How does a receptor potential lead to an action potential in a sensory neuron?

In human sensory reception, the transformation of a stimulus into a nerve impulse occurs through a process of transduction. When a physical stimulus—such as pressure, temperature, or chemical energy—acts upon a sensory receptor, it triggers the activation of ion channels within the cell membrane.12

The opening of these channels allows for a flux of ions, typically sodium, to enter the cell. This influx creates a receptor potential, which is a graded local potential that shifts the membrane toward a less negative, or depolarized, state.123

The transition from a receptor potential to an action potential depends on the amplitude of this depolarization:

• Threshold Potential: If the receptor potential is strong enough to reach a critical threshold potential, it triggers the full activation of voltage-dependent sodium channels.41
• Generation of Impulse: Once this threshold is met, sodium floods into the cell, causing a rapid reversal of membrane polarity. This event constitutes the action potential, or nerve impulse, which is then propagated along the nerve fibre to the central nervous system.452
• Graded Response: If the stimulus is too weak to reach the threshold, the local potential dissipates through passive spread, and no action potential is generated.1