Topic 6- Receptors (Pacinian corpuscle & the retina) Flashcards
Which stimulus does the Pacinian corpuscle respond to, and where is it found?
Pressure= stimulus
Pacinian corpuscle is found= deep in skin… fingers & feet.
Describe the structure of the Pacinian corpuscle
- consists of single sensory neurones wrapped with layers of tissue (lamellae)
- separated via gel.
- sensory neurone has special channel proteins in plasma membrane.
Describe the structure and function of these channel proteins in the plasma membranes of the sensory neurones.
- These proteins are stretch mediated sodium ion channels.
- Means that the Na+ channels are closed unless they are stretched/ deformed.
- Pressure has to be applied to pull and stretch open the Na+ channels to allow Na+ to diffuse in.
Explain what happens in the resting state?
Pacinian corpuscle
- in the resting state= no pressure stimulus
- No deformed layers and the plasma membrane is also not deformed
- Channels= too narrow in the stretch mediated Na+ channels and so Na+ cannot diffuse in.
- stays at RESTING POTENTIAL.
Explain how an action potential is generated
Pacinian corpuscle
- Pressure is applied
- Pushes down on sensory neurone and stretches and deforms the plasma membrane.
- The stretch mediated Na+ channels would be open (where the plasma membrane is stretched).. Na+ can diffuse in.
- More +VE ions inside compared to outside neuron.
- If there is enough +ve ions so Na+ ions diffusing in…. it can exceed threshold and a response can occur.
- because a action potential has been generated.
Name the two photoreceptors in the eye and state where they are found.
1) Rods & Cones
2) Found in retina
Describe the structure of the Rods and state their function.
Rods are called rods because of their shape.
- Cannot distinguish between different wavelengths of light.
- processes black & white images.
- can detect light at very low intensities.
- because of the concept of retinal convergence.
Explain how rod cells create a generator potential
- Absorbs light intensity.
- Protein/pigment inside rod cells called rhodopsin.
- if a big enough light intensity is absorbed then that pigment is broken down.
- breaking down of the pigment triggers an action potential.
- If enough pigment is broken down to meet the threshold( in bipolar cells), then an action potential can occur.
Why can rod cells function at very low light intensities
(in the dark)?
How is this an advantage?
1) It doesn’t take a lot of light to be absorbed to break down rhodopsin.
2) There’s also a reason why in the dark we can see black and white= retinal convergence.
- Multiple rod cells connecting to a single bipolar cell.
- This is an example of summation…
- because it adds together the pigment broken down by each of the rod cells and then together it produces an action potential.
- An advantage because we are still able to see in black and white even at low light intensities- survival mechanism.
What is the disadvantage of retinal convergence.
1) It provides a low visual acuity
- you don’t have as accurate vision in lower light intensities.
- because there are multiple rod cells connected to a bipolar cell.. your brain cannot distinguish between the separate light sources.
- because it could be a light source triggered by any one of these 3 rod cells
- so 2 light sources together cannot be seen as separate.
1) How many types of cone cells are there and what is their structure?
2) How are they different and how are they similar?
3) why can we see more than the 3 colours?
1) There are 3 types of cone cells….. cone shape
2) They all contain iodopsin pigment but they are in different COLOURS.
- Red
- Green
- Blue
3) depending on the proportion of the different cone cells which are stimulated we perceive a whole range of colours.
When is iodopsin broken down?
Only if there is a high light intensity
- reason for not seeing colours when it is quite dark, because there is not enough light energy to break the iodopsin and trigger an action potential.
-
Why does iodopsin require high light intensity to be broken down?
1) Because iodopsin requires more light energy to be broken down.
2) No retinal convergence.. therefore no spatial summation….
only 1 cone cell connected to 1 bipolar cell.
What is an advantage of cone cells not having retinal convergence
1) Cones give high visual acuity.
- because there’s only 1 cone cell connected to 1 bipolar cell… the brain can distinguish between separate light sources…
- sharper clearer vision in colour
Describe the distribution of the rods and cone cells throughout the retina.
It is uneven.
- The fovea is directly opposite the lens….. receives the highest intensity of light.
- Most cone cells are located near the fovea because they only respond to high light intensities.
- rod cells= further away from the fovea because they respond to lower light intensities.
