Layers of retina (deep to superficial)
Pigment layer
- 1 layer of cells melanin contacting epithelium
- Absorbs stray light
Neural layer
Photoreceptor layer
- Rod cells
- Stimulated under dim light
- Not colour vision
- More sensitive to dark light
- Located in the periphery of the retina.
- Cone cells
- Stimulated under bright light
- At bright light rhodopsin in cone cells replenishes faster than rod cells. So that rod cells will not have readily available rhodopsin to provide signal
- Colour vision
- Has 3 types Blue/Green/Red
- Located
- Only type of photoreceptor present in the fovea.
- At the posterior pole of the eye is a yellowish spot, the macula lutea, the center of which is a depression called the fovea centralis
- Adaptations of the fovea to mediate the highest visual acuity in the retina.
- Neurons of the inner layer of retina are actually displaced laterally to the side of the fovea to minimize light scattering on the way to the receptors.
- The ratio of photoreceptors to ganglion cells falls dramatically.
- Most foveal receptors synapse on only one bipolar cell, which synapses on only one ganglion cell.
- Because each ganglion cell is devoted to a very small portion of the visual field, central vision has high resolution.
- In other words, the receptive field of a foveal ganglion cell (i.e., the region of stimulus space that can activate it) is small.
- At the periphery, the ratio of receptors to ganglion cells is high; thus, each ganglion cell has a large receptive field. The large receptive field reduces the spatial resolution of the peripheral portion of the retina but increases its sensitivity because more photoreceptors collect light for a ganglion cell.
- The magnitude of phototransduction amplification varies with the prevailing levels of illumination (light adaptation). At low levels of illumination, photoreceptors are the most sensitive to light.
- As levels of illumination increase, sensitivity decreases (due to reduction in calcium currents in the rod outer segment), preventing the receptors from saturating and thereby greatly extending the range of light intensities over which they operate.
Bipolar cell layer
- Bipolar cells
- Amacrine cells
- Horizontal cells
Ganglion cell layer (ganglion cells)
- Ganglion cells
- The output of the retina is determined by ganglion cells which can generate action potentials and give rise to optic nerve.
- 2 types
- Ganglion cells concerned with details of image formation
- 99% of all ganglion cells
- Receive input from rods and cones via synaptic relays through the layers of the retina
- Function
- Circadian rhythms
- Pupillary light reflex.
- Melanopsin-containing ganglion cells
- < 1% of all ganglion cells
- Are intrinsically sensitive to light
- Will generate action potentials (even without rods/cones, particularly blue light);
- Function
- Are not concerned with image formation
- Maintaining circadian rhythm
- Via connections to the suprachiasmatic and pretectal nuclei
- This type of ganglion cell explains why those blind due to rod/cone disease (e.g., retinitis pigmentosa) may still have an intact pupillary reflex and maintain circadian rhythm.
Receptive fields
- To allow for better contrast by turning on or off adjacent bipolar cells
- The difference between the receptive fields of bipolar cells and neurons of the primary visual cortex are that
- Bipolar cells have a circular receptive field,
- Neurons of the primary visual cortex have rectangular receptive fields.
- Receptive fields are due to the presence of two types of bipolar cells working with the horizontal cells
- Two types of bipolar cells
- On bipolar cells
- Light hits cone cells → cone cells hyperpolarise → dec. Glutamate release from cone cells → dec stimulation of on bipolar cells → bipolar cells depolarises
- Off Bipolar cells
- Light hits cone cells → cone cells hyperpolarise → dec. Glutamate release from cone cells → dec stimulation of off bipolar cells → bipolar cells hyperpolarises
- Horizontal cells
- Having the ability to switch on and switch off cone cells
- When horizontal cells are stimulated (green arrow) by cone cells they send back a inhibitory signal (red arrows)
- Two situations for the on Bipolar cell
- Receptive fields are made up of two circles
- Small inner circle: called centre
- Large outer circle: called surround
- Together they make two types of receptive fields
- Turns on (inc firing rate) when
- Centre is stimulated with light
- Surround is not stimulated with light
- Turns off (dec firing rate) when
- Centre is not stimulated with light &
- Surround is not stimulated with light OR
- Centre is not stimulated with light &
- Surround is stimulated with light
- Back to baseline (normal firing rate)
- Centre is not stimulated with light &
- Surround is not stimulated with light OR
- Centre is stimulated with light &
- Surround is stimulated with light
- When Centre has light and surround has no light
- Centre cone cell is stimulated; surround cone cells are not stimulated
- Centre cone is hyperpolarized; surround cones are depolarized
- Centre cone does not release glutamate; surround cones releases glutamate
- The horizontal cell are stimulated;
- Inhibitory signals are sent back to cone cells by horizontal cells;
- Bipolar cells are not stimulated by glutamate
- Bipolar cells depolarizes and sends signal down the ganglion cells
- When Centre has no light and surround has light
- Centre cone cell is not sti; surround cone cells are sti
- Centre cone is depolarized; surround cones are hyperpolazried
- Centre cone release glutamate; surround cones do not release glutamate
- The horizontal cells are not stimulated;
- No inhibition by horizontal cells;
- Bipolar cells are stimulated by glutamate
- Bipolar cells hyperpolarizes and no signal is sent down to the ganglion cells
- The opposite of the On Centre
On centre
Off centre
Images
- The eye is the only place where glutamate is inhibitory
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