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,

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.

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,

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.

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.

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.)

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.