A2 Stimuli and Responses

Question 1

What are pacinian corpuscles, where are they found and what do they detect?

Answer 1

• Mechanoreceptors
• Detect mechanical stimuli - pressure and vibration etc.
• Found in skin

Question 2

Describe the basic structure of a pacinian corpuscle

Answer 2

Contain a sensory nerve ending wrapped in layers of connective tissue called lamellae

Question 3

Describe how a generator potential is established in a pacinian corpuscle

Answer 3

• It is stimulated by pressure on skin, deforming lamellae and pressing on sensory nerve ending
• Causes sensory neurone’s cell membrane to stretch, deforming stretch-mediated sodium ion channels
• Channels open - Na ions diffuse into cell, creating a generator potential
• GP reaches threshold - triggers AP

Question 4

Where in the eye are there most photoreceptors?

Answer 4

The fovea

Question 5

How do nerve impulses travel from the eye to the brain?

Answer 5

Impulses from photoreceptors carried from retina to brain by optic nerve - bundle of neurones.

Blind spot where optic nerve leaves - no photoreceptors cells - not sensitive to light

Question 6

How do photoreceptors convert light into electrical impulses?

Answer 6

• Light absorbed by light-sensitive optical pigments in photoreceptor
• Light bleaches pigment - alters membrane permeability to Na ions
• GP created if threshold is reached - nerve impulse sent along bipolar neurone
• BN connect photoreceptors to optic nerve - takes impulse to brain

Question 7

What are the differences between rod and cone cells

Answer 7

1. Rods found in peripheral parts of retina. Cones packed together in fovea
2. Contain different optical pigments - sensitive to different wavelengths
3. Rods give info in black and white. Cones give info in colour.

3 types of cones: red, green and blue sensitive

• Contain different optical pigment
• When stimulated in different proportions, see different colours

Question 8

Describe the difference in sensitivity between rod and cone cells

Answer 8

Rods: very sensitive to light (dim light) - many rods join one neurone - many weak GP’s combine reach threshold to trigger an AP

Cones: less sensitive to light (bright light) - one cone joins one neurone - more light to reach threshold and trigger AP

Question 9

Describe the difference in visual acuity between rod and cone cells

Answer 9

Rods: low visual acuity - many rods join one neurone - light from two points close together can’t be told apart

Cones: high visual acuity - cones are close together and one cone joins one neurone. Light from two points hits two cones - triggers 2 AP’s - go to brain so they can be distinguished as two separate points