6.1.2 Receptors

What is the outer structural component of a Pacinian corpuscle, and what is it made of?

Lamellae – these are concentric layers of connective tissue that surround the sensory neurone ending.

Describe the structure of the Pacinian Corpuscle.

A sensory neurone ending wrapped in lamellae of connective tissue.

What structure is found at the very centre of the Pacinian corpuscle?

The sensory neurone ending (the terminal of the sensory neurone).

What specific type of ion channel is present in the membrane of the sensory neurone ending?

Stretch-mediated sodium ion (Na⁺) channels.

In its resting state (no pressure), what is the state of these stretch-mediated Na⁺ channels?

They are closed.

What is the fluid-like substance indicated in the diagram, located between the lamellae?

Gel.

What structure continues from the corpuscle to transmit the signal to the CNS?

The sensory neurone axon.

What insulating layer surrounds the axon, and what cells form it?

The myelin sheath, formed by Schwann cells.

What is the specific type of stimulus that a Pacinian corpuscle detects, and what is the initial physical effect?

It detects a mechanical stimulus (e.g., pressure, vibration). This pressure deforms the lamellae.

What is the direct consequence of this deformation on the neurone's membrane?

The deformation stretches and opens the stretch-mediated sodium ion (Na⁺) channels in the sensory neurone ending's membrane.

What happens when these channels open?

Sodium ions (Na⁺) diffuse into the sensory neurone, moving down their electrochemical gradient.

How does the corpuscle code for the intensity (strength) of the pressure stimulus?

Greater pressure causes more deformation, which opens more stretch-mediated Na⁺ channels, allowing more Na⁺ ions to enter the sensory neurone.

What is the electrical result of Na⁺ influx?

It causes depolarisation of the sensory neurone ending (the membrane potential becomes less negative/more positive).

What is the name of the local depolarisation that is established?

A generator potential.

Under what condition does this generator potential lead to a transmitted signal?

If the generator potential is large enough to reach the threshold potential, it triggers an action potential in the sensory neurone.

What does the Pacinian corpuscle illustrate about receptor specificity?

Receptors respond only to specific stimuli. The Pacinian corpuscle is specific to mechanical pressure.

What does it illustrate about the general process of sensory transduction?

The stimulation of a receptor leads to the establishment of a generator potential.

What key principle of nerve signalling does it demonstrate?

The all-or-nothing principle. An action potential is only sent if the generator potential reaches threshold.

How are photoreceptors (rods, cones..) stimulated?

• Light-sensitive optical pigments in photoreceptors absorb light, bleaching them.

• Bleaching increases membrane permeability to sodium ions.

• Sodium ions diffuse in, moving down their electrochemical gradient.

• A generator potential is established.

• If generator potential is large enough to exceed the threshold, an impulse travels along the bipolar neurone to the optic nerve.

Which photoreceptor is more sensitive to light, rods or cones?

Rods are more sensitive to light than cones.

What are bipolar neurones?

Neurones that connect photoreceptors to the optic nerve.

How are rods connected to the bipolar and ganglion neurones?

Several rods are connected to a single bipolar neurone, which is then connected to a single ganglion neurone.

What is the functional advantage of this multiple connection (convergence) for rods?

It allows for spatial summation.

Weak stimuli from many rods can be combined (summated) at the bipolar neurone.

This releases enough neurotransmitter to reach the threshold potential and generate an action potential in the ganglion cell, even in dim light.

Which photoreceptor is less sensitive to light, rods or cones?

Cones are less sensitive to light than rods.

How are cones connected to the bipolar and ganglion neurones?

Each cone is connected to its own single bipolar neurone, which connects to its own single ganglion neurone (a one-to-one connection).

What is the functional consequence of this one-to-one connection for light sensitivity?

There is no spatial summation.

A single cone must be stimulated strongly enough on its own to reach the threshold and generate an action potential.

This requires brighter light.

Define ‘visual acuity’

The ability to visually distinguish points which are close together.

Which photoreceptor provides lower visual acuity (sharpness/resolution), rods or cones?

Rods give lower visual acuity.

What is the structural reason for the low acuity of rods?

Because several rods are connected to a single bipolar neurone.

Light from two separate points that stimulates two different rods in the same group will be perceived as a single point of light by the brain.

Which photoreceptor provides higher visual acuity (sharpness/resolution), rods or cones?

Cones give higher visual acuity.

What is the structural reason for the high acuity of cones?

Because each cone is connected to its own single bipolar neurone, which connects to its own single ganglion neurone.

Light from two separate points stimulates two separate cones, sending two separate sets of impulses to the brain, generating two action potentials, allowing them to be distinguished.

What type of vision do rods provide?

Monochromatic vision (shades of grey/black and white).

What is the pigment-based reason rods cannot detect colour?

There is only one type of rod cell, containing a single type of visual pigment (rhodopsin). This pigment absorbs a broad range of wavelengths but does not allow colour discrimination.

What type of vision do cones provide?

Colour vision.

How many types of cone cells are there, and what are they called?

Three types of cone cells: red-sensitive, green-sensitive, and blue-sensitive cones.

What is different about the pigments in each cone type?

Each type contains a different optical pigment (iodopsin) that absorbs light most effectively at a different range of wavelengths (red, green, or blue light).

How does the brain perceive the full range of colours?

Different wavelengths of light stimulate the three cone types in different proportions. The brain interprets the relative stimulation of these three types to perceive the full spectrum of colours.