Receptors - Pacinian Corpuscle — AQA A-level Biology

SPEC 3.6.1.2 - RECEPTORS

Receptors: Pacinian Corpuscle & Human Retina

How specific stimuli are detected and converted into generator potentials — pressure sensing and vision

What Is a Receptor?

Receptor

A cell or structure that detects a specific stimulus and converts it into an electrical signal (a generator potential). Receptors respond only to specific stimuli — they are stimulus-specific.

The AQA spec uses the Pacinian corpuscle to illustrate two fundamental principles that apply to all receptors:

Principle 1

Receptors respond only to specific stimuli. The Pacinian corpuscle responds to deep pressure and vibration — not to light, temperature, or chemicals.

Principle 2

Stimulation of a receptor leads to the establishment of a generator potential. The physical stimulus is transduced into a change in membrane potential.

Transduction

Receptors are transducers — they convert one form of energy (mechanical, light, chemical) into an electrical signal. This principle applies to every receptor in the body, from the Pacinian corpuscle in the skin to rod cells in the retina.

Pacinian Corpuscle — Structure

Pacinian Corpuscle

A mechanoreceptor found in deep skin tissue (and other tissues) that detects deep pressure and vibration. It consists of a single sensory neurone ending surrounded by concentric layers (lamellae) of connective tissue.

Generator Potential Mechanism

When pressure is applied to the Pacinian corpuscle, a sequence of events converts the mechanical stimulus into an electrical signal:

1

Mechanical pressure deforms the concentric lamellae of connective tissue.

2

Deformation stretches the membrane of the sensory neurone ending, opening stretch-mediated sodium ion channels.

3

Na⁺ ions flow into the sensory neurone down their electrochemical gradient — depolarising the membrane.

4

This depolarisation is the generator potential. It is graded — larger pressure → more channels open → larger generator potential.

5

If the generator potential reaches threshold, an action potential is triggered in the sensory neurone and propagates to the CNS.

Examiner Alert

It is the deformation of the lamellae that opens the channels — not direct contact with the neurone. The lamellae transmit and focus the pressure onto the neurone ending. Always state: deformation → stretch-mediated channels → Na⁺ influx → generator potential.

Generator Potential — Key Properties

Generator Potential

A change in the membrane potential of a receptor cell caused by stimulation. It is graded in size according to stimulus intensity. If it reaches the threshold level, it triggers an action potential in the associated sensory neurone.

Generator Potential — Graded

The generator potential is graded: the stronger the stimulus, the larger the depolarisation. Larger pressure → more stretch-mediated channels open → more Na⁺ enters → larger generator potential.

If the generator potential does not reach threshold → no action potential fires.

Action Potential — All-or-Nothing

Once the generator potential reaches threshold, an action potential fires. Action potentials are all-or-nothing — they are always the same amplitude regardless of stimulus strength.

Stimulus intensity is coded by the frequency of action potentials — not their size.

Coding Stimulus Intensity

Stronger stimulus → larger generator potential → threshold reached more frequently → higher frequency of action potentials → perceived as more intense. The size of each action potential is always the same — only the frequency changes.

The Human Retina

The retina contains two types of photoreceptor cells — rod cells and cone cells — which transduce light energy into electrical signals. Their differences in structure, pigment, and wiring explain all differences in visual sensitivity, colour vision, and acuity.

Rod Cell

A photoreceptor in the retina that is highly sensitive to low light intensities. Contains the pigment rhodopsin. Many rods converge onto a single bipolar cell and then a single ganglion cell — producing high sensitivity but low visual acuity. Found throughout the retina but absent from the fovea centralis.

Cone Cell

A photoreceptor in the retina that is sensitive to bright light and enables colour vision. Contains iodopsin (three types sensitive to red, green, or blue wavelengths). Each cone connects to its own bipolar and ganglion cell — producing high visual acuity but low sensitivity. Concentrated in the fovea.

The Core Principle

The differences between rod and cone vision are entirely explained by two things: (1) their optical pigments, and (2) the connections they make via bipolar cells to ganglion cells in the optic nerve.

Rod vs Cone — Retinal Wiring Diagram

Rod Cells — High Sensitivity Explained

Rod cells can detect very dim light because of retinal convergence — many rods connect to a single bipolar cell, which connects to a single ganglion cell.

1

Each rod cell contains rhodopsin — the photosensitive pigment bleached by light, generating a small electrical change in the rod.

2

Many rod cells (sometimes hundreds) all synapse onto a single bipolar cell.

3

Signals from many rods are summated at the bipolar cell — even small amounts of light from many rods can combine to reach the threshold of the bipolar and then ganglion cell.

4

An action potential is generated in the ganglion cell axon → travels via the optic nerve to the brain.

Why Low Acuity?

Because many rods share one ganglion cell, the brain cannot determine which rod was stimulated. It only knows that the ganglion cell fired — not precisely where the light fell on the retina. This produces a blurry, low-resolution image — low visual acuity.

Cone Cells — Acuity and Colour Explained

Cone cells are concentrated in the fovea (the central region of the retina). Each cone connects to its own individual bipolar and ganglion cell.

1

Each cone cell contains iodopsin — one of three variants sensitive to different wavelengths: red (~560 nm), green (~530 nm), or blue (~420 nm).

2

Colour is perceived by comparing the relative stimulation of the three cone types — the brain interprets the pattern as a specific colour.

3

One cone → one bipolar → one ganglion cell. No summation occurs.

4

The brain can determine exactly which ganglion cell fired → knows precisely where on the retina the light fell → high visual acuity.

Why Low Sensitivity?

Without summation, a single cone must receive enough light by itself to reach the threshold of its own bipolar/ganglion cell. In dim light, the small signal from one cone is insufficient — cones require bright light to function.

Rods vs Cones — Complete Comparison

FeatureRod CellsCone Cells

PigmentRhodopsinIodopsin (3 types: red, green, blue)

Light sensitivityHigh — respond to dim lightLow — require bright light

Visual acuityLow — poor resolutionHigh — sharp, detailed vision

Colour visionNo — black and white onlyYes — three iodopsin types enable colour

ConvergenceMany rods → one bipolar cell → one ganglion cellOne cone → one bipolar → one ganglion cell

Location in retinaThroughout retina; absent from fovea centralisConcentrated in the fovea

Adaptive advantageNight vision / peripheral visionFine detail and colour discrimination in bright light

The Unifying Explanation

All differences in rod vs cone function are explained by two things: optical pigment (rhodopsin vs iodopsin types) and convergence wiring (many-to-one vs one-to-one). Learn to trace any question about sensitivity, acuity, or colour back to one of these two factors.

Retina — Location of Rods and Cones

Common Exam Errors

Examiner Alert 1 — Lamellae deformation

Students say "pressure opens sodium channels directly." This misses the mechanism. It is the deformation of the lamellae that physically opens the stretch-mediated sodium ion channels in the sensory neurone membrane. Always include the lamellae.

Examiner Alert 2 — Generator potential vs action potential

Students confuse the two. The generator potential is graded (size varies with stimulus). The action potential is all-or-nothing (always the same size). Only the frequency of action potentials changes with stimulus intensity.

Examiner Alert 3 — Rods absent from fovea

Rods are absent from the fovea centralis — not from the whole retina. Rods are actually found throughout most of the retina. Be precise: "Rods are absent from the fovea centralis where cones are concentrated."

Examiner Alert 4 — Naming pigments

Rods contain rhodopsin; cones contain iodopsin (three types). Students commonly reverse these or fail to state there are three types of iodopsin. The three types are what enable colour discrimination.

DEEP Framework — Extended Answer

Model answer for: "Explain why rod cells have higher sensitivity to light but lower visual acuity than cone cells." [5 marks]

DEEP Chain

D

Define: Visual sensitivity is the ability to detect dim light. Visual acuity is the ability to resolve fine detail — to distinguish two nearby points as separate.

E

Explain mechanism (sensitivity): Many rod cells converge onto a single bipolar cell and then a single ganglion cell. Signals from many rods are summated at the bipolar cell. Even small amounts of light absorbed by multiple rods can combine to reach the threshold of the ganglion cell, triggering an action potential. Rods also contain rhodopsin, which is bleached by very low light intensities.

E

Explain mechanism (acuity): Because many rods share one ganglion cell, the brain cannot determine which rod was stimulated — only that the ganglion cell fired. This means the brain cannot pinpoint exactly where on the retina the light fell, giving poor spatial resolution (low acuity). In contrast, each cone connects to its own ganglion cell, so the brain can resolve the exact position of each light stimulus.

P

Practical link: In dim light, looking slightly to the side of a faint object (using peripheral rods) makes it more visible than looking directly at it (which uses the fovea where rods are absent). This demonstrates the sensitivity-acuity trade-off.

Practice Question

A student examines a section through human skin and identifies a Pacinian corpuscle. She applies a light touch stimulus, then a firm pressure stimulus, to the area of skin above the Pacinian corpuscle.

(a) The light touch does not produce a generator potential. Explain why, in terms of the structure of the Pacinian corpuscle. [2 marks]

(b) Explain how a firm pressure stimulus leads to the generation of an action potential in the sensory neurone. [4 marks]

(c) Explain how the nervous system codes for the difference in intensity between a light touch and a firm pressure, given that action potentials are all-or-nothing. [2 marks]

a:

Light touch does not sufficiently deform the concentric lamellae of the Pacinian corpuscle. Therefore the stretch-mediated sodium ion channels in the sensory neurone membrane are not opened (or opened insufficiently) and no Na⁺ influx occurs — so no generator potential is established, or the generator potential does not reach threshold.

b:

Firm pressure deforms the lamellae, which physically stretches the membrane of the sensory neurone ending. This opens the stretch-mediated sodium ion channels. Na⁺ ions flow into the sensory neurone down their electrochemical gradient, depolarising the membrane. This depolarisation (generator potential) reaches threshold, triggering an action potential that propagates along the sensory neurone to the CNS.

c:

Firm pressure produces a larger generator potential than light touch (because more channels are opened and more Na⁺ enters). A larger generator potential reaches threshold more frequently → action potentials are generated at a higher frequency. The nervous system codes stimulus intensity as the frequency of action potentials, not the size of individual action potentials (which are always the same — all-or-nothing).

Synoptic Links

Synoptic Link — Paper 3

Generator potential → Spec 3.6.2 (Action Potentials): The generator potential is the bridge between the receptor and the nervous system. The next lesson covers the action potential mechanism in detail — resting potential, depolarisation, repolarisation, refractory period. Understanding the generator potential as a graded precursor to the all-or-nothing action potential is essential for joining up these two topics.

Synoptic Link — Paper 3

Retinal convergence → Spec 3.2 (Cell membranes): Rod and cone cells transduce light via changes in membrane potential. The signal transmission through bipolar to ganglion cells involves synaptic transmission (covered in Lesson 6.05). A synoptic question could ask you to trace a signal from a photon hitting a rod cell all the way to an action potential in the optic nerve.

Synoptic Link — Essay

Receptors and the "specific stimulus" principle: The principle that receptors respond to specific stimuli applies across the specification — from the Pacinian corpuscle to chemoreceptors in the aorta (Lesson 6.03) to osmoreceptors in the hypothalamus (Lesson 6.08). Any essay on "survival and response" can use this principle as a unifying thread.

Summary

Key Concepts — Lesson 6.02

ConceptKey Point

Receptor principleReceptors respond only to specific stimuli; stimulation leads to a generator potential

Pacinian corpuscleMechanoreceptor; concentric lamellae surround sensory neurone ending; detects deep pressure

Generator potential mechanismPressure → lamellae deformed → stretch-mediated Na⁺ channels open → Na⁺ influx → depolarisation (generator potential)

Generator potentialGraded (size varies with stimulus); triggers action potential if it reaches threshold

Action potentialAll-or-nothing; stimulus intensity coded by frequency of action potentials

RodsRhodopsin; high sensitivity; low acuity; convergence (many → one ganglion); throughout retina

ConesIodopsin (3 types); low sensitivity; high acuity; one-to-one wiring; fovea; colour vision

Top Exam Tip

For the Pacinian corpuscle: always trace the mechanism in sequence — lamellae → stretch-mediated channels → Na⁺ → generator potential → threshold → action potential. For rods vs cones: every difference in function traces back to either the optical pigment or the convergence wiring. Examiners will specifically ask you to explain why — not just state what.