Physiology of vision Flashcards
Go over the structure of the eye
Appreciate these 4 points in order to see an object:
- The pattern of the object must fall on the vision receptors (rods and cones in the retina)
- The amount of light entering the eye must be regulated (too much light will “bleach out” the signals)
- The energy from the waves of photons must be transduced into electrical signals
- The brain must receive and interpret the signals
Describe the cellular structure of the retina and the pathway for signal transmission of light
Retina is comprised of 3 main types of cells:
- Ganglion cells
- Bipolar cells
- Photoreceptors
Light essentially passes through the ganglion and bipolar cells to reach the photoreceptors where electromagnetic radiation is converted to neural signals (transduction!)
Transmitted to the bipolar and then ganglion cells
What are the 2 important parts of a photoreceptor in which phototransduction takes place?
The rod and cone photoreceptors
Describe the process of phototransduction (process where light is converted into electrical signals)
Unstimulated photoreceptors have a depolarised resting membrane potential (Vm) of -20 (this is very different from other neurons)
When light hits the photoreceptors it results in hyperpolarisation as Na+ channels close and there is eflux of K+ greater now than the influx of Na+ resulting in signal production
Note only ganglion (and some amacrine cells) produce AP’s, all other cells produce graded changes in membrane potential
Why is an unstimulated photoreceptors Vm depolarised ?
Because of the ‘dark current’ A cGMP-gated Na+ channel that is open in the dark and closes in the light (recall that Na+ unflux causes depolarisation ==> when unstimulated this is occurring)
Define what visual acuity is and what it is determined by
- Refers to the clarity of someones vision
- Determined by photoreceptor spacing and refractive power
Describe the retinal receptive field
- The part of the retina that needs to be stimulated to elicit AP’s from a ganglion cell is the cell’s receptive field.
- Retinal receptive fields are small and concentric and correspond to the part of the world that the cells can “see”.
What type of light do rod photoreceptors help us sense ?
Seeing in Dim light
What type of light do cone photoreceptors help us sense ?
seeing in normal daylight
Explain the basis of colour discrimination in vision
Basically there is different sub-types of cone photoreceptors which pick up different wavelengths corresponding to different colours of light
Define retinotopy
This is the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream
Give an overview of the visual pathway
- Vision is generated by photoreceptors in the retina
- Signals leave the eye through the optic nerves - the fibres carrying info from the temporal visual field (bascially the outside haves of each eyes vision, the fibres which carry this info however are called the nasal fibres which can be confusing) decussate at the optic chiasm.
- After the chaism the axons are called the optic tracts which go to the lateral genitculate nucleus where axons synapse (LNG is located in the midbrain hence CN III connects to the midbrain)
- From here axons from the LGN fan out as the optic radiations and travel to the primary visual cortex in the occipital lobe
Where is the primary visual cortex located ?
In and around either side of the calcarine sulcus in the occipital lobe
Go over the visual field defects:
- A – monocular blindness – lesion in optic nerve
- B – bitemporal hemianopia – lesion in optic chiasm
- C – left homonymous hemianopia – lesion in optic tract
- D – superior homonymous quadrantanopia - lesion in temporal lobe of optic radiation
- E – left homonymous hemianopia with foveal sparing – lesion in occipital cortex
