Receptor Specificity and Potentials Revision Notes | A Level AQA Biology

1. Why Receptors Are Selective — The Starting Point

Your skin is bombarded with information every second — pressure, temperature, pain, vibration — yet each type of receptor responds to only one of these. That selectivity is not accidental; it is the foundation of how your nervous system makes sense of the world.

This lesson unpacks exactly how receptors detect specific stimuli, how that detection generates an electrical signal, and why some parts of your skin are far more sensitive than others. Every one of these ideas appears directly in AQA exam questions.

2. Receptor Specificity — One Receptor, One Stimulus

A receptor responds only to its adequate stimulus — the specific form of energy or chemical change it is structurally adapted to detect. Photoreceptors in the retina respond to light; Pacinian corpuscles respond to deep pressure; thermoreceptors respond to temperature change. Each receptor type has a membrane structure that is only deformed, opened, or activated by one particular stimulus.

Think of it like a lock and key. The receptor protein or membrane structure has a specific shape that only the correct stimulus can interact with. A Pacinian corpuscle, for example, has concentric layers of connective tissue surrounding a sensory nerve ending; mechanical deformation of those layers — and only that — stretches the membrane and opens stretch-mediated ion channels. Light, heat, or sound cannot do this.

Receptor specificity matters because it allows the brain to interpret the source of a signal. When your optic nerve fires, your brain interprets that as light — even if the eye is pressed mechanically. The brain trusts the identity of the receptor, not the nature of the stimulus that happened to trigger it. This principle is called the labelled line concept, and it is why specificity is so critical to accurate sensory coding.

3. From Stimulus to Signal — The Generator Potential

Once the correct stimulus reaches a receptor, the next question is: how does a physical or chemical event become an electrical signal? The answer is the generator potential.

When a receptor is stimulated, the stimulus causes a change in the permeability of the receptor cell membrane to ions — most commonly sodium ions (Na⁺). Sodium ions diffuse into the cell down their electrochemical gradient, making the inside of the membrane less negative. This local depolarisation is the generator potential.

The generator potential is graded: a stronger stimulus causes a larger depolarisation. If the generator potential reaches the threshold of the associated sensory neurone, it triggers an action potential, which then travels along the neurone to the central nervous system. A sub-threshold generator potential decays without producing an action potential — the stimulus is effectively ignored.

Worked example: Sequence: Stimulus to Action Potential in a Pacinian Corpuscle

Describe the sequence of events from mechanical pressure being applied to a Pacinian corpuscle through to an action potential being generated.

1. Pressure compresses the concentric connective tissue layers surrounding the sensory nerve ending. — This is the adequate stimulus for a Pacinian corpuscle — mechanical deformation.
2. Deformation of the membrane stretches stretch-mediated sodium ion channels, causing them to open. — The structural change in the membrane directly gates the channels — no second messenger is needed here.
3. Na⁺ ions move into the cell down their electrochemical gradient (concentration gradient and electrical gradient both favour inward movement). — The inside of the cell becomes less negative — this is depolarisation.
4. The local depolarisation of the receptor membrane is the generator potential. Its magnitude depends on stimulus intensity. — A stronger stimulus opens more channels, producing a larger generator potential.
5. If the generator potential is large enough to reach the threshold of the sensory neurone membrane, voltage-gated Na⁺ channels open and an action potential is initiated. — The action potential is all-or-nothing; the generator potential is graded. This distinction is a common exam focus.

Answer: Pressure → membrane deformation → stretch-mediated Na⁺ channels open → Na⁺ influx → generator potential (graded depolarisation) → if threshold reached, action potential fired in sensory neurone.

Notice the distinction between the generator potential and the action potential. The generator potential is graded — it varies in size with stimulus strength. The action potential is all-or-nothing — once threshold is reached, it fires at full size regardless of how large the generator potential was. Stimulus intensity is then coded by the frequency of action potentials, not their size.

4. Temperature Receptors — Sensitivity in Human Skin

Human skin contains two populations of thermoreceptors: warm receptors, which increase their firing rate as temperature rises (typically active between about 30°C and 45°C), and cold receptors, which increase their firing rate as temperature falls (typically active between about 10°C and 35°C). There is an overlap zone where both types fire simultaneously, which helps the brain make fine discriminations around normal skin temperature.

The sensitivity of these receptors is highest in the range of temperatures close to normal skin surface temperature (around 32–34°C). Small deviations from this baseline produce relatively large changes in firing frequency — the receptor is most sensitive where it matters most physiologically. At extreme temperatures, pain receptors (nociceptors) are also activated, adding a protective warning signal.

Temperature receptors are also found at different depths in the skin. Receptors closer to the surface respond more rapidly to environmental temperature changes, while deeper receptors reflect core tissue temperature more accurately. This layered arrangement gives the nervous system both fast and slow temperature information simultaneously.

5. Touch Receptors — Habituation and Resolution

Touch receptors in the skin have two properties that are particularly important for AQA: habituation and resolution. These are distinct concepts and examiners test them separately, so it is worth being precise about each one.

Habituation refers to the decrease in the frequency of action potentials fired by a receptor when a constant, unchanging stimulus is maintained over time. You experience this every time you put on clothing — within seconds, you stop noticing the fabric against your skin. The receptor has not been damaged; it has adapted to the sustained stimulus and reduced its output. This prevents the nervous system from being overwhelmed by irrelevant background information, freeing attention for new or changing stimuli.

At the cellular level, habituation in touch receptors is linked to a reduction in the rate of Na⁺ channel opening in response to a maintained stimulus. The initial deformation of the receptor membrane opens channels and produces a generator potential, but as the deformation is held constant, fewer channels continue to open, the generator potential decreases, and action potential frequency falls. Rapidly adapting receptors (such as Meissner's corpuscles) habituate quickly; slowly adapting receptors (such as Merkel's discs) maintain their response for longer.

Resolution describes how well touch receptors can distinguish between two separate points of contact. If two points are pressed simultaneously onto the skin, they are perceived as two separate stimuli only if each point stimulates a different receptor (or at least a different receptive field). The receptive field of a receptor is the area of skin within which a stimulus will activate that receptor.

Areas with high receptor density and small receptive fields — such as the fingertips and lips — have high resolution. Two points only 2–3 mm apart can be distinguished as separate. Areas with low receptor density and large receptive fields — such as the back — have low resolution. Two points may need to be several centimetres apart before they are perceived as separate. This is why you can thread a needle with your fingertips but cannot feel individual stitches on your back.

Resolution and habituation are related but distinct: resolution is about spatial discrimination (can two points be told apart?), while habituation is about temporal adaptation (does the response decrease over time with a constant stimulus?). Examiners will use these terms precisely, and so should you.

6. Connecting the Concepts — How Receptor Properties Shape Sensory Information

Earlier, we established that receptor specificity ensures each receptor type responds only to its adequate stimulus. Now we can see how the additional properties of sensitivity, habituation, and resolution further shape the quality of sensory information reaching the brain.

Sensitivity determines the minimum stimulus intensity needed to produce a generator potential that reaches threshold — a more sensitive receptor fires at lower stimulus intensities. Habituation filters out sustained, unchanging stimuli so the nervous system can prioritise change and novelty. Resolution determines the spatial precision of touch information — high resolution allows fine motor tasks and detailed tactile perception.

Together, these properties mean that sensory information reaching the brain is already processed and filtered at the receptor level. The brain does not receive raw, unprocessed data — it receives a curated signal shaped by specificity, sensitivity, adaptation, and spatial coding. This is an important conceptual point for evaluation questions.

7. Exam Technique — Writing for Marks

AQA questions on this topic tend to use four command words: describe, explain, compare, and evaluate. Each demands a different response structure, and confusing them is one of the most common ways students lose marks.

For describe questions — for example, 'Describe how a generator potential is produced' — give a sequential account of what happens without explaining why. State that the stimulus opens ion channels, that Na⁺ ions enter, and that the membrane depolarises. No 'because' is needed. For explain questions, you must add the mechanism: 'Na⁺ ions enter because they move down their electrochemical gradient.' The word 'because' is your signal that a mechanism is required.

For compare questions about receptor resolution or sensitivity, always address both sides explicitly. Do not describe one receptor type and then say 'the other is different.' State the similarity or difference directly: 'Fingertip receptors have smaller receptive fields than back receptors, meaning two-point discrimination is greater in the fingertip.' Paired comparative sentences score marks; isolated descriptions do not.