receptors

Question 1

Describe the basic structure of a Pacinian corpuscle

Answer 1

lamellae
sensory neurone ending
sensory neurone axon
myelin sheath
stretch mediated ion channel
gel

Question 2

Describe how a generator potential is established in a Pacinian corpuscle

Answer 2

1. Mechanical stimulus eg. pressure deforms
   lamellae and stretch- mediated sodium ion
   (Na+) channels
2. So Na+ channels in membrane open and Na+
   diffuse into sensory neurone
   ○ Greater pressure causes more Na+channels to open and more Na+ to enter
3. This causes depolarisation, leading to a
   generator potential
   ○ If generator potential reaches threshold
   it triggers an action potential

Question 3

Explain what the Pacinian corpuscle illustrates

Answer 3

● Receptors respond only to specific stimuli
○ Pacinian corpuscle only responds to mechanical pressure
● Stimulation of a receptor leads to the establishment of a generator potential
○ When threshold is reached, action potential sent (all-or-nothing principle)

Question 4

Explain the differences in sensitivity to light for rods & cones in the retina

Answer 4

Rods are more sensitive to light
● Several rods connected to a single neurone
● Spatial summation to reach / overcome threshold (as enough
neurotransmitter released) to generate an action potential

Cones are less sensitive to light
● Each cone connected to a
single neurone
● No spatial summation

Question 5

Explain the differences in visual acuity for rods & cones in the retina

Answer 5

Rods give lower visual acuity
● Several rods connected to a single neurone
● So several rods send a single set of impulses
to brain (so can’t distinguish between
separate sources of light)

Cones give higher visual acuity
● Each cone connected to a single neurone
● Cones send separate (sets of) impulses to brain
(so can distinguish between 2 separate sources
of light)

Question 6

Explain the differences in sensitivity to colour for rods & cones in the retina

Answer 6

Rods allow
monochromatic vision
● 1 type of rod / 1
pigment

Cones allow colour vision
● 3 types of cones - red-, green- and blue-sensitive
● With different optical pigments → absorb different wavelengths
● Stimulating different combinations of cones gives range of colour perception