Physiology of Visual System 2

Seeing

• 130 million photoreceptors in retina transform light intensity/color patterns into chemical and electrical signals which trigger activity of retinal cells.

• Information of 130 million photoreceptors is compressed to 1 million highly specialized ganglion cells.

• 1 million ganglion cells in the retina form the optic nerve and transmit visual information to the visual cortex.

Clinical ophthalmoscope view of the retinal inner surface:

shows a nasally located optic disc (our blind spot) and a temporally located macula/fovea (focal point) in each eye

Optic disc causes blind spot:

our brain fills in for it just like the perceived boundary lines in this illusion

Rays MUST PASS THROUGH:

• Cornea

• Anterior Chamber

• Pupil

• Lens

• Vitreous

• Inner Retinal Layers (Ganglion Cells, Amacrine, Bipolar and Horizontal Cells)

• → to reach the Photoreceptors

Rays DO NOT HAVE TO PASS THROUGH:

• Retinal Pigment Epithelium

• Choriocapillaris

• Sclera

• → to reach the Photoreceptors

Cone Photoreceptors

• Color Specific & High Acuity

• Cones are concentrated in the FOVEA

Rod Photoreceptors

• No Color Specificity, low acuity but High Sensitivity

• Rods are present in the peripheral retina

3 classes of cones: A trichromat person has normal colour vision

• S-cones: short-wavelength sensitive

• M-cones: middle-wavelength sensitive

• L-cones: long-wavelength sensitive

Monochromats

totally colour blind with one cone pathway plus rod pathway

Dichromats

a cone photopigment missing thus has only two cone channels

Anomalous Trichromats

all three cone photopigments with one cone photopigment anomalous in having a shifted peak sensitivity

Retinal Structure

The retinal photoreceptors, situated in the back of the retina, behind all the other transparent neural retinal cell layers but anterior to the Retinal Pigment Epithelium (RPE), have a close physical and functional relationship with the RPE.

Photoreceptor Function

Retinal photoreceptors serve to convert photonic energy to chemical signals by virtue of their special cellular structure consisting of an inner cell body and an outer segment of photopigment

Photoreceptor Renewal

• Stacks of discs of visual pigment molecules of photoreceptors are constantly renewed.

• New discs added at the base of the outer segment.

• Old discs displaced up the outer segment pinched off at tips and engulfed by Retinal Pigment Epithelium on a diurnal cycle peaking in the morning.

Photoreceptor Cascade

• In dark, Rhodopsin is unstimulated, cGMP is available, cGMP binds to cation channels and cation channels are kept open by cGMP which depolarizes the photoreceptor

• In light, Rhodopsin is stimulated, starting a cascade involving Transducin and phosphodiesterase (PDE)

• In light, with Rhodopsin stimulated, the cascade involving Transducin and PDE becomes more activated.

• In light, with Rhodopsin stimulated, the cascade involving Transducin and PDE leads to PDE degrading cGMP.

• In light, with Rhodopsin stimulated, the cascade involving Transducin and PDE degrades cGMP. cGMP levels begin to decrease, cation channels begin to close and start to hyperpolarize the photoreceptor.

• In light, with the whole Rhodopsin cascade stimulated, the cascade involving Transducin, PDE and degradation of all cGMP causes cation channels to fully close and photoreceptor is hyperpolarized.

In dark:

• Rhodopsin is unstimulated

• cGMP is available

• cGMP binds to cation channels

• cation channels are kept open by cGMP which depolarizes the photoreceptor

In light:

• Rhodopsin is stimulated

• leading to a cascade involving Transducin and PDE

• degrades cGMP

• since cGMP is not available, cation channels close and hyperpolarize the photoreceptor

Retinal Receptive Fields

• Light stimulation and hyperpolarization of photoreceptors results in impulses being sent to visual cortex via bipolar cells and retinal ganglion cells and their nerve fibers.

• Photoreceptor hyperpolarization leads to decreased release of inhibitory neurotransmitter (such as glutamate) onto downstream retinal neurons.

• Decreased release of inhibitory neurotransmitter onto downstream retinal neurons effectively leads to increased action potentials in these downstream neurons (in other words - “removal of inhibition”).

Rod to ganglion cell connections:

show high convergence (less cell to cell connections like less pixels on a camera)

Cone to ganglion cell connections:

show low convergence (more cell to cell connections like more pixels on a camera)

Horizontal Cells

• connect photoreceptors laterally and dampen cells surrounding the center of maximal receptive stimulation

• inhibit surrounding “less stimulated” cells and effect “lateral inhibition”

Cone cells ARE:

Receptive fields for GCs

Ganglion Cells:

SERVE cone based receptive fields

Cone photoreceptors often synapse with two types of ganglion cells:

On center and Off center ganglion cells:

• On and Off center ganglion cells offer opposite responses determined by the sensitivity of the cone receptive fields they serve.

Lateral Inhibition

• Peripheral (vs foveal) cones connect to more horizontal cells which further inhibit those neighbouring cells getting weaker stimuli

• This optimizes acuity

Visual Pathways From Eye to Brain

Optic nerve → optic chiasm → optic tract → lateral geniculate body (thalamus) → visual cortex (occipital lobe)

• Temporal axons stay temporal

• Nasal axons cross at optic chiasm

• Collateral pathways leave the thalamus and go to the midbrain

Cranial Nerve III

• controls pupillary constriction

• also controls ciliary body and lens accommodation