Study Notes on the Pacinian Corpuscle and Nervous System

Introduction to Pacinian Corpuscle

This study guide provides an in-depth analysis of the Pacinian corpuscle, a specialized receptor essential for detecting mechanical stimuli. Receptors are the primary interface between an organism and its environment, functioning as transducers that convert various forms of energy into electrical impulses (nerve impulses). This process is vital for coordinating survival responses.

Nervous System Overview

Key Sections of the Nervous System

The nervous system is organized into two primary divisions:

1. Central Nervous System (CNS): Comprised of the brain and spinal cord, the CNS acts as the integration center. It receives sensory information, processes it, and coordinates an appropriate motor response.

2. Peripheral Nervous System (PNS): This includes all the nerves outside the CNS. It is further divided into:

   • Sensory Neurons: Carry impulses from receptors to the CNS.

   • Motor Neurons: Carry impulses from the CNS to effectors (muscles or glands).

Receptors as Transducers

Definition and Role

Receptors are specialized cells or nerve endings that detect specific stimuli (changes in energy levels in the environment). A key characteristic of all receptors is that they act as transducers: they convert the energy of the stimulus (e.g., light, heat, or mechanical pressure) into a form of electrical energy called a generator potential.

Core Receptor Types (AQA Specification)

1. Pacinian Corpuscle: Responds to mechanical pressure (mechanoreceptor).

2. Rods: Photoreceptors in the retina sensitive to low-intensity light, providing black-and-white vision.

3. Cones: Photoreceptors in the retina sensitive to high-intensity light and different wavelengths, providing color vision.

Membrane Potentials and Action Potentials

Resting Potential

In a resting state, the membrane of the sensory neuron is polarized. This means there is a difference in charge across the membrane, typically around $-70 \text{ mV}$. This state is maintained by the active transport of sodium ions ($\text{Na}^{+}$) out of the cell and potassium ions ($\text{K}^{+}$) into the cell via the sodium-potassium pump.

The Generator Potential

When a stimulus is detected, the permeability of the receptor membrane changes, allowing an influx of ions. This creates a localized change in potential difference known as a generator potential. The size of the generator potential is proportional to the intensity of the stimulus.

Threshold and the All-or-Nothing Principle

For a nerve impulse to be sent to the CNS, the generator potential must reach a certain level called the threshold (typically around $-55 \text{ mV}$).

• If the threshold is reached, an action potential is triggered.

• If the stimulus is too weak, the threshold is not reached, and no impulse is sent. This is known as the all-or-nothing principle.

In-Depth Look at Pacinian Corpuscle

Structural Composition

• Concentric Lamellae: The receptor consists of a single sensory neuron ending surrounded by many layers of connective tissue called lamellae, which look similar to the layers of an onion.

• Viscous Gel: The gaps between the lamellae are filled with a thick, viscous gel that helps transmit and distribute mechanical pressure.

• Sensory Neuron: The center of the corpuscle contains the unmyelinated end of a sensory neuron. However, the section of the neuron leaving the corpuscle is myelinated to ensure rapid impulse transmission.

The Role of Stretch-Mediated Sodium Channels

The plasma membrane of the sensory neuron in a Pacinian corpuscle contains unique protein channels called stretch-mediated sodium ion channels. Their permeability to sodium changes when they are physically deformed (stretched).

Mechanism of Action: From Pressure to Impulse

1. Resting State: In the absence of pressure, the stretch-mediated sodium channels are too narrow to allow $\text{Na}^{+}$ ions to pass through. The neuron remains at its resting potential of $-70 \text{ mV}$.

2. Mechanical Deformation: When pressure is applied, the corpuscle is compressed, causing the lamellae to deform. This physical change stretches the membrane of the sensory neuron at the center.

3. Channel Opening: The stretching widens the sodium channels, making the membrane suddenly more permeable to sodium ions.

4. Depolarization (Generator Potential): $\text{Na}^{+}$ ions diffuse rapidly into the neurone down their electrochemical gradient. This influx makes the inside of the membrane less negative, creating a generator potential.

5. Action Potential Initiation: If the pressure is sufficient, the resulting generator potential reaches the threshold voltage. This triggers the opening of voltage-gated sodium channels further down the axon, initiating an action potential.

6. Transmission: The action potential is then propagated along the sensory neuron via saltatory conduction (jumping between nodes of Ranvier) toward the Central Nervous System.

Summary of Key Concepts

• Sensory Specificity: The Pacinian corpuscle only responds to mechanical pressure; it will not respond to light or chemicals because they do not deform the stretch-mediated channels.

• Transduction: It successfully converts mechanical energy into electrical energy.

• Adaptation: The layers of gel and tissue allow the receptor to respond primarily to changes in pressure rather than constant pressure, which is why we "forget" we are wearing clothes after a few minutes.