Sensory Receptors and the Pacinian Corpuscle

Every signal that reaches a neurone began as some form of environmental energy — light striking the retina, sound vibrating the eardrum, pressure deforming the skin. Converting that energy into the electrical language of the nervous system is the job of sensory receptors. This lesson explores how receptors act as transducers, focusing on the Pacinian corpuscle as the named example required by OCR specification 5.1.3(c).

Key Definitions:

• Sensory receptor — a specialised cell or structure that detects a specific stimulus and converts its energy into a nerve impulse.
• Transducer — any device or structure that converts one form of energy into another; sensory receptors convert stimulus energy into electrical energy.
• Generator potential — a graded change in membrane potential in a receptor that, if large enough, triggers an action potential.
• Threshold — the membrane potential (~−55 mV) at which voltage-gated Na⁺ channels open, producing an action potential.

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Receptors as Transducers

An engineer designing a microphone solves the same problem as evolution designing a sensory receptor: a pressure wave must be converted into an electrical signal. Each receptor is tuned to one form of energy — light for photoreceptors, stretch for muscle spindles, chemicals for taste receptors, temperature for thermoreceptors, and pressure for mechanoreceptors such as the Pacinian corpuscle.

OCR identifies several major categories of receptor. You should be familiar with them even though detailed anatomy is not required for each:

Receptor	Modality detected	Location
Photoreceptor (rod / cone)	Light	Retina of eye
Chemoreceptor	Chemicals	Taste buds, olfactory epithelium, carotid body
Thermoreceptor	Temperature	Skin, hypothalamus
Mechanoreceptor	Pressure, vibration, stretch	Skin, muscle spindles, cochlea
Pacinian corpuscle	Pressure / vibration	Deep layers of skin, especially fingers, soles, genitals and joints

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Structure of the Pacinian Corpuscle

The Pacinian corpuscle is a spherical or oval body about 0.5–2 mm across — large enough to see with a dissecting microscope. It lies deep in the dermis of the skin and in connective tissue surrounding joints. Its structure is beautifully designed for its function:

• A single sensory neurone ending sits at the core, unmyelinated at its tip.
• Surrounding the nerve ending are concentric layers of connective tissue separated by gel-like fluid — like the layers of an onion.
• The whole corpuscle resembles a tiny shock absorber: the layers cushion the nerve ending from steady pressure, allowing it to respond to rapid changes in pressure (vibration) rather than constant force.

flowchart TB
    subgraph Pacinian corpuscle
    L[Concentric connective tissue lamellae]
    G[Fluid-filled gaps]
    NE[Unmyelinated nerve ending<br/>with stretch-mediated Na+ channels]
    end
    NE --> AX[Myelinated axon to CNS]

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The Stretch-Mediated Sodium Channel

Embedded in the plasma membrane of the nerve ending are a special class of protein called stretch-mediated sodium ion channels. In an unstimulated corpuscle these channels are closed and the membrane carries the usual resting potential of about −70 mV. When pressure deforms the corpuscle, the lamellae slide, the membrane stretches and the channels change conformation — opening and allowing Na⁺ to flow into the axon down its electrochemical gradient.

This inward movement of positive charge depolarises the membrane, producing a generator potential (also called a receptor potential). The size of the generator potential depends on the strength of the stimulus: a small deformation produces a small generator potential, a larger deformation produces a larger one. Unlike an action potential, a generator potential is graded, not all-or-nothing.

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From Generator Potential to Action Potential

If the generator potential is large enough to bring the neighbouring myelinated axon membrane to threshold (~−55 mV), voltage-gated Na⁺ channels open and an action potential is fired. If the generator potential is below threshold, no action potential occurs — the stimulus is too weak to be registered.

The relationship between stimulus strength and nervous output is therefore as follows:

• Stronger stimulus → greater membrane deformation → more stretch-mediated Na⁺ channels open → larger generator potential.
• Larger generator potential → threshold reached more rapidly → action potentials fire at higher frequency.