1) Describe the connectivity and synaptic quality between photoreceptors and bipolar cells.
Photoreceptors, which are located in the back of the retina, detect light. There are two types of receptors: rods and cones. Rods are used for scotopic or night vision, whereas cones are used for photopic or color vision. For most of the retina, rods out number the cones, except for in the fovea, which is the location for the highest visual acuity.
During the process of phototransduction when light is involved, 11-cis-retinal, derived from Vitamin A, is combined with opsin to form rhodopsin. Light absorption by rhodopsin triggers a rapid series of steps, in which retinal changes shape, by going from 11-cis to all-trans, and eventually releasing opsin. The opsin that is released activates the G protein known as transducin. Transducin then activates phosphodiesterase, which works to break down cGMP and close the sodium channel. This closure of the channel prevents sodium from entering, thus leading to a hyperpolarization of the photoreceptor cell. Thus, voltage-gated calcium channels close in synaptic terminals, causing no glutamate to be released.
For phototransduction for the dark current, cGMP gated channels remain open, allowing for a sodium influx, leading to depolarization of the photoreceptor cell. The voltage-gated calcium channels will open in the synaptic terminals and thus, the neurotransmitter glutamate will be released.
For both the dark and light currents, the photoreceptors do not have action potentials, only receptor potentials.
Photoreceptors can then synapse on bipolar cells of which there are two types: on-type and off-type cells. On-type cells depolarize when light is present whereas off-type cells depolarize in darkness. Their receptiveness to the graded potentials from photoreceptors depends on the different receptors on these bipolar cells. On-type bipolar cells have metabotropic receptors (mGluR6) and off-type have ionotropic receptors (AMPA). Thus, when there is light, the photoreceptors are hyperpolarized. This leads to less release of glutamate. The decreased release of glutamate means less metabotropic receptors on on-type cells are activated and thus lower inhibitory effects. Sodium channels are kept open and depolarization occurs. Thus, on-type bipolar cells depolarize when there is light. This is a sign-inverting pathway.
On the other hand, when there is darkness, photoreceptors are depolarized. More glutamate is released and binds to ionotropic receptors on off-type bipolar cells. This leads to depolarization of off-type bipolar cells. This is a sign-preserving pathway.
2) Describe the connectivity between photoreceptors and horizontal cells
Horizontal cells help us to visualize a greater contrast in off-center or on-center receptive fields via lateral inhibition. They connect the peripheral photoreceptors to the center photoreceptors. The main function (synaptic quality) of the horizontal cells is to inhibit the neighboring center-photoreceptor. This action is carried out in the presence of glutamate which is directly provided by the surrounding-photoreceptor.
If glutamate is provided by the surrounding-photoreceptor, the horizontal cell will be depolarized (activated) and release GABA to the neighboring photoreceptor. Horizontal cells synapse directly on the center-photoreceptor. GABA inhibits the neighboring center-photoreceptors, which further reduces the outflow of glutamate directed to the bipolar cell. This is an example of an on-center/off-surround condition. However, photoreceptors only provide glutamate to horizontal cells when it is depolarized in a dark condition.
In an off-center/on-surround condition, the surrounding photoreceptor will be hyperpolarized and unable to provide enough glutamate to the horizontal cell. The horizontal cell is hyperpolarized (inhibited) in this case and will not provide enough GABA to inhibit the neighboring center-photoreceptor. Horizontal cells work to enhance the visual stimuli in the given condition, making the light appear brighter or absence of light appear darker. It is important to keep in mind that horizontal cells have receptor potentials and not action potentials.
3) Describe the responses of center-surround antagonistic receptive fields under differential lighting conditions.
The visual system has a unique organization for the detection for the detection and clearing up of the light we encounter on a day to day basis. The receptive field for light is organized in a manner where there is a center that contains photoreceptors and a surround can that contains photoreceptors. When light shines on the different areas it creates light or dark effect depending on where the light is located. The photoreceptors have associated bipolar cells have antagonistic functions in the presence of light in the center vs. light in the surround.
On-Center/ Off-Surround Receptive Field:
In the presence of light in the center and darkness in the surround. The center photoreceptors hyperpolarize in the presence of light and do not release glutamate from the synapse. The decreased release of glutamate then decreases the inhibitory effect glutamate has on the on center bipolar cell. In turn, the on-center bipolar cell depolarizes and releases glutamate. Darkness in the surround will depolarize the surround photoreceptors and release glutamate. The glutamate from the surround photoreceptors stimulate the connected horizontal cell to release GABA onto the adjacent center photoreceptor. This in turn increases the inhibitory effect on the release of glutamate in the center photoreceptors. The on-center bipolar cell then releases an increased amount of glutamate from the synapse to send to the retinal ganglion cells to the brain. The effects of light in the surround and darkness in the center allow for increased contrast of the light from the dark stimulus, by making the light brighter that it actually is.
Off-Center/ On-Surround Receptive Field:
In the presence of light in the surround and darkness in the center, the effect is the opposite than what was stated for on center off surround receptive field. The center photoreceptor becomes depolarized from the darkness, allowing for the release of glutamate from the synapse. The increased glutamate acts on the on-center bipolar cell in an inhibitory fashion. This causes a decrease in glutamate from the on-center bipolar cell. The light in the surround hyperpolarizes the surround photoreceptor and decreases the release of glutamate. The decrease in glutamate therefore inhibits the inhibitor which is the horizontal cell and decreases the GABA released onto the center photoreceptor. In turn, more glutamate is released from the center photoreceptor further increasing the amount of glutamate released from the on-center bipolar cell. This allows for contrast to enhance the darkness that is being received in the center.
4) Describe the construction of orientation-selective receptive fields.
After all the information discussed above reaches the primary visual cortex, a few things must occur in order for it to be pieced together and processed as an an image. The circular center-surround visual receptive fields generated by horizontal cells must be combined to make a comprehensive image of the outside world. The horizontal cell gathers information from many surround cells and transmits it to one center cell. The circular center-surround fields generated overlap and form a continuous bar shaped image. This image that is made up of many circular receptive fields can be of the “on center/off surround” or “off center/on surround” type. These new images are arranged in different orientations in the primary visual cortex and can only be processed as a complete image if they are presented to a place in the cortex that processes that specific orientation of image. The configuration of the receptive field must match the configuration of the stimulus in order to be processed by the primary visual cortex.
5) Describe the construction of motion- (direction-) selective receptive fields.
The motion selective fields are composed of a number receptive fields merging together to produce an orientation selective, rod-like receptive field. The “bars” must have a sufficient amount of space in the arrangements to report a moving stimulus. The space between the receptive fields for motion is required because in order for motion to be detected, a stimulus has to excite one receptive field and then traverse to excite another while leaving the previous receptive field. This will indicate that the stimulus is in a new location and no longer in the field of view that excited the original receptive field that was activated. Motion will be detected when successive receptive fields are activated as the previous field is no longer stimulated. Moreover, the receptive fields all have to respond to stimuli moving in the same direction. Receptive fields for motion project to complex neurons in the striate cortex that respond to stimuli in a bidirectional manner, meaning the neuron will be depolarized when the stimulus moves in the preferred direction and hyperpolarized when the stimulus moves in the opposite direction, as opposed to the edge detectors which are inhibited by orthogonal stimuli.
6) Distinguish P- and M-cell pathways.
The magnocellular pathway is the “where” pathway because M-cells determine the position of an object, including its spatial location and movement. The parvocellular pathway is the “what” pathway because P-cells are color sensitive and are good for recognizing the identity of a particular object. P-type, M-type, and konio cells make up 95% of the ganglion cells. While the other 5% have not yet been well characterized, 5% of the ganglion cells are made up of konio cells, 10% M-type, and 90% P-type.
M-cells and P-cells are two types of retinal ganglion cells that have different pathways in the brain. Both Magnocellular (M-cell) and Parvocellular (P-cell) visual paths originate in the retina. However, they project onto different layers of the lateral geniculate nucleus. M-type ganglion cells of the retinal project to the last four layers while the P-type ganglion cells project to the first two layers. These layers are separated by small konio ganglion cells that function in providing color vision. After projecting onto the appropriate layers of the lateral geniculate nucleus, both M- and P-cell pathways project onto the primary visual cortex or the striate cortex. M-cells project into layer 4C-alpha and then to layer 4B while P-cells project into layer 4C-beta. After the primary visual cortex, there are two visual processing streams. The dorsal stream projects to the parietal cortex and is involved in visual motion detection and spatial orientation. The ventral stream projects to the temporal temporal cortex and is involved in perception of the visual world, face and object recognition.
7) Explain the retino-geniculate-cortical projections and why particular lesions therein might lead to distinct visual field deficiencies, e.g., hemianopsias.
Overall visual pathway: Retina? optic nerve? optic chiasm: nasal fibers cross, temporal stay ipsilateral ? optic tract ? Lateral Geniculate Nucleus of the thalamus ? optic radiations: Meyer’s loop and direct ? primary visual cortex.
For the optic radiations, Meyer’s loop takes a detour through the temporal lobe and synapses onto the lower gyrus of the calcarine sulcus, the lingual gyrus. The direct pathway stays parietal and synapses onto the upper gyrus, the cuneus. In addition, the lingual gyrus receives information from the lower retina, which is the upper visual field. The cuneus gyrus is the opposite.
To appreciate the effects of lesions within the visual system, it is essential to first consider the sensory information conveyed at each location where a lesion might occur, and the specific role of that location as it relates to the perception of images in the visual field. One of the first places susceptible to neurological damage in the visual system is the optic nerve. The optic nerve contains all retinal input from a single ipsilateral eye. Hence, it is reasonable to suspect that complete lesioning of the right optic nerve would result in right monocular blindness. This is true because the lateral (temporal) half of the retina–which carries the nasal constituent of the left visual hemifield and remains ipsilateral as it passes through the optic chiasm into the optic tract–as well as the medial half of the retina–which carries the temporal constituent of the right visual hemifield and decussates across the optic chiasm– are both incapacitated. When the optic chiasm is sagittally lesioned towards the middle of the crossing, the upshot is bitemporal hemianopsia since the only fibers that decussate are the nasal fibers. However, if the optic chiasm is impinged at the lateral edges of the decussating nerves (i.e., as potentially seen in patients with pituitary tumors), binasal hemianopsia results. Continuing posteriorly, the next possible site of significant injury to the visual system is the optic tract. The optic tracts carry retinal input from both eyes (ipsilateral temporal fibers and contralateral nasal fibers) and thus all the information for an entire contralateral hemifield of vision. A lesion to the right optic tract would cause bilateral loss of the left hemifield, or left homonymous hemianopsia. Further along the visual pathway are the optic radiations, which split vertically on each side into two divisions. The upper divisions contain information regarding the inferior quadrants of the visual field, whereas the lower divisions–Meyer’s Loop–contain that of the superior quadrants. Loss of function in Meyer’s loop causes superior quadrantic anopsia, and inferior quadrantic anopsia in the upper fibers. Finally, a similar effect as the one described occurs for lesioning of the upper and lower banks of the calcarine fissure of the occipital lobe, except only with foveal sparing (i.e., quadrantic visual loss is incomplete near the center of the image).