2. 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

3. 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

4. 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

5. Pacinian corpuscles
in the dermis of
the skin

6. 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

7. 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

8. 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…)

9. 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

10. 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

11. 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)

12. 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