3.6.1.2 Receptors (A-level only)

Question 1

Comment on the specificity of receptors.

Answer 1

Receptors only detect one particular stimulus - light, pressure or [glucose] etc.
Many different types of receptor that each detect a different type of stimulus.
Some receptors are cells (photoreceptors) whereas others are proteins on cell surface membranes (glucose receptors are proteins
found in cell membranes of some pancreatic
cells).

Question 2

How do receptors initiate a response to a stimulus? (comment on action potentials).

Answer 2

1. Potential difference across membrane.
2. Potential difference at rest = resting potential.
3. Stimulus detected, cell membrane excited and becomes more permeable - more ion movement across membrane - affecting the potential difference.
   => change in pd due to a stimulus = generator potential.
4. Bigger stimulus excites membrane more, causing a larger charge in pd —> bigger generator potential produced.
5. Action potential ( = electrical impulse along neurone) triggered if gp reaches threshold level. Action potentials are all the same size so strength of a stimulus is measured by the frequency of action potentials.
6. If stimulus is too weak, gp won’t reach the threshold level so there is no action potential.

Question 3

What are Pacinian Corpuscles? Where are they found?

Answer 3

= Mechanoreceptors (detect mechanical stimuli such as pressure and vibrations).
Found in the skin.
Contain a sensory nerve ending, wrapped in layers of connective tissue called lamellae.

Question 4

How do Pacinian Corpuscles work?

Answer 4

1. When PC stimulated, the lamellae are deformed and press on the sensory nerve ending.
2. This causes the sensory neurone’s cell membrane to stretch, deforming the stretch-mediated Na+ ion channels. Channels open and Na+ diffuse into cell, creating a generator potential.
3. If the generator potential reaches the threshold level, it triggers an action potential.

Question 5

How is light received by the eye?

Answer 5

1. Light enters the eye through the pupil - amount of light entering is controlled by the iris muscles.
2. Light rays focused by lens onto retina, which lines the inside of the eye. The retina contains photoreceptor cells which detect light.
3. Fovea = area of the retina with many photoreceptors.
4. Nerve impulses from photoreceptor cells carried from retina to brain by the optic nerve, which is a bundle of neurones.
5. Where the optic nerve leaves the eye is called the blind spot - no photoreceptor cells so not sensitive to light.

Question 6

How do photoreceptors convert light into an electrical impulse?

Answer 6

1. Light hits photoreceptor cells and is absorbed by light-sensitive optical pigments.
2. Light bleaches the pigments, causing a chemical change and altering the membrane permeability to Na+ ions.
3. Generator potential created and if reaches the threshold, an action potential is sent along a bipolar neurone (connected to both photoreceptor and optic nerve, which takes impulses to the brain).
4. Two types of photoreceptor cells in humans - rods and cones.

Question 7

Where are rods mainly found?

Answer 7

Rods mainly found in the peripheral parts of the retina.

Question 8

Comment on the colour of rods.

Answer 8

Rods and cones contain different optical pigments making them sensitive to different wavelengths of light.
Rods only give information in black and white (monochromatic vision).

Question 9

Comment on the sensitivity of rods.

Answer 9

Rods are very sensitive to light - fire action potentials in dim light.
—> this is because many rods join one neurone so many weak generator potentials combine to reach the threshold and trigger an action potential.

Question 10

Define visual acuity.

Answer 10

= The ability to tell apart points that are close together.

Question 11

Comment on the visual acuity of rods.

Answer 11

Rods give low visual acuity because cones are close together and one cone joins one neurone —> light from two points close together can’t be told apart.

Question 12

Where are cones found?

Answer 12

Cones are found packed together in the fovea.

Question 13

Comment on the colour of cones.

Answer 13

Rods and cones contain different optical pigments making them sensitive to different wavelengths of light.
Cones give information in colour (trichromatic vision).
There are 3 types of cones, each containing a different optical pigment —> red - / green - / blue-sensitive.
When the three different types of cones are stimulated in different proportions, we see different colours.

Question 14

Comment on the sensitivity of cones.

Answer 14

Cones are less sensitive to light than rods (only fire action potentials in bright light).
This is because one cone joins one neurone, so it takes more light (i.e. a bigger stimulus) to reach the threshold and trigger an action potential.

Question 15

Comment on the visual acuity of cones.

Answer 15

Cones give high visual acuity because they are packed close together, and because one cone joins one neurone.
Therefore, when light from two points hits two cones, two action potentials (one from each cone) go to the brain —> can distinguish two points that are close together as two separate
points.