Receptors

Question 1

Describe how a generator potential is established in a Pacinian corpuscle

Answer 1

1. Mechanical stimulus - 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 2

Explain what the Pacinian corpuscle illustrates

Answer 2

● 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 3

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

Answer 3

Rods are more sensitive to light
● Several rods connected to a single neurone
● Spatial summation to reach 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 4

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

Answer 4

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 5

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

Answer 5

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

Question 6

Cardiac muscle is myogenic. What does this mean?

Answer 6

It can contract and relax without receiving electrical impulses from nerves

Question 7

Describe the myogenic stimulation of the heart and transmission of a
subsequent wave of electrical activity

Answer 7

1 Sinoatrial node (SAN) acts as pacemaker → releases regular waves of electrical activity across atria
○ Causing atria to contract simultaneously
2 Non-conducting tissue between atria / ventricles prevents impulse passing directly to ventricles
○ Preventing immediate contraction of ventricles
3 Waves of electrical activity reach atrioventricular node (AVN) which delays impulse
○ Allowing atria to fully contract and empty before ventricles contract
4 AVN sends wave of electrical activity down bundle of His, conducting wave between ventricles to apex where it branches into Purkyne tissue
○ Causing ventricles to contract simultaneously from the base up

Question 8

Describe the roles of chemoreceptors, pressure receptors, the autonomic
nervous system and effectors in controlling heart rate

Answer 8

chemoreceptors detect blood [rise / fall] in blood CO2 conc. or [fall/ rise] in blood pH Send impulses to medulla / cardiac control centre Which send more frequent impulses to SAN along [sympathetic /parasympathetic] neurones
So [more / less] frequent impulses sent from SAN and to / from AVN
So cardiac muscle contracts [more / less] frequently
So heart rate [increases / decrease

Question 9

Where are chemoreceptors and pressure receptors located

Answer 9

aorta and carotid arteries