Receptors (A-level only) Flashcards
2 features of receptors
Specificity
Generator potential
Specificity
Receptors only respond to specific stimuli.
Examples of these stimuli include:
Light.
Temperature.
Pressure.
This means that a receptor that responds to light will not respond to temperature or pressure.
Generator potentials
Receptors connect with sensory neurones.
When stimulated, the receptor creates a generator potential in the sensory neurone.
An example where stimulation of the receptor creates a generator potential is in the Pacinian corpuscle.
Pacinian corpuscle
The Pacinian corpuscle is a mechanoreceptor found in the skin.
Mechanoreceptors respond to changes in pressure to establish a generator potential.
Resting potential- Pacinian Corpuscle
The Pacinian corpuscle consists of concentric rings of connective tissue that surround a sensory neurone.
When the corpuscle is not being stimulated it is at resting state.
At resting state, the charge inside the neurone is more negative than outside (-70mV).
This is because there are more Na+ ions outside the neurone than inside.
A difference in charge across the cell membrane is called the potential difference.
Stimulation of the receptor- Pacinian Corpuscle
When pressure is applied to the Pacinian corpuscle, the rings of connective tissue apply pressure on the sensory neurone.
The sensory neurone has stretch mediated Na+ channels, these channels normally restrict the movement of Na+ ions.
Applied pressure causes the stretch-mediated Na+ channels to open.
Generator potential- Pacinian Corpuscle
Na+ ions flood into the sensory neurone through the open Na+ channels.
There are now more Na+ ions inside the neurone than outside.
The charge inside the neurone becomes more positive than outside, so the potential difference has changed.
The generator potential has been established.
Action potential- Pacinian Corpuscle
If the generator potential reaches the threshold level (about -50mV) then an action potential is produced in the sensory neurone.
Eye- Sensitivity to light
Rod cells -
Highly sensitive to light.
Cone cells -
Less sensitive to light.
Eye- Visual acuity
Visual acuity is the ability to distinguish between close objects or two points.
Rod cells -
Low visual acuity.
Cone cells -
High visual acuity.
Eye- Number and distribution
Rod cells -
Highly numerous.
Evenly distributed on the retina but absent in the fovea.
Cone cells -
Fewer cells than rod cells.
Distributed mainly at a single point in the retina called the fovea.
Eye- Pigment
Rod cells -
Use a pigment called Rhodopsin.
Rhodopsin detects light and dark.
Rod cells are monochromatic - only detect one wavelength of light.
Cone cells -
Use a pigment called Iodopsin.
Iodopsin detects colour.
Cone cells are trichromatic - divided into three types and each responds to a different wavelength of light, either red, blue or green.
Photoreceptor to generator potential 4 steps:
Detecting light
Bipolar neurone
Sensitivity to light
Spacial summation
Detecting light
Light is absorbed by the pigments in photoreceptor cells:
Rhodopsin is the pigment in rod cells
Iodopsin is the pigment in cone cells.
Absorption of light induces a change in the membrane permeability of the pigments.
This causes Na+ ions to flood into the cell and a generator potential is established.
If the generator potential reaches the threshold, a nerve impulse flows along a bipolar neurone.
Bipolar neurone
Each photoreceptor synapses (forms a junction) with a relay neurone called a bipolar neurone.
Each bipolar neurone synapses with a sensory neurone called a ganglion cell.
Axons of ganglions leave the eye via the optic nerve to send a signal to the brain.
An axon is the long, extended cell body of a nerve cell.
Sensitivity to light
Differences in sensitivity to light are due to differences in how rod and cone cells connect to bipolar neurones.
Each cone cell synapses with a single bipolar neurone.
Sufficient light must stimulate the cone cell to generate an action potential in the bipolar neurone.
Several rod cells synapse with the same bipolar neurone.
Light stimulating a single rod cell may not be sufficient to generate an action potential in the bipolar neurone.
Spatial summation
Several rod cells synapse with the same bipolar neurone.
This means that the cumulative stimulation of more than one rod cell can create an action potential in the bipolar neurone.
This is called spatial summation.
Spatial summation results in retinal convergence.
This is the idea that several rod cells generate a signal in a single sensory neurone.
