Receptors are transducers The Nature of Receptors Receptors are transducers Receptors transduce the external energy of the stimulus into the code of the nerve impulse The application of a stimulus to a receptor leads to the formation of a generator potential, whose magnitude is determined by the stimulus intensity

The function of a generator potential is to Sensory receptors monitor changes in either the external or internal environments A stimulus triggers the formation of a generator potential across the receptor cell membrane The function of a generator potential is to trigger an action potential in a sensory neuron

The Pacinian Corpuscle Pacinian corpuscles are mechanoreceptors found in skin and certain mucous membranes Pacinian corpuscles respond to pressure, or any kind of mechanical stimulus that causes deformation of the receptor The Pacinian corpuscle consists of a single sensory nerve fibre, the end of which is enclosed in a multi-layered connective tissue capsule The nerve fibre is myelinated except for the nerve ending, which is within the central core of the Pacinian corpuscle No voltage-gated sodium channels are found in the nerve ending, which is sensitive to mechanical compression; voltage-gated channels are however present at the nodes of Ranvier in the myelinated region of the corpuscle’s nerve fibre

Pacinian corpuscles in the dermis of the skin

Concentric lamellae of connective tissue, with Structure of a Pacinian Corpuscle Concentric lamellae of connective tissue, with a viscous gel between them Non-myelinated end of sensory nerve fibre within the central core Single sensory nerve fibre Myelin sheath The Pacinian corpuscle consists of a single sensory nerve fibre, the terminal region of which is enclosed in a multi-layered connective tissue capsule The nerve fibre is myelinated except for the terminal region which is within the central core of the Pacinian corpuscle

Stretch-mediated sodium ion channels, within the non-myelinated terminal of the sensory nerve fibre, open in response to an applied pressure The influx of sodium ions results in a change in membrane potential called the generator potential; the magnitude of the generator potential is graded and, unlike the action potential, varies with the stimulus intensity

Generator Potentials A generator potential is a graded change in the electrical properties of a receptor cell membrane A generator potential, like an action potential, is a temporary change in the membrane potential The generator potential, however, VARIES with the strength and duration of the stimulus; it does not display the ‘all or nothing’ response characteristic of action potentials Most generator potentials are created when a stimulus triggers the opening of sodium ion channels, that result in depolarisation of the receptor cell membrane One exception to this action is the effect of a light stimulus on the photoreceptor cells of the retina (more later…)

The resulting change in membrane potential is the generator potential An applied pressure causes deformation of the connective tissue capsule, and stretching of the surface membrane of the non-myelinated portion of the sensory neuron Stretch-mediated sodium ion channels in the surface membrane of the neuron open, and sodium ions diffuse into the neuron The resulting change in membrane potential is the generator potential

If the generator potential reaches If a microelectrode penetrates the myelinated region of the sensory fibre, a typical action potential is recorded An applied pressure triggers the formation of a generator potential across the non-myelinated sensory nerve terminal, due to the opening of stretch-mediated sodium ion channels If the generator potential reaches threshold, it triggers local currents of sufficient strength to stimulate the opening of voltage-gated sodium ion channels at the first node of the sensory fibre

Pressure deforms the connective tissue capsule Stretch-mediated sodium ion channels in the non-myelinated, sensory fibre terminal open and depolarise the membrane The greater the pressure, the more stretch-mediated sodium ion channels open and the greater the magnitude of the generator potential (recordings from X)

As the viscous gel flows back to its original position between During compression, the viscous gel between the concentric lamellae moves, and allows the nerve ending to resume its normal shape As the pressure is removed, the whole corpuscle resumes its original shape with the nerve terminal becoming deformed in the process Compression No deformation of nerve ending No generator potential No action potentials No Compression Deformation of nerve ending Generator potential Action potentials As the viscous gel flows back to its original position between the lamellae, the corpuscle returns to its resting, fully uncompressed state