2.3 Ganglion and Bipolar Cells |
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| Ganglion Cells receive visual information from Photoreceptors
(Rods & Cones) via various intermediate cells such as Bipolar
Cells, Amacrine Cells, and Horizontal Cells. Their axons form the Optic
Nerve and connect approximately for 80% to the LGN (Lateral Geniculate Nucleus) and for 20% to
the SC (Superior Colliculus).
There are 5 main types of Ganglion cells: 1. Midget Cells: - "Midget" based on the small sizes of their dendritic trees and cell bodies.
- 80% of retinal ganglion cells.
Project to the Parvocellular layers of the LGN: P-cells
Mostly in the Fovea and receiving mainly Cones input.
Process Colour (Red & Green), Form and Fine details.
2. Parasol Cells: - "Parasol" based on the large sizes of their dendritic trees and cell bodies.
- 10% of retinal ganglion cells.
- Project to the Magnocellular layers of the LGN: M-cells
- Mostly in the Periheral-retina and receiving mainly Rods input.
- Process Motion and Orientation.
3. Bistratified Cells:
- "Konio" means “as small as dust”, verry small sized.
- 10% of retinal ganglion cells
- Project to the Koniocellular layers of the LGN: K-cells
- Very large receptive fields that only have
centers (no surrounds) and receive input from intermediate numbers of Rods and Cones.
- Process Colour (Blue & Yellow)
4. Photosensitive cells: - They
contain their own photopigment, melanopsin, which makes them respond
directly to light even in the absence of Rods and Cones.
- Contribute to the regulation of pupil size and other behavioral responses to ambient lighting conditions.
- Project
to the Circadian Pacemaker of the brain: the Supra Chiasmatic Nucleus
(SCN) (Circadian Rhythm = 24-hour cycle in the physiological processes
of living beings)
5. Other Cells projecting to the LGN for eye movements (saccades)
- Projecting to the LGN and cells making connections with the Edinger-Westphal Nucleus
- This EWN supplies fibers to the eye, constricting the pupil and accommodating the lens.
| | | Some Bipolar Cells are depolarised by light (on-bipolar
cells) and others are hyperpolarised by light (off-bipolar cells).
Bipolar cells synapse with Ganglion cells, in which action potentials
can be generated, so that each Ganglion cell has a circular receptive
field comprised of many Photoreceptors. | | - On-centre cell: A central
excitatory area surrounded by an inhibitory area, forming an on-centre,
off-surround receptive field.
- Off-centre cell: A
central inhibitory area surrounded by an excitatory area, forming an
off-centre, on-surround receptive field.
- Combined type of cell has a receptive field that is both stimulated and inhibited by light.
The diagram shows the relative firing rate of "on-" and "off-centre Ganglion cells" in the light and dark: | | |
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