Question at position 24 What triggers the depolarization of an olfactory receptor cell?Transport of odorant molecules into the cytoplasmInflux of sodium and calcium ionsEfflux of potassium ions from the cellBinding of neurotransmitters to post-synaptic receptorsConversion of GTP into GDP

Question

BlackTom AI · Accepted Answer

To understand what triggers the depolarization of an olfactory receptor cell, we need to follow the typical transduction cascade in olfactory neurons.
Option 1: 'Transport of odorant molecules into the cytoplasm' — In olfactory transduction, odorants bind to receptors on the cilia of olfactory receptor neurons rather than needing to be transported into the cytoplasm for depolarization. This option misstates the key early event that leads to membrane potential change.
Option 2: 'Influx of sodium and calcium ions' — This is the core mechanism. Odorant binding activates a G-protein signaling cascade that opens nonselective cation channels permeable to Na+ and Ca2+. The inward flow of these cations depolarizes the membrane and generates the receptor potential, leading toward action potential firing.
Option 3: 'Efflux of potassium ions from the cell' — Efflux of K+ would tend to hyperpolarize or reduce excitability, not depolarize. Therefore, this would oppose the process that creates receptor potential.
Option 4: 'Binding of neurotransmitters to post-synaptic receptors' — Olfactory receptor neurons signal to downstream neurons via action potentials; while neurotransmitter binding occurs at synapses downstream, the initial depolarization in the olfactory receptor cell is triggered by ion influx through receptor-channel pathways, not by neurotransmitter binding to post-synaptic receptors.
Option 5: 'Conversion of GTP into GDP' — The G-protein cycle involves GTP hydrolysis to GDP as part of signal transduction, but this chemical step is not by itself the direct trigger of depolarization. It is part of the underlying signaling cascade, yet it does not alone cause the immediate depolarization event which results from cation influx.
In summary, the primary depolarizing event is the entry of positive ions, particularly Na+ and Ca2+, through cation channels opened by the odorant-receptor signaling cascade. The other options describe related or downstream aspects or events that do not directly produce the depolarizing receptor potential.

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