Neurotransmitters

Introduction image

Neurotransmitters also refer as “Chemical messengers” that is composed of molecules like amino acids, amines, purines and neuropeptides. Its activity can inhibit or excite the production of a nerve impulse. Its synthesis occurs endogenously by the presynaptic-neuron and releases out on membrane stimulation. Neurotransmitter ensures neurotransmission by transmitting an action potential beyond the chemical synapse.

Our nervous system uses this as a carrier molecule that attaches with the specific receptors to trigger the opening of voltage-gated channels that in turn allow the movement of ions beyond the membrane that stimulates the action potential to move althrough the axon.

Content: Neurotransmitters

  1. Definition
  2. Location
  3. Functions
  4. Features and Facts
  5. Ideal Properties
  6. Types
  7. Role of Neurotransmitters

Definition of Neurotransmitter

It can define as the endogenous chemical agents whose synthesis occurs within the neurons by the association of ER and Golgi body that helps in providing active precursors for the synthesis of neurotransmitter and packaging it into the vesicles, respectively. The vesicles encircling the neurotransmitters are known as “Synaptic vesicle”. The neurotransmitter comes into action when the synaptic vesicles rupture after coming in contact with the plasma membrane. Its release can either excite or inhibit the nerve signal transmission to the adjoining neuron. These chemical messengers bind with the corresponding receptors of the postsynaptic neuron.

Location

Neurotransmitters are packaged within the small spherical bundles known as synaptic vesicles and are found towards the periphery of the axon’s endings. In the case of chemical synapses, the synaptic vesicles are small and round, whereas in electrical synapses these vesicles can be pleomorphic in shape.

Neurotransmitters Function

Neurotransmitter performs the following tasks in a nervous system.

Functions of neurotransmitter

Carrier agent: Neurotransmitter acts like a carrier molecule, which releases beyond the axolemma along with the nerve signal. Its role is not over yet, as it also propagates the nerve signal to the adjoining or postsynaptic neuron after specific binding with the receptors site.

Booster: Neurotransmitter boosts the conduction of nerve signal from neuron to neuron or neuron to the target cell.

Modulatory factor: Besides, accelerating and inhibiting the conduction of action potential, it also functions to balance the signal between the neurons.

Features and Facts

Neurotransmitters share the following characteristics and facts that are mentioned below:

  • Neurotransmitters are part of chemical neurotransmission.
  • It plays a fundamental role in physical (muscle movement, heart rate, etc.) and psychological functions (mood, fear, etc.) in our body.
  • Neurotransmitter was first described by a scientist named Otto Loewi in the year 1926 by conducting experiments on the vagus nerves of frogs, after which he confirmed that the neurons could communicate by releasing chemicals.
  • Excitatory, inhibitory and modulatory neurotransmitters are the three different modes of action performed by the neurotransmitter.
  • There are approximately 1000-10000 of neurotransmitters per synaptic vesicle.
  • It continually drives to withhold our brain in function.
  • Increase or decrease in the level of neurotransmitter production may influence physical and psychological activities of our body.
  • The deficiency of neurotransmitters is linked with certain neurodegenerative diseases. For example, lack of ACH (Acetylcholine) and glutamate can cause neurodegenerative Alzheimer’s disease.
  • An activity of a neurotransmitter may get affected by the ingestion of certain drugs like dopamine and even by the action of other neurotransmitters.
  • The neurotransmitters are produced by the amino acid precursors and enzyme system inside a neuron.

Ideal Properties of Neurotransmitter

To recognize the neurotransmitter, the following points you should keep in mind.

  1. For its synthesis, the endoplasmic reticulum must contribute to the production of active precursors and enzymes necessary for the synthesis of neurotransmitters.
  2. A Golgi body of neuron must package these neurotransmitters around a protein covering in the form of small vesicles to protect it against from degeneration.
  3. There must be an adequate number of neurotransmitters whose binding with the postsynaptic receptors can stimulate a response.
  4. For neurotransmission, the chemical messenger should diffuse beyond the presynaptic neuron, and the postsynaptic neuron should comprise of cell receptors to which a chemical messenger can bind.
  5. There should have reuptake systems so that free neurotransmitters in the synaptic cleft junction can move back into the presynaptic neuron so as to prevent the loss of neurotransmitters.

Neurotransmitters Type

Based on the functional role inside a nerve cell, a neurotransmitter can classify into the following kinds:
types of neurotransmitters

Excitatory Neurotransmitters

From the name, it’s quite clear that this kind of chemical messenger will excite or promote an action potential to move beyond presynaptic neuron. In simple words, the excitatory neurotransmitter can persuade the target cell or neuron into action. The chemical messengers (for example acetylcholine) can activate cation channels (like Na+ and Ca2+), and are thus depolarizing can also define as the excitatory neurotransmitters.

It includes chemical messengers like histamine, glutamate, epinephrine, norepinephrine, acetylcholine, dopamine etc. It also modulates the transmembrane ion flow through different channels to facilitate the generation and propagation of a nerve signal.

Inhibitory Neurotransmitters

It’s a form of chemical messenger that hinders the firing an action potential across the presynaptic membrane. Merely, an inhibitory neurotransmitter can dissuade the target cell or neuron into action. The chemical messengers (for example GABA) can activate anion channels (like Cl), and are thus hyperpolarizing can also define as inhibitory neurotransmitters. It includes chemical messengers like GABA, taurine, acetylcholine, dopamine etc. It does not facilitate the generation and propagation of a nerve impulse.

Modulatory Neurotransmitters

It also refers as “Neuromodulators” as they regulate the functioning of other neurons. Some of the neuromodulators include dopamine, histamine, acetylcholine etc. This kind of neurotransmitter does not participate in neurotransmission. It can neither reabsorbed by the presynaptic neuron nor broken down into the metabolites. Modulatory neurotransmitter influences the overall activity level of the brain.

What do they do?

The synaptic vesicle consists around thousand of neurotransmitters, arranged towards the bulbous endings. The axon’s terminal region considers as the active zones of a neuron. The moving down of action potential results into a fusion of synaptic vesicles with the axon’s plasma membrane. As a result of integration, the chemicals messengers will disperse beyond the membrane and bind with the receptor sites of the postsynaptic neuron. The receptor proteins bring out certain modifications in the chemical messenger by activating ion channels. The cell receptors also mediate the transformation of chemical information into electrical form.
synthesis of acetylcholine

After the regulation of ion channels, the neurotransmitter and receptor binding will exhilarate ion fluxes beyond the membrane. Depending upon the type of channels and ions fluxed into the membrane can either cause depolarization and hyperpolarization. A depolarized state is the state of an inner membrane having positive charges. A hyperpolarized state is the state of an inner layer having negative charges, by the influx of chloride ions via chloride gated channels.
Role of neurotransmitters

If a cell reaches a threshold of depolarization, then it will trigger the influx of more electropositive sodium ions into the electronegative cytosol via sodium voltage-gated channels. Then, Depolarization of membrane will stimulate the propagation of an action potential. When an action potential reaches the region of bulbous swelling, the arrival of positive charges triggers the opening of calcium gated channels. These channels then allow the movement of calcium ions into the membrane. The integration of axolemma and vesicles will disperse out the chemical messengers into the synaptic cleft, and the process then repeats similarly.

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