Receptors

Question 1

How does the CNS recieve information?

Answer 1

• the central nervous system receives sensory information from its internal and external environment through a variety of different and specific receptors
• sensory reception is the function of these receptors, whereas sensory perception involves making sense of the information from the receptors
• this is largely a function of the brain
• we shall now look in detain at one receptor- the pacinian corpuscle

Question 2

What are the features of the pacinian corpuscle?

Answer 2

• it responds only to mechanical pressure
• it converts the energy of the stimulus into a nervous impulse known as a generator potential
  -found deep in the skin
• the single sensory neurone of a pacinian corpuscle is at the centre of layers of tissue, each separated by a gel

Question 3

how does this paninian corpuscle change the mechanical energy of the stimulus into a generator potential?

Answer 3

• the sensory neurone ending at the centre of the pacinian corpuscle has a sodium channel in its plasma membrane
• this is called a stretch-mediated sodium channeled it becomes permeable when stretched
• the stretch-mediated sodium channels of the membrane around the neurone of a pacinian corpuscle are too narrow to allow sodium ions to pass along them, in this state, the neurone of the pacinian corpuscle has a resting potential
• when pressure is applied to the pacinian corpuscle, it is deformed and the membrane around its neurone becomes stretched
• this stretching widens the sodium channels in the membrane and sodium ions diffuse into the neurone
• the influx of sodium ions depolarises the membrane, producing a generator potential
• the generator potential in turn creates an action potential that is passed to the CNS

Question 4

What are the light receptors in the eye?

Answer 4

• the light receptor cells are found on the retina
• there are two main types of light receptors are rod cells and cone cells
• both convert light energy into a nerve impulse

Question 5

What are rod cells?

Answer 5

• rod cells cannot distinguish different wavelengths of light so images are seen only in black and white
• rod cells are more numerous than cone cells
• rod cells are used to detect light of very low intensity
• a certain threshold value has to be exceeded before a generator potential is created in the bipolar cells to which they are connected
• as a number of rod cells are connected to a single bipolar cell = retinal convergence, there is a much greater chance that the threshold value will be exceeded than if only a single rod cell were connected to each bipolar cell
• this is due to summation
• as a result rod cells allow is to see in low light intensity
• a consequence of many rod cells linking to a single bipolar cell is that light received by rod cells sharing the same neurone will only generate a single impulse grovelling to the brain regardless of how many of the neurones are stimulated
• this means that, in perception, the brain cannot distinguish between the separate sources of light that stimulated them
• two dots close together cannot be resolved and so will appear as a single blob
• rod cells therefore give low visual activity

Question 6

What are cone cells?

Answer 6

• there are three types of cone cells that respond to different wavelengths of light
• this allows us to see in colour
• each has its own bipolar cell, to a sensory neurone so nerve impulses can be distinguished by the brain which means there is a high resolution
• they don’t respond to low light intensities
• they have the pigment idopopsin that must be broken down to create a generator potential
• they are located in the foeva which receives the most light intensity

Question 7

What are the differences between rod cells and cone cells?

Answer 7

• rod shaped vs. cone shaped
• there are more rod cells than cone cells
• there are more rod cells at the periphery of the retina, absent at the fovea vs. fewer cone cells at the periphery of the retina, concentrated at the fovea
• rod cells give poor visual acuity vs. cone cells give good visual acuity
• rod cells are sensitive to low intensity light vs. cone cells are not sensitive to low intensity light
• there are only one type of rod cells vs. there are three types of cone cells each respond to different wavelengths of light