Receptors are specific to a single type of stimulus and produce a generator potential when stimulated. The Pacinian corpuscle as a receptor that responds to changes in mechanical pressure. The role of rod and cone cells in the retina, the differences in sensitivity and visual acuity, and the distribution of rods and cones across the retina.

What this dot point is asking

AQA wants you to explain that receptors are specific and produce a generator potential, describe how the Pacinian corpuscle detects pressure, and compare rod and cone cells in sensitivity, visual acuity and distribution.

The answer

Receptors and the generator potential

A receptor is a specialised cell or sensory neurone ending that detects a specific stimulus and acts as a transducer, converting the energy of the stimulus into a nerve impulse.

Two key properties:

• Receptors are specific to a single type of stimulus (a pressure receptor responds only to pressure, a light receptor only to light).
• When stimulated, the receptor membrane becomes more permeable to sodium ions, producing a small depolarisation called a generator potential. A larger stimulus produces a larger generator potential; if it reaches threshold, an action potential is triggered in the sensory neurone.

The Pacinian corpuscle

The Pacinian corpuscle is a receptor in the skin (and joints and tendons) that responds to changes in mechanical pressure. It is the ending of a single sensory neurone surrounded by concentric layers (lamellae) of connective tissue separated by gel.

How it works:

1. In its resting state, the stretch-mediated sodium channels in the neurone membrane are too narrow to let sodium in.
2. When pressure is applied, the corpuscle is deformed and the membrane stretches.
3. The stretch-mediated sodium channels widen, so Na+ diffuses in.
4. This depolarises the membrane, producing a generator potential.
5. A large enough generator potential reaches threshold and triggers an action potential that passes along the sensory neurone.

Rod and cone cells

The retina contains two types of light receptor (photoreceptor): rods and cones. They differ in sensitivity, the type of vision they give, and their distribution.

Rod cells

• Very sensitive to light (work in dim light) because many rods connect to one bipolar neurone (retinal convergence), so their generator potentials sum (spatial summation) to reach threshold even in low light.
• Give low visual acuity (cannot resolve fine detail) because many rods share one neurone, so two nearby points produce only one impulse.
• Contain the pigment rhodopsin, which gives only black-and-white (monochrome) vision.
• Found mostly in the peripheral retina; absent from the fovea.

Cone cells

• Less sensitive (work only in bright light) because each cone usually connects to its own bipolar neurone, so there is no summation; a higher light intensity is needed to reach threshold.
• Give high visual acuity because each cone sends a separate impulse, so two close points are resolved as two.
• Three types contain iodopsin pigments sensitive to red, green or blue light, giving colour vision.
• Concentrated at the fovea (directly behind the lens), where light is focused.

Examples in context

Example 1. Night vision and the periphery of the retina. Pilots and astronomers use averted vision, looking slightly to the side of a dim object, because the rod-rich peripheral retina is far more sensitive in low light than the cone-rich fovea. This applied technique is a direct consequence of rod sensitivity arising from retinal convergence and summation.

Example 2. Vibration sensing in the fingertips. Pacinian corpuscles deep in the skin of the fingertips respond to vibration when handling tools or reading Braille. Because they detect changes in pressure rather than steady pressure, they let the nervous system register texture and movement against the skin, illustrating the receptor as a transducer of mechanical energy into impulses.

Try this

Q1. Explain how a Pacinian corpuscle produces a generator potential when pressure is applied. [3 marks]

• Cue. Pressure deforms the corpuscle and stretches the membrane; stretch-mediated sodium channels widen; Na+ diffuses in, depolarising the membrane to produce a generator potential.

Q2. Compare rod and cone cells with respect to sensitivity and the type of vision they provide. [3 marks]

• Cue. Rods are more sensitive (work in dim light) and give monochrome vision; cones are less sensitive (need bright light) and give colour vision (three types).

Q3. Explain why visual acuity is greater in the fovea than in the peripheral retina. [3 marks]

• Cue. The fovea is rich in cones, each connected to its own neurone, so close points are resolved separately; the periphery has rods sharing neurones, so close points cannot be distinguished.