Introduction to Neurotransmitters and Receptors

 Introduction to Neurotransmitters and Receptors.


Neurotransmitters:

  • The chemical released by neurons at terminal ends in response to the generated action potential is called a “neurotransmitter.”

  • The released neurotransmitter helps transmit the nerve impulse.

  • The neurotransmitter released into the synapse attaches to the receptors present on the effector organ or on the next neuron to produce response or transmit the impulse.

  • They are synthesized using raw materials and specific enzymes.

  • Neurotransmitters are stored in the “vesicles”.

  • The storage vesicles rupture in response to the action potential and the neurotransmitter is released at the synapse.

  • Most neurotransmitters released at synapse are retaken up by the neuron and the process is called “reuptake mechanism.”

  • Different types of neurotransmitters are as follows,

  1. Amino acids.

  2. Biogenic Amines.

  3. Neuropeptides.

  • Amino Acids:

    • Many amino acids act as neurotransmitters.

    • There are specific nerves that use specific amino acids as neurotransmitters, they are called by the name of that amino acid.

    • E.g. GABA (Gamma Amino Butyric Acid): GABAergic neurons.

    • Glutamate: Glutamatergic neurons.

    • Glycine: Glycinergic neurons.

  • Biogenic Amines:

    • These are specific small molecules derived from amino acids.

    • They retain an amino group and most lack carboxyl group of amino acids.

    • They play a variety of roles through different receptors.

    • E.g. Adrenaline acts through subtypes of Alpha and Beta receptors.

    • Acetylcholine acts through subtypes of nicotinic and muscarinic receptors.

    • Dopamine acts through different dopamine receptors.

  • Neuropeptides: 

    • These are smaller protein molecules. (Contains 50 or less amino acids.)

    • Their action is long lasting compared to other neurotransmitters.

    • E.g TRH (Thyrotropin Releasing Hormone).

    • Endorphins.

    • Angiotensin II.

Receptors:

  • Receptors are protein molecules in the target cell or on its surface that bind ligands. 

  • Receptor is a cellular macromolecule, whose function is to recognize and respond to chemical signals.

  • They can be broadly classified on basis of their location as,

    1. Cell Surface Receptors.

    2. Internal Receptors.

  • Different types of Cell surface receptors are as follows,

    1. Ligand Gated Ion Channel Receptors.

    2. G-Protein Coupled receptors 

    3. Enzyme Linked receptors 

  1. Ligand Gated  Ion Channel Receptors:

  • These receptors bind to a ligand and open a channel through the membrane that allows specific ions to pass through

  • When a ligand binds to the extracellular region of the channel, there is a conformational change in the protein’s structure that allows ions such as sodium, calcium, magnesium, and hydrogen to pass through.

  • e.g. 

    • Nicotinic Ach Receptor,

    • GABAA Receptor.

    • 5HT3 Receptor.

  1. G-Protein Coupled receptors:

  • G-protein-linked receptors bind a ligand and activate a membrane protein called a G-protein

  • The activated G-protein then interacts with either an ion channel or an enzyme in the membrane. All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own specific extracellular domain and G-protein-binding site.

  • e.g. 

    • Adrenorecptors.

    • Muscarinic Ach Receptor,

    • Histamine Receptors.

  1. Enzyme Linked receptors:

  • Enzyme-linked receptors are cell-surface receptors with intracellular domains that are associated with an enzyme. 

  • In some cases, the intracellular domain of the receptor itself is an enzyme.

  • When a ligand binds to the extracellular domain, a signal is transferred through the membrane and activates the enzyme, which sets off a chain of events within the cell that eventually leads to a response. 

  • An example of this type of enzyme-linked receptor is the tyrosine kinase receptor ( Insulin Receptors, Cytokine Receptors etc.)

B) Internal Receptors:

  • Internal receptors, also known as intracellular or cytoplasmic receptors. 

  • They are found in the cytoplasm of the cell and respond to hydrophobic ligand molecules that are able to travel across the plasma membrane. 

  • Once inside the cell, many of these molecules bind to proteins that act as regulators of mRNA synthesis to mediate gene expression. 

  • Internal receptors can directly influence gene expression without having to pass the signal on to other receptors or messengers.

  • e.g.

    •  Steroid receptors,

    • Thyroid hormone receptors, 

    • Vitamin D receptors.

Commonly asked questions:

  1. Write a note on Neurotransmitters.

  2. What do you mean by a Receptor? Classify receptors with examples.

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