage concentrations of certain neurotransmitters by either increasing or decreasing them, thus treating certain diseases.

For example, Myasthenia
gravis is treated by acetylcholinesterase inhibitors which increase the level
of acetylcholine. They do so by inhibiting the degradation of acetylcholine by
the enzyme acetylcholinesterase. Neostigmine is an example of such a drug.

Another one, Parkinson’s
disease is treated by Levodopa which is given along with carbidopa to increase
dopamine availability to the brain. This combination improved a lot of
symptoms.

Similarly, Selective serotonin reuptake inhibitors increase the concentration of neurotransmitter serotonin thus helping in the treatment of generalized anxiety disorder and depression.

Epilepsy treatment is
based on reducing the hyperactivity of the brain by giving medication that
interacts with GABA receptors.

Beta-blockers work by
controlling the concentration of nor-epinephrine. They are used in the
treatment of migraine, heart diseases, and glaucoma. (8)

The Bottom line

When we want to understand the diverse world of our body which is controlled by our brain, we need to know about neurotransmission. Due to neurotransmission, billions of neurons are connected chemically.

They are sending tons of information to each other on a daily basis. So, we are talking, texting, writing, calling and the list goes on. All of this is happening because of accuracy in neurotransmission at various levels of the brain. Any impairment and in a blink of an eye we will be suffering from a significant dysfunction in our daily activities.

References

1. https://www.ncbi.nlm.nih.gov/books/NBK539894/
2. https://en.wikipedia.org/wiki/Neurotransmission
3. https://teaching.ncl.ac.uk/bms/wiki/index.php/Summation
4. https://www.kenhub.com/en/library/anatomy/neurotransmitters
5. https://www.medscape.com/viewarticle/860177_5
6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4133642/
7. https://www.mayoclinic.org/diseases-conditions/alzheimers-disease/symptoms-causes/syc-20350447
8. https://www.medicalnewstoday.com/articles/173068.php