Magnocellular and Parvocellular pathways1.pptx

ophthalmology

1. Magnocellular and Parvocellular pathways Prepared and presented by Dr. Gracy P. Matarweh

2. Magnocellular and Parvocellular Pathways Goals: Magnocellular pathway and its function Parvocellular pathway and its function Differences

3. Magnocellular pathway Parasol ganglion cells project from the retina via the optic nerve to the two most ventral layers of the lateral geniculate nucleus of the thalamus, to the magnocellular cells . From the LGN, the M pathway continues by sending information to the interblob regions of the 4Cα layer of the V1 region of the visual cortex, also called the "striate cortex"

4. Parasol ganglion cells (M cells) the first step in the magnocellular pathway. They are a type of retinal ganglion cell make up roughly 10% of all retinal ganglion cells. Large cell bodies Large branching dendrite networks thick, heavily myelinated axons.  Fast conduction velocities Innervated by large receptive fields Receive NO information about color Contribute information about motion and depth of objects

5. Magnocellular cells (M-cells) Neurons located within the Adina magnocellular layer (layer 1 and 2 ) of the lateral geniculate nucleus . broadband because their field center and surround receive input from the same mixture of cone types Part of the visual system Termed magnocellular because they have a relatively larger size than parvocellular cells

6. Function provide useful static, depth, and motion information . have high light/dark contrast detection ,  and are more sensitive at low spatial frequencies than high spatial frequencies detecting changes in luminance, and thus provide useful information for performing visual search tasks and detecting edges . providing information about the location of objects.

7. Function detect quick changes in the position of an object.This is the basis for detecting motion. The information sent to the  intraparietal sulcus  (IPS) of the posterior parietal cortex allows the magnocellular pathway to direct attention and guide saccadic eye movements to follow important moving objects in the visual field. IPS sends information to parts of the frontal lobe that allows the hands and arms to adjust their movements to correctly grasp objects based on their size, position, and location

8. Parvocellular pathway Midget cells originate in the ganglion cell layer of the retina, and project to the parvocellular layers of the lateral geniculate nucleus (LGN). The axons of midget cells travel through the optic nerve and optic tract, ultimately synapsing with parvocellular cells in the LGN  The electrically-encoded visual information leaves the parvocellular cells via relay cells in the optic radiations, traveling to the  primary visual cortex  layer 4A and 4C-β.

9. Midget cells small size  dendritic trees and cell bodies 80% of retinal ganglion cells receive inputs from relatively few rods and cones slow conduction velocity respond to changes in color but respond only weakly to changes in contrast unless the change is great  simple center-surround receptive fields, where the center may be either ON or OFF while the surround is the opposite.

10. Parvocellular cells are neurons located within the four dorsal parvocellular layers of the lateral geniculate nucleus (LGN) of the  thalamus (layer 3,4,5 and 6 ). Most parvo cells have color-opponent center surround receptive fields , e.g., a red on-center and a green off-surround. small size of the cell compared to the larger magnocellular cells . sensitive to colour capable of discriminating fine details

11. Parasol vs. Midget cells Parasol Cell (M cells) Midget Cell (P cells) RGC Type Magnocellular Pathway Parvocellular Pathway Pathway it's involved in Large Small Cell body size Complex Less complex Dendritic tree ~1.6 ms ~2 ms Conduction rate "Where" objects are; "How" to grasp the objects "What" objects are according to fine detail Function in visual system Low Medium to high Sensitivity to spatial frequency High Low Temporal frequency Achromatic Red-green opponency Color opponency

12. Thank you for your kind attention