the visual system Flashcards
Cell Types in the Retina, LGN, and Cortex
Give a gross Anatomy of the eye
Pupil: opening where light enters the eye
Sclera: White of the eye
Iris: Gives colour to eyes
Cornea: Glassy transparent extrenal surface of the eye
optic nerve: bundle of axons from the retina
Cell Types in the Retina, LGN, and Cortex
What are the main cell types in the retina?
Photoreceptors (rods and cones), bipolar cells, horizontal cells, amacrine cells, and ganglion cells.
Cell Types in the Retina, LGN, and Cortex
What are the two main types of ganglion cells?
M-type (Magnocellular) and P-type (Parvocellular) ganglion cells.
Cell Types in the Retina, LGN, and Cortex
What are the main cell types in the lateral geniculate nucleus (LGN)?
Magnocellular cells (motion, low detail), Parvocellular cells (fine detail, color), and Koniocellular cells (additional color processing).
Cell Types in the Retina, LGN, and Cortex
What is the significance of ocular dominance columns in the visual cortex?
They represent input from either the left or right eye and are critical for binocular vision.
What is Emmetropic and Presbyopia
Emmetropic is a normal good eye 20/20 vision
Presbyopia = out lens hardes with age and ciliary muscles weaken. This causes decreased ability in accomodation.
What is Hyperopia and Myopia
Hyperopia: Far sightedness; refractive power insufficient for close objects
Myopia: Near sightedness; refractice power too strong for distant objects
Vision: How does it work?
In order to see an object
- 1- the pattern of the object must fall on the vision receptors (rods and
cones in the retina) → accommodation
- 2- the amount of light entering the eye must be regulated (too much
light will “bleach out” the signals)
- 3- the energy from the waves of photons must be transduced into
electrical signals
- 4- The brain must receive and interpret the signals
Dark Current and Phototransduction
What is the dark current?
The steady influx of Na+ into photoreceptor cells in the dark, keeping them depolarized.
Dark Current and Phototransduction
What happens when light hits a photoreceptor?
cGMP levels drop, Na+ channels close, the cell hyperpolarizes, and neurotransmitter release decreases.
Dark Current and Phototransduction
What molecule is responsible for detecting light in rods?
Rhodopsin (composed of opsin and retinal).
Dark Current and Phototransduction
What is the sequence of events in phototransduction?
Light → Retinal changes from 11-cis to all-trans → Activates transducin → Activates phosphodiesterase → Decreases cGMP → Closes Na+ channels → Hyperpolarization.
Photoreceptor Structure
Converts electromagnetic radiation to neural
signals (transduction!)
- Four main regions:
- Outer segment
- Inner segment
- Cell body
- Synaptic terminal
Types of photoreceptors:
* Rods and cones
dark current summary
- Open in the dark
- Closes in response to light
- Nucleotide-gated channel (opened
by cGMP) - Permeable to Na+
- Keeps photoreceptor Vm more
positive than most neurons
→ Steady release of neurotransmitter
Glutamate Receptors in the ON and OFF Pathways
What is the role of glutamate in the ON and OFF pathways?
Glutamate is continuously released in darkness. Light reduces glutamate release.
Glutamate Receptors in the ON and OFF Pathways
How do ON bipolar cells respond to light?
They have metabotropic glutamate receptors (mGluR6) that hyperpolarize in response to glutamate, so they depolarize when light reduces glutamate
Glutamate Receptors in the ON and OFF Pathways
How do OFF bipolar cells respond to light?
They have ionotropic AMPA/kainate receptors that depolarize in response to glutamate, so they hyperpolarize when light reduces glutamate.
Visual Acuity Ability to distinguish two nearby points is determined by what
Determined
largely by photoreceptor spacing and refractive power
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