Receptors: Pacinian Corpuscles, Rods and Cones

A Pacinian corpuscle has a sensory neurone ending surrounded by layers of connective tissue. Pressure deforms the structure and the sensory membrane, affecting stretch-mediated sodium channels. Sodium entry changes the membrane potential, producing a generator potential.

The size of the generator potential depends on the stimulus and is graded. If depolarisation reaches the relevant threshold, action potentials can be generated in the sensory neurone. A stronger stimulus can increase firing frequency, but does not make each action potential indefinitely taller. Distinguish the receptor’s local graded signal from the propagated impulse.

Rods are highly sensitive to low light but do not provide colour discrimination in the same way as the different cone types. Cone types have different optical pigments with different spectral sensitivities, supporting colour vision. Cones generally require brighter light to respond effectively.

An optical pigment absorbs particular wavelengths; it is not a coloured filter that simply labels a completed image. Differences in pigments and their responses contribute to what can be detected, while later neural connections determine how those signals are combined and interpreted.

Many rod signals can converge onto a shared pathway, allowing weak signals to combine and improve sensitivity. This convergence reduces the ability to identify which individual rod was stimulated, limiting spatial detail. In the foveal cone pathways, less convergence supports greater visual acuity.

Sensitivity and acuity therefore involve a trade-off: detecting dim light is not identical to distinguishing two close points. Avoid claiming that rods are better at every aspect of vision because they detect lower light levels. Use receptor distribution, pigment differences and neural wiring to explain the measured property.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.