6.7 Neurophysiology of Vision Flashcards
What are the 3 components of a visual field
The left eye monocular vision, the binocular vision and the right eye monocular
Describe the anatomical components of the retina
The macula is the part of the retina that gives central vision and the fovea is a small depression within the macula where visual acuity is the highest. The optic disk is the region where the ganglion cell axons leave the retina to form the optic nerve. It is also characterized by the entrance and exit of ophthalmic arteries and veins that supply the retina. Each retina has 2 hemi retina, a nasal retina side and a temporal retina side.
What is acuity
Visual clarity, how easily you can distinguish between objects
What are the photoreceptors in the retina
Rods and cones are the two types of photoreceptor cells in the retina, and they play a critical role in vision by converting light into electrical signals that are sent to the brain
Describe the distribution of rods and cones in the retina
There are many more rods (90 million) than there are cones (4.5 million). There is low cone density throughout the retina but then a sharp peak in the centre of the retina.There is a high rod density throughout the retina but then a sharp decline in the fovea.
Where is the acuity of the cone system increased
At the fovea, the acuity of the cone system is increased by the displacement of the overlying cellular layers and blood vessels so that light is subjected to a minimum of scattering before it strikes the photoreceptors.
What are some characteristics of cones
Cones are not very sensitive to light and they do not have many photosensitive pigments. They are mostly active during the day and allow you to see colours. They give a high acuity of vision
What are some characteristics of rods
Rods are very sensitive to light and have lots of photosensitive pigments. They are mostly active during the night and dont allow you to see colour. They give a low acuity of vision.
What are the 5 types of neurons in the retina
photoreceptors (rods and cones), bipolar cells, ganglion cells, horizontal cells and amacrine cells
What nervous system is the retina part of
The central nervous system
Describe the laminar organisation of the retina
There is a 3 neuron chain, photoreceptors, then bipolar cells, then ganglion cells which then transmit signals to the optic nerve. In the photoreceptor layer, the rods and cones react to light information and the signal is transmitted to the bipolar layer and then the ganglion layer. The ganglion layer is the only layer that sends action potentials, the other layers produce graded potentials. The horizontal and amacrine cells mediate communication between the cells.
Describe retinal signal transduction in rods responding to both light and dark.
In the dark, there is a high concentration of cGMP in rods which opens cGMP gated Na channels, Na enters rods and membrane depolarises and glutamate nt is released at the synapse with bipolar cells.
In the light the light will activate a pigment which then activates phophodiesterase which converts the cGMP to GMP, closing the cGMP gated Na channels, so the membrane hyperpolarises and glutamate release is limited.
So in the dark, photoreceptors depolarise and release lots of glutamate, in the light, photoreceptors hyperpolarise and dont release much glutamate
Describe the types of cones
In cones, there are different photopigments that are sensitive to lights of different wavelengths. There are short wavelength/blue cones, medium wavelength/green cones and long wavelength/red cones
Describe bipolar ‘on’ and ‘off’ centres
In the presence of light, the photoreceptors (specifically the rods) hyperpolarise, causing less glutamate to be released at the synapses with the bipolar cells.
-some bipolar cells have on centers with metabotropic glutamate receptors, they are activated and depolarises in presence of light (and lack of glutamate being released to them) and release lots of glutamate to the on centre ganglion cells which in turn fire lots of action potentials
-some bipolar cells have off centers with ionotropic glutamate receptors, they are deactivated and hyperpolarise in the presence of light (and lack of glutamate being released to them) and hence do not release much glutamate onto the off centre ganglion cells, so the ganglions do not fire many action potentials
What is a receptive field
A receptive field is an area which, when stimulated by light, causes a change in the membrane potential of the cell, in the receptive field centre there is a direct synaptic input from photoreceptors onto the bipolar cell and in the receptive field surround there is indirect input via horizontal cells
What happens if a receptive field center is exposed to light
If the receptive field center is exposed to light, the photoreceptor will hyperpolarise, causing less glutamate release hence the on center bipolar cells will depolarise
What happens if the receptive field surround is not exposed to light.
If the receptive field surround is not exposed to light, the photoreceptor will depolarise and glutamate will be released onto the horizontal cells causing them also to depolarise, the horizontal cells release inhibitory neurotransmitters like glycine, this prevents glutamate from being released from the photoreceptors and hence the off center bipolar cells are hyperpolarised
What are the 3 types of ganglion cells
M type - 5% , P type - 90% and non M non P cells (5%)
Compare M type and P type ganglion
M type has a larger receptive field, conducts action potentials more rapidly in the optic nerve, is more sensitive to low contrast stimuli, luminance and movement and have a transient burst of action potentials. P type continuously fire action potentials and respond to changes in red/green colour contrast
Describe colour opponent cells
Most P-type and some Non-M non-P cells are sensitive to light wavelength differences; these cells are called colour-opponent cells because the response to one colour/wavelength in the receptive field centre is inhibited by showing another colour/wavelength in the receptive field surround. As M type ganglion cells dont have any colour opposition hence they dont play any role in processing colour
Describe the structure of retinofugal projection
Optic nerves from both eyes merge at the optic chiasm and then seperate again into optic tracts. At the chiasm, nerves from nasal retina side cross over and project into the contralateral hemisphere but the temporal retina nerves do not cross and remain ipsilateral (no decussation). Most of the axons of the optic tract innervate the lateral geniculate nucleus (LGN) on each side. The axons then project from the LGN to the primary visual cortex, this is known as optic radiation. The information processed in V1 then sent to V2,3,4 before it is then sent to many other brain regions.
Describe some visual recognition areas in the temporal cortex
-the lateral occipital area (for object and shape processing)
-the parahippocampal place area (for place processing)
-fusiform face area (for face processing)
Describe some conditions caused by lesions to visual processing areas
visual agnosia- lesion to higher processing areas the visual processing stream (inferotemporal cortex area TE), patients can see but dont recognise what they see
hemineglect syndrome- lesion in dorsal parietal pathway, patients can only see half of the true image
prospagnosia - disorder of not recognising familiar faces, due to lesion in the fusiform face area
