Topic 6—A: Stimuli and responses- 4. Receptors

Question 1

How are receptors specific?

Answer 1

• They only detect one particular stimulus
  Example- light

Question 2

What do receptors in the nervous system convert the energy of the stimulus into?

Answer 2

• They convert the energy of the stimulus into the electrical energy used by neurones

Question 3

The resting potential

Answer 3

• When a nervous system receptor is in its resting state there’s a difference in charge between the inside and the outside of the cell
• The inside is negatively charged relative to the outside
• This means there’s a voltage across the membrane
• Voltage is also known as potential difference
• The potential difference when a cell is at rest is called its resting potential
• The resting potential is generated by ion pumps and ion channels

Question 4

Generator potential

Answer 4

• When a stimulus is detected, the cell membrane is excited and becomes more permeable allowing more ions to move in and out of the cell- altering the potential difference
• The change in potential difference due to a stimulus is called the generator potential
• a bigger stimulus excites the membrane more causing a bigger movement of ions and a bigger change in potential difference- so a bigger generator potential is produced

Question 5

Action potential

Answer 5

• If the generator potential is big enough, it will trigger an action potential- an electrical impulse along a neurone
• An action potential is only triggered if the generator potential reaches a certain level called the thresh hold level
• action potentials are all one size so the strength of the stimulus is measured by the frequency of action potentials
• If the stimulus is too weak, the generator potential won’t reach the threshold so there’s no action potential

Question 6

Pacinian corpuscles

Answer 6

• They are mechano receptors
• They detect mechanical stimuli e.g. pressure and vibrations
• found in your skin
• Pacinian corpuscles contain the end of a sensory neurone called a sensory nerve ending
• The sensory nerve ending is wrapped in loads of layers of connective tissue called Lamellae

Question 7

What happens when a pacinian corpuscle is stimulated?

Answer 7

• pressure distorts the pacinian corpmuscles
• deforming stretch mediated na+ channels
• channels open and sodium ions diffuse into the cell creating a generator potential
• If the generator potential reaches the threshold, it triggers an action potential

Question 8

Photoreceptors

Answer 8

• Receptors in your eye that detects light
• Light enters the eye through the pupil and the amount of light that enters is controlled by the muscles of the iris
• Light rays are focused by the lens onto the retina which lines the inside of the eye
• The retina contains the photoreceptor cells
• Fovea is an area of the retina where there are lots of photoreceptors
• Nerve impulses from the photoreceptor cells are carried from the retina to the brain by the optic nerve which is a bundle of neurones
• Where the optic nerve leaves the eye it is called the blindspot- there aren’t any photoreceptor cells so it’s not sensitive to light

Question 9

How photoreceptors work?

Answer 9

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

Question 10

In the human eye, what are the two types of photo receptor?

Answer 10

1. Rods
2. Cones

Question 11

Where are rods mainly found?

Answer 11

• In the peripheral parts of the retina

Question 12

Where are cones mainly found?

Answer 12

• Packed together in the fovea

Question 13

Why are rod and cones sensitive to different wavelengths of light?

Answer 13

• They contain different optical pigments

Question 14

What do rods only give information in?

Answer 14

• Black and white (monochromatic vision)

Question 15

What do cones give information in?

Answer 15

• In colour (trichromatic vision)

Question 16

What are the three types of cones containing a different optical pigment?

Answer 16

1. Red sensitive.
2. Green sensitive.
3. Blue sensitive.
   - When they’re stimulated in different proportions, you see different colours

Question 17

Sensitivity (rods)

Answer 17

• Rods cells are very sensitive to light (they work well in dim light)
• This is because many rods join one bipolar neurone so many week generator potentials combine to reach the threshold and trigger and action potential

Question 18

Sensitivity (cones)

Answer 18

• Cones are less sensitive than rods (they work best in bright lights)
• This is because one cone joins one bipolar neurone so it takes more light to reach the threshold and trigger an action potential

Question 19

Definition of visual acuity

Answer 19

• It is the ability to tell apart points that are close together

Question 20

Visual acuity (rods)

Answer 20

• Rods give low visual acuity because many rods join the same bipolar neurone which means light from two points close together can’t be told apart

Question 21

Visual acuity (cones)

Answer 21

• cones give high visual acuity because cones are close together and one cone joins one bipolar neurone
• When light from two points hits two cones, two action potentials one from each cone goes to the brain- so you can distinguish two points that are close together as two separate points

Question 22

Rods

Answer 22

• Mainly located in the peripheral parts of the retina
• Give information in black and white
• Many rods join one bipolar neurone
• High sensitivity to light
• Give low visual acuity
• contains pigment Rhodopsin

Question 23

Cones

Answer 23

• Mainly located in the fovea
• Give information in colour
• One cone joins one bipolar neurone
• Low sensitivity to light
• give high visual acuity
• Contains pigment Iodopsin

Question 24

How do rods allow us to see in lowlight intensities?

Answer 24

• Lower threshold potential (less Rhodopsin) needs to be broken down to initiate an action potential
• several rods share a sensory neurone

Question 25

What is always open during resting potential?

Answer 25

- k+ channels are open so potassium can leave - Pump is going the whole time - Na+ channels are closed

Question 25

What is open when there is a stimulus?

Answer 25

- Na+ channels

Question 26

What is open when there is an action potential?

Answer 26

- More na+ channels

Question 27

Resting potential (channels)

Answer 27

- Outside the neurone= positive due to presence of Na+ and k+ - Inside of neurone= negative due to presence of anions - Gated na+ channels are closed so little diffusion of na+ into neurone - k+ channels are open, so k+ can diffuse out of neurone - Na+ and k+ pump, pumping 3 na+ out of neurone and 2k+ into neurone

Question 28

Generator potential (channels)

Answer 28

- Outside of neurone- gets more negative due to loss of na+ and k+ ions - Inside the neurone is more positive due to the presence of na+ and K+ ions - Stimulus causes gated na+ channels to open, increasing the permeability of the membrane to na+ - 2k+ na+ diffuses down its concentration gradient into the neurone making potential difference more positive if there is enough na+ diffusing across membrane then threshold potential is reached and membrane is depolarised