Receptors

Question 1

what is the role of a transducer?

Answer 1

to convert one form of energy into another form

receptors in the nervous system convert the energy of the stimulus into electrical energy used by neurones

Question 2

what is the resting potential?

Answer 2

when a receptor has not been stimulated it is in its resting state.
- there is a difference in charge betwen the inside and outside of cell
- inside more negatively chrged
- voltage across the membrane (a potential difference)

Question 3

what is the generator potential?

Answer 3

• generator potential = a change in potential difference due to a stimulus
• 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)

Question 4

what is an action potentials?

Answer 4

if the generator potential reaches the threshold value it will trigger an action potential

Question 5

where are pacinian corpuscles found?

Answer 5

• occur deep in the skin and are most abundant on the fingers and soles of feet
• also occur in joints, ligaments and tendons

they enable the organisms to know which joints are changing direction

Question 6

what does a pacinian corpuscle look like?

Answer 6

• made up of layers of tissues each separated by a viscous gel
• the sensory neurone is at the centre of the layers

Question 7

how do pacinian corpuscle respond to mechanical pressure (stimulus)?

Answer 7

1. in resting state the stretch mediated sodium channels of the membrane are too narrow to allow sodium ions to diffuse into the sensory neurone
2. when pressure is applied the pacinian corpuscle lamellae becomes deformed and the membrane around the neurone becomes stretched
3. this stretching widens the sodium channels and so opens the sodium ions channels and allows sodium ions to diffuse into the neurone
4. this influx changes the potential and depolarise the membrane producing a gnerator potential
5. generator potential create an action potential

The greater the pressure, the more sodium ion channels open so more sodium ions will diffuse into the sensory neurone making it more likely that the threshold will be reached

Question 8

what are the two types of photoreceptors found in the retina of the eye?

Answer 8

rods and cones

both act as transducers converting light energy into electrical energy

Question 9

label the diagram of the eye

Answer 9

Question 10

how abundant are rods and cones?

Answer 10

• rods have greater numbers than cone cells (approx 120 million in each eye)
• less cone cells with 6 million in each eye

Question 11

how are rods and cones distributed?

Answer 11

• rods are mainly found at the periphary of the retina, absent at the fovea
• cones are concentrated at the fovea and fewer at periphery of retina

Question 12

what is visual acuity?

Answer 12

the ability to tell apart points that are close together

Question 13

what is the visual acuity of rods and cones and why?

Answer 13

Rods have low visual acuity
- because many rods are connected to a single bipolar cell
- light that is received by rod cells sharing the same neurone will only generate a single action potential travelling to the brain regardless of how many neurones are stimulated
- the brain cannot distinguish between separate sources of light that stimulated them
- two dots close together cannot be resolved and so appear as a single blob

Cones have high visual acuity
- cones are connected to their own separate bipolar cell connected to a single sensory neurone
- each cone cells sharing sends a separate set of action potentials to the brain
- results in clearer/sharper vision

Question 14

what light intensities are rods and cones sensitive to?

Answer 14

Rods
- detect light at low intensities (see in the dark)
- Many rods connect to a single neurone. A threshold value must be reached before a generator potential is created in a single bipolar cell and there is a much greater chance that the threshold value will be exceeded due to spatial summation, allowing us to see in low light intensities
known as retinal convergence

Cones
- only respond to high light intensities
- each cone cell connects to a separate bipolar cell connected to a single sensory neurone
- means the stimulation of a number of cone cells cannot be combined to help exceed the threshold value and so cannot create generator potential
No retinal convergence

Question 15

what pigment is in rods and cones?

Answer 15

The pigments in rod cells is rhodopsin and must be broken down in order to create a generator potential. There is enough energy from low light intensity to do this.

The pigment in cones is iodopsin which requires high light intensity to break it down. The 3 different types of cone cells have 3 different types of iodopsin (red sensitive, blue sensitive and green sensitive

Question 16

what wavelengths of light do rods and cones distinguish?

Answer 16

• rods cannot distinguish different wavelengths of light so image is in black and white
• there are 3 different types of cones each responding to a different wavelength of light
  1. red sensitive - L cones (longer wavelengths of light)
  2. green sensitive - M cones (medium wavelenghts of light)
  3. blue sensitive - S cones (shorter wavelengths of light)

Question 17

compare rods and cones for the following:
- shape
- abundance
- distribution
- visual acuity
- light sensitivity
- how many types
- pigment
- does it show retinal convergence?

Answer 17