Pacinian Corpuscle Structure and Function Revision Notes | A Level AQA Biology

1. Why the Pacinian Corpuscle Matters

Every time you pick up a pen, your nervous system detects the pressure without you consciously thinking about it. That detection begins at a receptor — and the Pacinian corpuscle is one of the clearest examples of how a physical stimulus becomes an electrical nerve signal.

The AQA specification uses the Pacinian corpuscle to illustrate a fundamental principle: receptors are transducers. They convert one form of energy — in this case, mechanical pressure — into an electrical generator potential. Understanding this one receptor gives you the conceptual toolkit to analyse any sensory receptor you encounter in an exam.

2. Structure of the Pacinian Corpuscle

Picture an onion cut in half. That layered, concentric appearance is exactly what a Pacinian corpuscle looks like in cross-section. The outer layers are called lamellae — concentric rings of connective tissue and fluid that surround the core of the receptor.

Running through the very centre of those lamellae is the ending of a sensory neurone. This unmyelinated nerve ending sits at the core, and its membrane contains the stretch-mediated sodium ion channels that are critical to the receptor's function. The whole structure is found deep in the skin, particularly in the fingertips, soles of the feet, and joints — locations where detecting pressure and vibration is most useful.

• Outer capsule: concentric layers of connective tissue (lamellae) surrounding a fluid-filled core
• Central core: contains the unmyelinated ending of a single sensory neurone
• Sensory neurone membrane: contains stretch-mediated (mechanically gated) sodium ion channels
• Location: deep in the dermis and subcutaneous tissue, especially in fingertips and joints

The layered structure is not decorative — it has a functional role. The lamellae act as a mechanical filter, transmitting rapid changes in pressure (such as vibration) to the central nerve ending while dampening sustained, unchanging pressure. This is why Pacinian corpuscles are particularly sensitive to changes in pressure rather than constant pressure.

3. From Pressure to Electrical Signal: Establishing the Generator Potential

Now that you can picture the structure, here is where it gets interesting: how does squeezing the corpuscle actually produce an electrical signal? The answer lies in what happens to the membrane of the sensory neurone ending when it is physically deformed.

At rest, the stretch-mediated sodium ion channels in the neurone membrane are closed. The membrane is polarised — the inside of the neurone is more negative than the outside, which is the normal resting potential. No net movement of sodium ions occurs across the membrane.

When pressure is applied to the corpuscle, the lamellae transmit that force inward, physically deforming the membrane of the sensory neurone ending. This deformation changes the shape of the stretch-mediated sodium ion channels, causing them to open. Sodium ions ($\text{Na}^+$) then move down their electrochemical gradient — from the high concentration outside the cell to the lower concentration inside — through these open channels.

The influx of positively charged sodium ions makes the inside of the membrane less negative. This local depolarisation is the generator potential. Crucially, the generator potential is graded: a stronger stimulus deforms more channels, more sodium ions enter, and the depolarisation is larger.

If the generator potential reaches the threshold level of depolarisation, it triggers an action potential in the sensory neurone, which then travels along the nerve fibre to the central nervous system. If the stimulus is too weak to reach threshold, no action potential is fired — this is the all-or-nothing principle of action potentials, which contrasts with the graded nature of the generator potential itself.

4. Stimulus Intensity and the Graded Generator Potential

One of the most important features of the generator potential is that it is graded — its size depends on the strength of the stimulus. This is fundamentally different from an action potential, which is all-or-nothing once threshold is reached.

A light touch deforms only a small region of the sensory neurone membrane. Fewer stretch-mediated channels open, fewer sodium ions enter, and the resulting depolarisation is small. A stronger press deforms a larger area of membrane, opens more channels simultaneously, and produces a larger generator potential. If that larger generator potential exceeds the threshold, an action potential fires.

Notice that once threshold is reached, the action potential itself does not get bigger with a stronger stimulus — instead, the frequency of action potentials increases. The graded generator potential is the mechanism by which stimulus intensity is encoded before that all-or-nothing threshold decision is made.

5. Putting It Together: A Step-by-Step Walkthrough

Exam questions on this topic often ask you to explain the full sequence from stimulus to generator potential. The step-by-step below models the level of detail and precise language that earns marks on AQA papers.

Worked example: Explaining How a Generator Potential is Established in a Pacinian Corpuscle

Explain how applying pressure to a Pacinian corpuscle leads to the establishment of a generator potential. [5 marks]

1. Pressure is applied to the outer capsule of the Pacinian corpuscle. — [B1] Name the stimulus (pressure/mechanical deformation). Examiners want you to be specific — 'a stimulus' alone is too vague.
2. The lamellae transmit the pressure inward, physically deforming the membrane of the sensory neurone ending at the core of the corpuscle. — [B1] You must state that it is the membrane of the sensory neurone ending that is deformed — not the lamellae themselves. This is a common point of confusion.
3. Deformation of the membrane causes stretch-mediated sodium ion channels to change shape and open. — [B1] Use the term 'stretch-mediated sodium ion channels' — this is the AQA specification language. Saying 'ion channels open' without specifying the type or mechanism loses a mark.
4. Sodium ions ($\text{Na}^+$) move into the sensory neurone down their electrochemical gradient through the open channels. — [B1] State both the direction (into the cell) and the reason (down the electrochemical gradient). Mentioning only 'sodium ions enter' without explaining why is a partial answer.
5. The influx of positively charged sodium ions depolarises the membrane locally, producing a generator potential. The size of this depolarisation is proportional to the number of channels opened, and therefore to the strength of the stimulus. — [B1] Stating that the generator potential is graded (proportional to stimulus strength) is the highest-order point here and distinguishes a top-band answer from a mid-band one.

Answer: Pressure deforms the lamellae, which deform the membrane of the sensory neurone ending. This causes stretch-mediated sodium ion channels to open. Sodium ions enter the cell down their electrochemical gradient, depolarising the membrane and establishing a graded generator potential whose size is proportional to stimulus strength.

6. Exam Technique and Common Mistakes

Questions on the Pacinian corpuscle appear regularly on AQA papers and tend to follow predictable patterns. The marks are almost always awarded for precise mechanistic language — not vague descriptions of 'signals being sent'.

The most reliable way to pick up every mark is to follow the causal chain: stimulus → membrane deformation → channel opening → ion movement → depolarisation → generator potential. Each arrow in that chain represents a potential mark. If you skip a step, you skip a mark.

One final point worth consolidating: the Pacinian corpuscle is an example of a receptor acting as a transducer. It converts mechanical energy (pressure) into electrical energy (a generator potential). This principle — that receptors transduce stimuli into generator potentials — applies across all sensory receptors on the AQA specification, making the Pacinian corpuscle a model you can reason from whenever you encounter an unfamiliar receptor in an exam question.