The Eye, Vision, and Visual Pathways

Sclera

connective tissue

Cornea

allows light to enter

Lens

focuses light on retina

Iris

pigmented smooth muscle

Pupil

hole in iris

optic disk

• blind spot 

  • optic nerve exits posterior surface of eyeball

  • no receptor cells at that location

visual filling

brain (visual cortex, V1) fills in the rest

The Retina

• Outer layer – Photoreceptors: rods (monochrome/ low light) and cones (color/bright light)

• Middle layer – Bipolar cells and amacrine cells

• Inner layer – Ganglion cells (generate APs)

• Amacrine cells & Horizontal cells modulate communication via lateral inhibition (inhibit neighboring cells)

macula lutea

a depression in the center of retina

Fovea contains

cones only.

To provide clear pathway to fovea

• the bipolar and ganglion cells must be displaced laterally.

• Activated in relatively bright light

• 1:1 communication with bipolar cells

Hence, in the light cones provide high spatial resolution

• Ratio of rods to cones increases as we move away from fovea

• Overall, rods outnumber cones about 20:1

Rods:

active in dim light and provide lower spatial resolution, due to high convergence onto bipolar cells

Neural Processing: convergence

• Degree of convergence between photoreceptors and bipolar cells is greater with rods than with cones. 

• Remember, in fovea & 

  macula (cones), there is little 

  convergence

• leading to greater visual acuity

• But in periphery, thousands of  

  rods converge on single 

  bipolar cell.

• Greater convergence allows

  greater sensitivity to light

  (spatial summation)

Phototransduction

• Signals are converted by the rods (1 type) and cones (3 types)

• Both photoreceptor types contain outer and inner segments

• Molecules that absorb light are found in the outer segment - disks

Disks contain photopigments which absorb light:

• They consist of light-absorbing pigments,  retinal (common to all photoreceptor types)  and a protein called an opsin

• Type of opsin determines what light wavelengths are absorbed

• Also in the disk is a G-protein –transducin- and the enzyme phosphodiesterase

Generating Optic Nerve Signals overveiw

• In dark, rods steadily release glutamate from basal end of cell

• Rods absorb light, glutamate secretion ceases

• Bipolar cells sensitive to on and off pulses of glutamate secretion

  • some bipolar cells inhibited by glutamate and excited when secretion stops

    • these cells excited by rising light intensities

  • other bipolar cells are excited by glutamate and respond when light intensity drops

• When bipolar cells detect fluctuations in light intensity, they stimulate ganglion cells directly or indirectly

• Ganglion cells are the only retinal cells that produce action potentials

• Ganglion cells respond to the bipolar cells with rising and falling firing frequencies

• via optic nerve, changes provide visual signals to the brain

Neural pathways from the eyes…

• bipolar cells of retina: 1^st-order neurons

• retinal ganglion cells: 2^nd-order neurons whose axons form optic nerve

• two optic nerves combine to form optic chiasm

• half the fibers cross over to the opposite side of the brain (hemidecussation, see next slide) and chiasm splits to form optic tracts

• optic tracts synapse onto 3^rd-order neurons in the lateral geniculate nucleus of the thalamus, which send their axons to the primary visual cortex of the occipital lobe (conscious vision)

Subcortical Projections from the Eye

• Immediately after the chiasm, some optic nerve fibers instead project to the hypothalamus: suprachiasmatic nucleus, for entraining circadian rhythms

• …and others project instead to structures in the midbrain:

• superior colliculi in the tectum: for reflexes orienting head and eye movements, and the pretectal nuclei: for the pupillary light reflex

outer eye contains

sclera

middle eye contains

lens

iris

pupil

inner eye cotains

retina

fovea

optic disk