6.1.2 Receptors Flashcards
Receptors:
Receptors respond only to specific stimuli.
Stimulation of a receptor leads to the establishment of a generator potential.
How does stimulation of a Pacinian corpuscle produces a generator potential?
Pacinian corpuscle has layers of connective tissue called lamellae surrounding the ending of a sensory neurone axon.
Pressure on the lamellae deforms stretch-mediated sodium ion channels in the neurone membrane, allowing sodium ions to diffuse in, depolarising the membrane and so establishing a generator potential.
Structure of the human retina:
Optical pigments
Light sensitive retinal cells contain optical pigments called opsins.
Opsins break down when they absorb light - a process called bleaching. The bleaching of opsins results in a generator potential.
ATP provides the energy to reform bleached opsins, which takes time.
Cone cells contain iodopsins.
Rod cells contain rhodopsin.
Rhodopsin bleaches at lower light intensities than iodopsins.
Rhodopsin bleaches regardless of the wavelength of light.
Iodopsins only bleach when they absorb specific ranges of wavelength. Human cone cell iodopsins are red light sensitive, green light sensitive or blue light sensitive.
Structure of the human retina:
Cone cells
Cone cells contain the pigment iodopsin.
The human retina contains three types of cone cells:
* red light sensitive cone cells
* green light sensitive cone cells
* blue light sensitive cone cells
Each has a slightly different iodopsin.
There is overlap in the sensitivity of the different cone cells, so the combined effect of all three pigments allows humans to observe all the colours of the visible spectrum.
Cone cells are less sensitive to light that rod cells, but because each cone cell synapses with a separate bipolar neurone, cone cells provide higher acuity.
When a person looks directly at an object, its image is focused on the fovea. The fovea contains more cone cells and less rod cells than other parts of the retina.
Structure of the human retina:
Rod cells
Rod cells contain the pigment rhodopsin.
Rod cells are more sensitive to light that cone cells but because many rod cells synapse with a single bipolar neurone (‘retinal convergence’), rod cells provide lower acuity.
Although there are few rod cells in the human fovea, there are many rod cells than other parts of our retina, meaning we have very good peripheral sensitivity to light.
Summation between rod cells which synapse with the same bipolar neurone means that individual rod cells only need to produce very small generator potentials - the sum of these generator potentials can reach threshold potential in the bipolar neurone.
