Receptors (A-level only): The basic structure of a Pacinian corpuscle.

The idea

A Pacinian corpuscle is an oval, onion-like structure roughly a millimetre long, found deep in the skin and in joints, ligaments and tendons. At its centre is the naked ending of a single sensory neurone, an unmyelinated axon terminal, and around this are many concentric rings of connective tissue lamellae separated by a viscous gel. The lamellae are the mechanical part of the receptor: pressure applied to the outside of the corpuscle deforms them, and that deformation is transmitted inwards to the membrane of the axon. The axon membrane carries stretch-mediated sodium ion channels, which are normally closed and open only when the membrane is stretched.

What earns the marks

• A single sensory neurone ending at the centre of the corpuscle.
• Concentric rings or layers of connective tissue lamellae surrounding it.
• Gel or fluid separating the lamellae.
• Stretch-mediated sodium ion channels in the membrane of the sensory neurone ending.
• Linking deformation of the lamellae to stretching of the neurone membrane.

Common mistakes and how to put them right

• Describing many neurone endings inside one corpuscle instead of a single one.

• Calling the lamellae layers of skin rather than connective tissue within the corpuscle.

• Placing the sodium ion channels in the lamellae rather than in the neurone's cell surface membrane.

• Omitting the channels altogether, so the structure cannot be linked to the generator potential.

• Labelling the central ending as myelinated, when the ending inside the corpuscle is not.

Exam tips

• Label a diagram with three things: the single sensory neurone ending, the lamellae, and the stretch-mediated sodium ion channels.
• Link every structural feature you name to a function, because structure-only answers rarely gain full marks.
• State where the corpuscle is found, deep in the skin or in joints and tendons, if the question asks for location.