15.1.4—Sensory receptor cells

A sensory receptor is a specialised cell, or a sensory neurone ending, that detects a particular type of stimulus. As a transducer it converts stimulus energy into a change in electrical state, beginning a sensory signal.

• A suitable stimulus changes the receptor cell or receptor ending and causes depolarisation, producing a receptor potential.
• A weak stimulus may not depolarise the receptor enough to activate the sensory neurone.
• If the change reaches the required threshold, the sensory neurone is activated and carries impulses towards the CNS.

Receptor specificity links the type of stimulus to the appropriate sense system: for example, light receptors detect light and taste-bud chemoreceptors detect chemical stimuli. The receptor potential is the graded transduction step; detailed action-potential stages and synaptic transmission are covered separately.

A receptor does not respond equally to every form of energy, and a weak stimulus does not automatically produce a nerve impulse. This card explains detection and threshold activation, not the full membrane-potential sequence or neurotransmitter release.