Ligand-gated Ion channels:
- Fastest response; used in neurotransmission.
- Binding of neurotransmitter causes opening of the channel's pore to ions; the cell depolarises (excitation) or hyperpolarises (inhibition).
- Cellular response results from the change in membrane potential, or from the change in intracellular ionic concentration.
- EXAMPLES: Nicotinic superfamily, glutamate superfamily, purinergic superfamily (ATP).
G-Protein Coupled Receptors:
- These are slower than the ligand-gated receptors and allow slower neurotransmission.
- Process of signal transduction:
- The G-protein trimer is bound to GDP.
- The agonist occupied receptor binds to the G-protein and GDP is exchanged for GTP. The trimer dissociates to α (which is bound to the GTP) and βγ.
- The α and βγ subunits reach and activate or inhibit effectors.
- GTP hydrolyses and the trimer forms again. The GTP hydrolysis rate determintes the length of action.
- EXAMPLES: Muscarinic ACh receptor, α and β adrenoceptors, GABA & glutamate metabotropic receptors.
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