A wide variety of stimuli, such as light, pressure, sound, temperature, or chemicals can start an action potential.

A receptor cell is a cell that responds to a stimulus by initiating an action potential.

Transducers convert the energy of stimuli such as light, heat, or sound into electrical impulses in neurones.

A chemoreceptor is a receptor cell that responds to chemical stimuli.

1. Sodium ions (Na⁺) diffuse into chemoreceptor cells through channel proteins in the cell-surface membrane
2. Increase in positive charge inside the cell due to influx of Na⁺
3. Depolarisation of the chemoreceptor cell-surface membrane
4. Voltage-gated calcium ion (Ca²⁺) channel proteins open
5. Ca²⁺ enters cell cytoplasm
6. Exocytosis of vesicles containing neurotransmitter

Neurotransmitters stimulate an action potential in the sensory neurone. This neurone transmits impulses to the taste centre in the cerebral cortex of the brain.

The receptor potential is a change in the normal resting potential across the membrane of a receptor cell, caused by a stimulus.

If the receptor potential is above the threshold, the receptor cell stimulates the sensory neurone to send impulses.

1. Photoreceptors - detect light intensity and colour, and are responsible for sight
2. Chemoreceptors - detect chemical potential, and are responsible for taste
3. Mechanoreceptors - detect pressure and touch
4. Proprioceptors - detect body movement and position
5. Thermoreceptors - detect temperature