Sight and sound Flashcards
What is the pathway of light into the eye?
Light crosses the rod and cone cells (photoreceptors) and goes to the back of the eye to the retinal ganglion cells.
The retinal ganglion cells have the axons and form the optic nerve, which synapses at the Lateral geniculate nucleus and goes to optic tract which goes to the visual cortex.
What are the structures of the eye?
The retinal ganglion cells in the retina are found in the blind spot.
In the middle of the retina is the macula, there are no blood vessels.
In the middle of the macula is a thinner part - the fovea - has high visual acuity, and light hits this part of the retina.
How do the cells connect?
Via chemical synapses, the horizontal and amacrine cells modulate transmission of information.
There are only action potentials in the Ganglion cells.
What are the segments of the photoreceptors?
Outer segment - membranous disks contain light sensitive photopigments which absorb light.
Inner segment - changes membrane potential.
Cell body
Synaptic terminals - changes release of Glutamate from synaptic terminals.
What are characteristics of rod cells?
Very sensitive to light
Important for good vision in low light situations.
Have many membranous disks - rhodopsin found here.
What are characteristics of cone cells?
Fewer membranous disks.
Contain cone opsins which can respond to red, green and blue.
Requires good light to see colour.
Mostly found in the fovea.
What is phototransduction in the dark?
Photoreceptors are already depolarised:
cyclicGMP gated cation channel open.
Inward Na+ current.
Photoreceptor membrane depolarised.
Release of glutamate to postsynaptic membrane, then goes to bipolar cells.
What is phototransduction in the light?
cGMP gated cation channel closed.
Stop inward Na+ current.
Photoreceptor membrane hyperpolarised.
Reduced or no release of glutamate.
How does Rhodopsin affect cGMP in the dark?
Rhodopsin is inactive so cGMP can bind to the receptors and open the cGMP gated sodium channel and Na+ enters the cell.
How does Rhodopsin affect cGMP in the light?
Light activates Rhodopsin, causes a conformational change which activates transducin (G-protein) which activates phosphodiesterase.
Phosphodiesterase breaks down cGMP into GMP, so there is less cGMP to bind to the cGMP gated sodium channels - less influx of Na+ - hyperpolarisation.
What happens to bipolar cells in the light?
On bipolar cells depolarise and release glutamate. Glutamate then travels to retinal ganglion cells.
Off bipolar cells hyperpolarise and reduce glutamate release.
Why do bipolar cells react differently?
Cells respond in different ways to glutamate because they express different glutamate receptors.
The rate of neurotransmitter release is proportional to membrane potential.
What is the sequence of events in the dark for off bipolar cells?
Photoreceptors are depolarised - release glutamate - binds bipolar cells - channel proteins open and off bipolar cells depolarise.
Action potential increases in retinal ganglion cells.
What is the sequence of events in the light for off bipolar cells?
Photoreceptors cGMP channels close - hyperpolarisation - reduced glutamate release.
Less binding to bipolar off cells - hyperpolarises.
Less firing of action potentials to ganglion cells.
What is the sequence of events in the dark for on bipolar cells?
Photoreceptors depolarised - release glutamate - inhibits on bipolar cells.
Bipolar cells hyperpolarise and reduce firing of action potentials to ganglion cells.
What is the sequence of events in the light for on bipolar cells?
Photoreceptor hyperpolarises - on bipolar cells are excited - depolarise - increased action potential firing of ganglion cells.
What is lateral inhibition?
Horizontal and amarcrine cells have lateral inhibition which is important for enhancing perception of edges and borders.
How does sound move through the ear?
Sound moves through canal and hits Tympanic membrane, causes ossicles to vibrate.
In the oval window, vibration moves fluid through the cochlear, where the Organ of Corti is found within the three chambers.
What are the three chambers of the cochlear?
Scala vestibuli and Scala tympani have low K+ and high Na+.
Scalal media - fluid is endolymph - has high K+ and low Na+.
What is the structure of the Organ of Corti?
Basilar membrane
Hair cells - contain stereocilia at top.
Tectorial membrane - gelatinous, on the top.
Movement or deflection moves whole section and causes stereocilia hair cells to move, and opens or closes K+ channels.
Where does sound move from in the Organ of Corti?
3 layers of outer hair cells, inside the tectorial membrane.
Outer hair cells outnumber inner hair cells - specialised epithelial cells.
Most synapses are in inner hair cells.
Synapse with bipolar cells in spiral ganglion, to the auditory nerve.
What is the movement of sound in the Basilar membrane?
Movement of fluid moves the basilar membrane, so can discern frequency of sound.
Moves the round window - fluid is not compressible.
How is an action potential generated in the ear?
In endolymph: Stereocilia is deflected by movement of fluid.
Mechanosensitive transduction K+ channels open.
K+ influx and depolarisation.
Activates VGCaC to open.
Neurotransmitter glutamate released to spiral ganglion neurite.
Sensory neuron action potential generated.
What are the K+ channels in the endolymph?
K+ channel is mechanically gated TRPA1 channel.
Tip link protein links K+ channels on adjacent stereocilia.
Movement back closes K+ channel.
