Biology – Control and coordination in mammals | e-Consult

The generation of an action potential in a sensory neuron, specifically within a taste bud chemoreceptor, is a complex process involving a series of precisely regulated ionic movements. It begins with the binding of a tastant molecule to receptor proteins on the taste receptor cell's membrane. This binding triggers a change in the membrane potential, typically a small depolarization, often referred to as a receptor potential.

This receptor potential can be graded, meaning its strength is proportional to the concentration of the tastant. If the receptor potential reaches a certain level of depolarization, known as the threshold potential (typically around -55mV), voltage-gated sodium channels begin to open. These channels are highly sensitive to changes in membrane potential.

The opening of voltage-gated sodium channels leads to a rapid influx of sodium ions (Na+) into the cell. This influx causes a rapid depolarization of the membrane, driving the membrane potential towards a positive value (e.g., +40mV). This rapid depolarization is the rising phase of the action potential.

As the membrane potential reaches its peak, the voltage-gated sodium channels begin to inactivate, preventing further influx of Na+ ions. Simultaneously, voltage-gated potassium channels start to open. This opening allows potassium ions (K+) to flow out of the cell. The efflux of positive K+ ions causes the membrane potential to become more negative, resulting in the repolarization phase of the action potential.

The potassium channels close, and the membrane potential returns to its resting potential (typically around -70mV). This hyperpolarization phase is a brief period where the membrane potential is more negative than the resting potential. Finally, the sodium-potassium pump actively transports Na+ out of the cell and K+ back in, restoring the original ionic gradients and preparing the neuron for the next action potential. The action potential then propagates down the axon, triggering the release of neurotransmitters at the synapse.