Signal transduction neurotransmitter binding

Natural or synthethic receptor ligands that induce a conformational change (active conformation) and a signal transduction process upon receptor binding. Agonists may act as typical hormones or neurotransmitters or they may confer paracrine functions, recognize bacterial, viral or other environmental constituents via activating their dedicated receptors. 

The way in which the effector works differs from case to case. By binding or interconversion, many receptors activate special mediator proteins, which then trigger a signal cascade (signal transduction see p. 384). Other receptors function as ion channels. This is particularly widespread in receptors for neurotransmitters (see p. 354). 

B. An antagonist binds to a receptor and prevents the action of an agonist. Choice A is wrong because this combination does initiate a signal transduction process. C and D are incorrect because both neurotransmitters and hormones work through their appropriate receptor to initiate signal transduction. 

The elevation of presynaptic Ca2+ causes synaptic vesicles to fuse with the plasma membrane and release neurotransmitter into the synaptic cleft. Neurotransmitters then diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane. This can result in the opening or closing of ion channels, thus changing the probability that the postsynaptic neuron will fire an action potential, or the triggering of biochemical cascades within the postsynaptic neuron (collectively termed signal transduction). In this manner, neuronal communication occurs as information is transmitted from the presynaptic membrane to the postsynaptic membrane, and thus from one neuron to another. 

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