RECEPTORS AND THE PACINIAN CORPUSCLE

WHAT DO RECEPTORS DO?

• Very few neurones are sensitive to stimuli directly.

• Receptors act as transducers and convert stimuli into nerve impulses.

HOW DO RECEPTORS WORK?

• A specific stimulus causes the membrane potential in the receptor to change (i.e. to depolarise), producing a generator potential.

• The more intense the stimulus, the higher the GP.

• If above the threshold, Ap generated and transmitted to sensory neurone via synaptic transmission.

PROCESS OF RECEPTORS:

• Stimulus.

• GP in receptor.

• AP in receptor.

• AP in sensory neurone.

WHAT IS A PACINIAN CORPUSCLE?

• Modified neurone, consisting of lamellae of connective tissue surrounding unmyelinated end of a myelinated sensory neurone.

• There is a viscous gel between layers.

• Mechanoreceptors.

WHERE ARE PACINIAN CORPUSCLES FOUND?

Deep in skin, around joints, genital areas, and internal organs.

WHAT TYPE OF NA+ CHANNEL DO PACINIAN CORPUSCLES HAVE?

• Stretch-mediated gated Na+ protein.

• Found in membrane of sensory nerve ending.

• Change shape when under pressure so that only then can Na+ ions pass through the membrane and initiate a generator potential.

• At rest, the cor[uscle is round and the Na+ channels are the wrong shape to let ions pass through.

• When under pressure, the membrane is stretched and as a result the Na+ channels open and the Na+ enters the neurone. This leads to a generator potential which, if large enough, leafs in turn to an action potential.

• The greater the pressure, the greater the generator potential.

• Once the pressure has been registered, no further action potentials are sent until the pressure is released. The the corpuscle temporarily springs into an elongated shape, deforming the mebrane once again, and setting off another burst of impulses.

PRESSURE?

1. Deformation of lamellae.

2. Stretch mediated Na+ channels open.

3. Na+ diffuses into axon down electrochemical gradient.

4. Membrane potential becomes less negative and a GP is induced.

5. If GP potential reaches threshold, then AP in sensory is induced.

6. AP sent to the CNS and interpreted as pressure,+