The Pacinian corpuscle

A Pacinian corpuscle is a pressure receptor found deep in the skin. It responds to firm pressure and vibration. It is made of many layers of connective tissue, called lamellae, wrapped around the end of a sensory neurone. Pressing on it squashes the layers, which stretches the neurone's membrane and opens stretch-mediated sodium ion channels. Sodium ions flood in and create a generator potential, which can trigger an action potential to the brain.

Why the Pacinian corpuscle matters for A-level

AQA's A-level Biology specification uses the Pacinian corpuscle as its example of a receptor, to show two things: that receptors respond only to specific stimuli, and that stimulating a receptor sets up a generator potential. It names the mechanism too: deformation of stretch-mediated sodium ion channels in the corpuscle leads to the generator potential.

Structure of a Pacinian corpuscle

A real corpuscle is oval, and in cross-section its layers look like the rings of a cut onion. The drawing shows that idea: rings of lamellae around the neurone ending.

How a Pacinian corpuscle works

1. At rest, the stretch-mediated sodium channels in the neurone's membrane are too narrow for sodium ions to pass. The membrane has its resting potential.
2. Pressure deforms the lamellae, which press on the neurone ending and stretch its membrane.
3. The stretch widens the sodium channels, so sodium ions (Na⁺) diffuse into the neurone.
4. The membrane depolarises. This change in potential is the generator potential.
5. If the generator potential reaches the threshold, an action potential is set off along the sensory neurone to the central nervous system.

A bigger pressure deforms the lamellae more, opens more channels, and makes a bigger generator potential. The generator potential is graded, but the action potentials it triggers are all the same size: a stronger stimulus shows up as a higher frequency of action potentials, not bigger ones.

The Pacinian corpuscle as a receptor

• Specific: it responds only to mechanical pressure. Light or heat don't deform the lamellae, so they don't stimulate it.
• A transducer: it converts one form of energy (mechanical) into another (electrical, a nerve impulse).
• Adapts quickly: under a steady pressure the lamellae settle and the generator potential fades, so the corpuscle mainly signals changes, such as vibration.

Pacinian corpuscles, rods and cones

AQA pairs the Pacinian corpuscle with the receptors in the retina. All of them turn a stimulus into a generator potential; they differ in what they respond to.

A reflex arc then links receptors like this one to effectors. See how a receptor fits into the bigger picture in the Human Physiology course, which covers neurones and reflexes.

Common mistakes

• Saying the corpuscle detects light touch. It responds to deep pressure and vibration.
• Calling the generator potential an action potential. The generator potential comes first and only triggers an action potential if it reaches the threshold.
• Saying stronger pressure makes bigger action potentials. It makes more frequent ones.
• Naming the wrong channels. They are stretch-mediated sodium ion channels, not voltage-gated ones, at this stage.

To test nervous coordination under time pressure, try today's A-level Biology: Year 2 short timed paper. Each paper mixes topics.

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