3.6.5: Receptors

Question 1

Suggest and explain how the interaction
between the circular and radial muscles could
cause the pupil to constrict (narrow)

Answer 1

1. Circular muscle contracts;
2. Radial muscle relaxes;

Question 2

Explain how a fovea allows an organism to see
in detail

Answer 2

1. High (visual) acuity;
2. (Each) cone is connected to a single neurone;
   Accept no retinal convergence.
   Accept ‘bipolar/nerve cell’ for neurone.
3. (Cones send) separate (sets of) impulses to brain;

Question 3

Explain how a high density of rod cells allows
an organism to have good night vision.

Answer 3

1. High (visual) sensitivity;
   Accept retinal convergence.
2. Several rods connected to a single neurone;
   Accept ‘bipolar/nerve cell’ for neurone
   Accept 2, ‘many’ or
3. Enough (neuro)transmitter to reach/overcome threshold
   OR
   Spatial summation to reach/overcome threshold; more for ‘several’

Question 4

Describe how stimulation of a Pacinian
corpuscle produces a generator potential

Answer 4

1. (There are) stretch-mediated sodium ion channels (in the membrane);
2. (Increased pressure) deforms/changes (sensory neurone/axon)
   membrane/lamella(e) OR (Increased pressure) deforms/changes sodium ion
   channels;
3. Sodium ion channels open;
4. Sodium ions diffuse in;
5. Depolarisation (leading to generator potential);