NEUR 3000 Exam 3

The eye, central visual system, and the ear

Pupil

• Regulates the amount of light entering the eye

• Constricts or dilates based on axis

Iris

• Pigmented, smooth muscle

• Contracts/relaxes to adjust pupil size

Aqueous Humor

• Clear, watery fluid that fills the anterior chamber of the eye, between the cornea and the lens

Extraocular muscles

• Muscles that move the eye around

Cornea

• Focuses light onto the retina, allowing us to see clearly.

Lens

• Focuses light onto the retina for clear vision at different distances.

Ciliary muscle

• muscles that push and pull on lens to alter shape

Vitreous Humor

• Fluid in posterior chamber between optic nerve and lens

Retina

• location of sensory receptor cells [photoreceptors]

Fovea

• region with highest density of photoreceptors

Visual Acuity

the ability of the eye to distinguish two points

• dependent on the density of photoreceptors

Connection of cells in retina

photoreceptors —> bipolar cells —> ganglion cells [their axons form the optic nerve]

Optic Disk

blind spot since no rods and cones, so there’s no detection of light

Cones

• need a lot of stimulation

• bright-light vision

• 3 types of opsin

  • red green and blue

Rods

• more sensitive to light

• low-light vision

• only expresses one opsin

• have more opsin in one photoreceptor

Peripheral vision has ________ for low light

high sensitivity

Central vision has _____ for bright light

high acuity

Rhodopsin wavelength

500 nm

Red-opsin wavelength

560 nm

Green-opsin wavelength

530 nm

Blue-opsin wavelength

430 nm

Transduction of no light [rods]

1. No light of 500 nm hitting rhodopsin

2. Guanylyl cyclase converts GMP to cGMP

3. cGMP-gated Na+ channels open

4. -normal next events-

5. glutamate released to retinal bipolar cell

Transduction of light [rods]

1. Light of 500 nm is hitting rhodopsin

2. Rhodopsin changes shape and its G-protein is activated

3. G-protein activates phosphodiesterase

4. Phosphodiesterase converts cGMP into GMP

5. cGMP-gated Na+ channels close

6. Na+ influx stops

7. The membrane potential stops depolarizing

8. K+ efflux through the leak K+ channel increases

9. The membrane potential hyperpolarizes

10. Voltage-gated Ca2+ channels close

11. Ca2+ influx stops

12. Docking proteins return to original shape

13. Glutamate release stops

Receptive field

any part of field that neuron can detect

Retinal Bipolar Cells [no light]

1. Glutamate-gated Na+ channels open

2. Na+ influx

3. Cell depolarization

Retinal Bipolar Cells [light]

1. Metabotropic glutamate receptors on bipolar cell somehow hyperpolarize the membrane, but no glutamate released

2. Depolarize when light

M-type ganglion cell (magnocellular)

• large receptive field

• bursts of rapidly conducted action potentials

• CONNECTED TO RODS

P-type ganglion cells (parvocellular)

• small

• 90% of ganglion cells

• sustained discharge of APs

• sensitive to wavelength of light

• CONNECTED TO CONES

NonM-nonP type

• Sensitive to wavelength of light

• color-opponent cells: enhances contrasts of colors

• CONNECTED TO CONES

Accessory Pathway for Superior Colliculus

project to superior colliculus —> project to pons —> project to motor neurons —> muscle cells of head, neck, and eyes —> reflexive orientation to visual stimuli

Accessory pathway for the supra-chiasmatic nucleus

project to the supra-chiasmatic nucleus —> project to pineal gland —> melatonin projection to blood —> conserve energy as we go to sleep (Circadian rhythm)

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