Topic 6: Organisms response: Pacinian Corpuscles

Pacinian Corpuscles

Mechanoreceptors in the skin that detect pressure and vibrations. It is a multicellular sensory receptor with the unmyelinated end of a single sensory neuron forming its core.

They contain the ending of a sensory neurone wrapped in layers of connective tissue called lamellae

The Pacinian corpuscles lamellae allow it to detect pressure and vibrations, demonstrating how receptors only respond to specific stimuli. There is viscous gel between each layer of tissue

When the Pacinian corpuscle is stimulated

1. The lamellae deform, pressing on the sensory neurone ending

2. This stretches the neurone’s membrane, causing it to change shape

3. This opens stretch-mediated sodium ion (Na+) channels in the membrane, increasing its permeability to Na+

4. Na+ diffuses into the neurone, depolarising it and resulting in a generator potential

5. If this signal reaches the threshold, an action potential is triggered

Receptors detect stimuli:

• receptors are specific - only detecting one type of stimulus

• receptors act as transducers - convert or transduce energy from one form to another e.g. heat to electrical, sound to electrical, light to electrical

There are many different types of receptors:

1. Cells e.g. photoreceptors - receptor cells that connect to the nervous system

2. Proteins of cell surface membranes e.g. glucose receptors - proteins found in the cell membranes of some pancreatic cells

As with all sensory receptors, Pacinian Corpuscles:

• are specific to a single type of stimulus - it is a pressure receptor (mechanoreceptor) that only responds to changes in mechanical pressure and vibrations

• produces a generator potential by acting as an energy transducer - mechanical energy of stimulus converted into a nervous impulse known as a generator potential

Where are they found?

• they occur deep in the skin

• most abundant on the fingers, soles of the feet and the external genitalia

• occurs in joints, ligaments, and tendons - enable the organisms to know which joints are changing direction

Structure of a Pacinian Corpuscle

• end of a single sensory neurone in the centre (naked)

• surrounded by layers of connective tissue = lamellae

• each layer is separated by gel

• encased within a capsule

• blood capillary around layers

How does the Pacinian corpuscle transduce the mechanical energy of the stimulus into a generator potential?

1. resting potential

2. generator potential

3. action potential

Resting potential

1. When receptors in its resting state (not being stimulated) we say there is a difference in charge between inside and outside of the cell - there is a voltage across the membrane

2. Voltage is also known as potential difference

3. The outside is positively charged compared to the inside. The membrane is polarised

4. Potential difference across the membrane is generated by ion pumps and ion channels

Generator potential

5. Change in potential difference due to stimulus. When a stimulus is detected, the cell membrane is excited, becomes more permeable, allowing more ions to move in or out of cell - altering the potential difference

Action potential

6. Bigger stimulus = bigger movement of ions = bigger change in potential difference = bigger generator potential. An action potential (an electrical impulse along a neurone) is only triggered if the generator potential reaches the threshold level

7. When an action potential occurs this part of the membrane is said to be depolarised - there is a temporary reversal in the charges either side of the membrane.

All action potentials cause the same changes in membrane potential, e.g. -70mV to +40mV

so it is the frequency of the action potentials which increases if te strength of the stimulus increases not the size

Initiating a generator potential

1. A pressure stimulus pushes on the lamellae which deforms the membrane of the sensory neurone

2. Stretch-mediated sodium ion channels in the membrane of the neurone open

3. Sodium ions diffuse into the neurone, depolarising it and creating a generator potential. The greater the pressure the more sodium ion channels open

4. If enough sodium ion enter the cell to reach threshold potential, then an action potential will be initiated. An impulse will move along the sensory neurone

Normal resting state

stretch-mediated sodium channels are too narrow to allow sodium ions to pass along them

Pressure applied

Pacinian Corpuscle is deformed, membrane round its neurone is stretched

Stretching

widens the sodium channels and sodium ions diffuse into the neurone