Receptors

Question 1

Common features of sensory receptors

Answer 1

Respond to specific stimuli + produce a generator potential by acting as a transducer
eg: Pacinian corpuscle, rods and cones

Question 2

What is the pacinian corpuscle + describe the structure

Answer 2

• Pressure receptors (respond to changes in mechanical pressure) located deep in skin (fingers + feet)
  -single sensory neurone at centre of layers of connective tissue, each separated by a gel
  -sensory neurone ending at centre, has a stretch mediated sodium channel in its membrane (permeability to sodium changes when they are deformed e.g stretching)

Question 3

Explain how the pacinian corpuscle works

Answer 3

• At rest, the stretch-mediated sodium channels of the membrane around the neuron are too narrow to allow Na+ to pass through - neurone has resting potential
• When pressure is applied, corpuscle becomes deformed + membrane stretches
• Stretching widens sodium channels allowing Na+ to diffuse into neurone
• Influx of Na+ changes potential of membrane, depolarising it, producing generator potential
• This creates an action potential (nerve electrical impulse) that passes along the neurone towards CNS

Question 4

What are the 2 types of photoreceptor cells + where are they located?

Answer 4

Rods + Cones - in retina

Question 5

Describe rod cells

Answer 5

Rod-shaped
Can’t distinguish different wavelengths of light + process images in black + white
Can detect light of very low intensity as many rod cells connect to one single bipolar neurone (retinal convergence)

Question 6

How is a generator potential created in rod cells?

Answer 6

-To create a generator potential the pigment of rod cells (rhodopsin) must be broken down by light energy. Theres enough energy from low-intensity light to cause this breakdown
-Enough pigment has to be broken down for the threshold to be met in the bipolar cell
-Threshold can be reached even in low light as many rod cells are connected to a single bipolar cell - example of summation

Question 7

Advantage + disadvantage of retinal convergence

Answer 7

Advantage:
- can see in black + white even at low light intensities (survival mechanism)
Disadvantage:
- brain can’t distinguish between separate sources of light that stimulated them. 2 light sources close together cannot be seen as separate so rod cells give low visual acuity

Question 8

Describe cone cells

Answer 8

Cone-shaped
3 types that contain different types of iodopsin pigment (red, green + blue) which all absorb different wavelengths of light
Iodopsin is only broken down if there is a high light intensity so action potentials can only be generated with enough light

Question 9

Why can’t we see colour in the dark?

Answer 9

One cone cells connects to a bipolar cell so cones can only respond to high light intensity

Question 10

What is the advantage of one cone cell connecting to one bipolar cell?

Answer 10

The brain can distinguish between separate sources of light so cone cells give a high visual acuity

Question 11

Explain the distribution of rods and cones in the retina

Answer 11

The distribution of rods + cones in retina is uneven
Light is focused by the lens on the fovea which will receive the highest intensity of light.
More cone cells are located near the fovea as they respond to high light intensities + rod cells further away as these respond to low light intensities