ReceptorsAQA A-Level Biology: Revision notes

A receptor is a cell or structure that detects a change in the environment. Each type of receptor is specific to one type of stimulus. Photoreceptors respond to light, mechanoreceptors to pressure or stretch, thermoreceptors to temperature and chemoreceptors to chemicals.

A stimulus changes the permeability of the receptor's membrane to ions. This changes the potential difference across the membrane and produces a generator potential. If the generator potential reaches the threshold, it triggers an action potential in the sensory neurone.

The Pacinian corpuscle is a pressure receptor found in the skin, especially of the fingers, soles of the feet and external genitalia. It consists of the ending of a sensory neurone wrapped in layers of connective tissue (lamellae) separated by gel.

It responds only to mechanical pressure: light, for example, does nothing, because it does not deform the membrane.

1. Pressure deforms the layers of the corpuscle and stretches the membrane of the neurone ending.
2. Stretch-mediated sodium ion channels widen.
3. Sodium ions diffuse into the neurone ending, depolarising the membrane and producing a generator potential.
4. A bigger stimulus opens more channels and gives a bigger generator potential.
5. If the generator potential reaches the threshold, voltage-gated sodium ion channels open and an action potential is sent along the sensory neurone.

Action potentials are all-or-nothing, so a stronger stimulus gives a higher frequency of action potentials, not larger ones.

Light passes through the retina to the photoreceptors, which are rods and cones. Light causes the breakdown of the optical pigment in a receptor, which produces a generator potential. Photoreceptors are connected to bipolar neurones, which connect to the neurones that make up the optic nerve.

Rods contain the pigment rhodopsin, are found mainly in the periphery of the retina and work in dim light. They give vision in black and white (shades of grey).

Cones contain iodopsin, are found mainly in the fovea and need bright light. There are three types, sensitive to red, green and blue light.

Sensitivity to light: many rods connect to a single bipolar neurone. The generator potentials of many rods add together (spatial summation) so the threshold is reached even in dim light. Rhodopsin is also broken down by low-intensity light. Cones each have their own bipolar neurone, so a single cone in dim light does not reach the threshold.

Visual acuity is the ability to tell two points apart. Each cone in the fovea has its own bipolar neurone, so adjacent points of light produce separate impulses: high acuity. Rods that share a bipolar neurone give a single impulse, so acuity is low.

Colour: the brain compares impulses from the three types of cone. Rods have one pigment, so they give no colour discrimination.