13. The Eye and Retina

The Eye and Retina

Eye Structure:

Image Formation:

• Retinal images are inverted.

  • Upper (superior) visual field → lower (inferior) hemiretina

  • Lateral (temporal) visual field → nasal (medial) hemiretina

Ophthalmoscopic Appearance of the Eye:

• Optic Disk

  • Location: where ganglion cell axons exit the eye to form the optic nerve

  • Does not have photoreceptors

  • Gives rise to a blind spot in the lateral (temporal) visual field bc. the optic disk is in the nasal (medial) hemi-retina

    • Blind spots are filled in by brain

• Macula

  • Yellowish-pigmented area near the center of the retina

  • Responsible for high-acuity, central vision

• Macular degeneration: in older adults resulting in a loss of vision in the central visual field

Pupillary Light Reflex:

• Pupil controls the amount of light reaching the retina

  • pupil continuously adjusts to ambient light levels

  • Consensual b/w L & R eye: shining light in one eye influences the pupils of both eyes

• Neural Circuit: the optic nerve (II) → midbrain (pretectal; allows signal to become bilateral) → midbrain (Edinger-Westphal) → oculomotor nerve (III)

Circuitry of the Retina - "Inside-Out" Organization:

• Photoreceptors face the back of the eye (anterior)

• Ganglion cell axons (superficial) emerge on the interior, leading to the blind spot

• The octopus retina is similar to ours but flipped around (photoreceptors=interior=no blind spot)

Circuitry of the Retina - Vertical Pathway:

• Photoreceptors (accept light) → Bipolar cells → Ganglion cells (project to forebrain)

Circuitry of the Retina - Horizontal (Lateral) Connections:

• Amacrine cells - spike

  • Receive input from bipolar cells

  • Synapse with ganglion cells, bipolar cells, and other amacrine cells

• Horizontal cells - no spike

  • Receive input from photoreceptors

  • Synapse with other photoreceptors and bipolar cells

• Only ganglion cells and amacrine cells produce action potentials

• Photorecep., bipolar cells, horizontal cells = non-spiking

Photoreceptors:

• Photoreceptors convert light to neural signals

• Regions:

  • Outer seg = photosensitive region

  • Inner seg= cellular support region

  • Synaptic terminal = neurotransmitter release

• 2 main types of photoreceptors

  • Rods = night vision (grey tones)

    • All rods respond to the same wavelength of light

    • Lots of photopigment bc. more membranous disks = more sensitive

  • Cones = color vision

    • Blue, Green, Red

    • Fewer photopigment

Distribution of Photoreceptors:

• Distribution of rods and cones varies from the fovea to the retinal periphery

• Peripheral Retina - good for detecting faint objects

  • more rods than cones

  • more photoreceptors than ganglion cells,

  • more sensitive to light

  • less sensitive to color (low acuity)

• Fovea

  • pit in the retina where inner layers are pushed aside

  • maximizing visual acuity

• Central Fovea

  • contains only cones

  • h1:1 ratio with ganglion cells

  • area of highest visual acuity (high acuity)

Phototransduction in Rods:

• Light-activated G-protein-coupled biochemical cascade

  • Rods: Rhodopsin

  • Cones: Phosphodiesterase

• Modulates cGMP-gated Na+ channels

  • Na+ channels open (dark) → photorecep. depol.

  • Na+ channels close (light) → photorecep. hyper.

Phototransduction - Photopigment:

• Light interacts with photopigment to produce a change in membrane potential

• Analogous to activity at G-protein-coupled neurotransmitter receptors but causes a decrease in second messenger by activating phosphodiesterase (cGMP → 5’GMP)

Phototransduction in Rods:

• Dark Current:

  • Rod outer segments = depolarized due to a steady influx of Na+ ions

• Light Current:

  • Photoreceptors hyperpolarize

• Photoreceptors release more glutamate when depolarized and less when hyperpolarized

Adaptation to Light Level:

• The human eye can function across a wide range of light levels (nine orders of magnitude)

  • In a given moment of time, eye can only sense a contrast ratio of one thousand

• Photoreceptors change their "operating point" to adapt to the ambient light level

• Dark Adaptation:

  • Takes approximately 20-30 minutes

  • 1. Cones (color sensitive) become more sensitive → eventually plateaus

  • 2. Rods (night vision: grey tones) become more sensitive → reach max. sens.

Receptive Field Organization:

• Receptive field: region of the sensory surface that, when stimulated, changes the membrane potential of the neuron

• Center-Surround RF: organization is common in many sensory systems

Receptive Field Organization in Photoreceptors (non-spiking):

• Off-Center RF:

  • inhibited (hyper) by stimulation in the center

  • excited (depol) by stimulation in the surround

• Produced by lateral inhibition at the level of the horizontal cell

Receptive Field Organization in Bipolar Cells (non-spiking):

• Center-Surround & Off-Center RF’s

• OFF-CENTER

  • hyperpolarize (-) = light stimulates the center

  • depolarize (+) = light stimulates the surround

  • ionotropic glutamate receptors (AMPA) = open Na+ channels

  • excitatory, sign-conserving synapse

  • increased light in center RF → hyper. voltage of photorecep. → lowers glutamate release from photorecep. → hyperpolarize bipolar cell (OFF)

• ON-CENTER

  • depolarize (+) = light stimulates the center

  • hyperpolarize (-)= light stimulates the surround

  • metabotropic glutamate receptors (mGluR6) = open K+ channels

  • inhibitory, sign-inverting synapse

  • increased light in center RF → hyper. voltage of photorecep. → lowers glutamate release from photorecp. → depol. bipolar cell (ON)

Receptive Field Organization in Ganglion Cells (AP=spiking):

• Center-surround receptive fields but produce spiking output (AP)

  • respond to spatial changes in illumination

  • implements a form of contrast enhancement

• On-center and Off-center ganglion cell types

  • Off: good for detecting edges not illumination

Afferent Pupillary Defect:

• Swinging flashlight test

  • One eye constricts normally w/ light

  • Other eye dilates w/ light