Paper 2 - Receptors

Question 1

What is a receptor?

Answer 1

A type of cell that detects a stimulus and sends an impulse along a sensory neurone to the CNS

Question 2

What is a stimulus?

Answer 2

A detectable change in the environment that can cause a response by the nervous system

Question 3

Structure of Pacinian Corpuscles

Answer 3

• Modified sensory neurone endings which consists of layers of connective tissue (lamellae) surrounding the end of a sensory neurone
• Viscous gel between the layers
• Corpuscle is stimulated by a change - firm pressure
• Known as mechanorceptors as they’re sensitive to changes in mechanical pressure

Question 4

What are photoreceptors?

Answer 4

Receptors in the eye that detect light.

Question 5

The eye

Answer 5

• Light enters the eye through the pupil and the amount of light that enters is controlled by the muscles of the iris.
• The light rays then focus on the retina which is found at the back of the eye.
• On the retina there’s an area where there are lots of condensed photoreceptors, known as the fovea.
• Nerve impulses from the photoreceptors are carried from the retina to the brain via the optic nerve.
• Where the optic nerve leaves the retina is known as the blind spot - no photoreceptors present there.

Question 6

Types of photoreceptors

Answer 6

1. Rods - allow vision in dim light
   Many rods synapse with one bipolar neurone = low acuity but high sensitivity.
2. Cones - sensitive to high light intensity

Question 7

When photoreceptors convert light into an electrical impulse

Answer 7

1. Light entered the eye, hits the photoreceptors and is absorbed by light sensitive optical pigments.
2. Light bleaches the pigments causing a chemical change and altering the membrane permeability to sodium ions.
3. A generator potential is created and if it reaches the threshold a nerve impulse is sent along a bipolar neurone
4. Bipolar neurons connect photorectops to optic nerve, which takes impulses to the brain.

Question 8

Process of action potential

Answer 8

1. Stimulus excited membrane - some Na+ channels open and Na+ diffuses into neurone.
2. When threshold is reached Na+ channels open and depolarisation occurs - membrane potential difference becomes positive and AP fired
3. At +30mv, Na+ channels close and K+ channels open so K+ diffuses out (repolarisation)
4. Hyper polarisation - membrane potential drops below resting potential as K+ channels are slow to close and some leak out.
5. Refractory period - no other APs can fire during this time
6. Na+|K+ pump resting potential by pumping 3 Na+ out for 2 K+ in