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.
