Vision

Front 1

Sclera

Back 1

Outer white layer of the eye, composed of fibrous tissue, covered by conjunctival epithelium

Front 2

Conjunctiva

Back 2

Stratified epithelium lining the inner eyelids and covering the sclera.

Front 3

Cornea

Back 3

Anterior, transparent structure, contributes to most of the eyes refractive power

Front 4

Choroid

Back 4

Vascular layer b/t sclera and retina, provides oxygen and nutrients to the outer retina

Front 5

Ciliary body

Back 5

Contains ciliary muscle involved in the lens accommodation response, epithelial coating produces aqueous humor

Front 6

Lens

Back 6

Transparent biconvex structure that alters curvature to change focal distance (accommodation)

Front 7

Iris

Back 7

Pigmented smooth muscle, controls pupil diameter.

Front 8

Aqueous humor

Back 8

Gelatinous, transparent fluid similar to interstitial fluid, located in anterior and posterior chambers, provides ocular pressure to inflate eyes and provides nutrients to avascularized structures.

Front 9

Retina

Back 9

Inner, neural layer in the posterior eye; responsible for phototransduction and transmission of visual signals to the brain.

Front 10

Fovea

Back 10

Depression in the inner retinal surface where light focused; cone photoreceptor-rich; responsible for sharp, central vision.

Front 11

Optic disc

Back 11

Where ganglion cell axons coalesce before exiting the eye; responsible for the ‘blind-spot’.

Front 12

Vitreous humor

Back 12

Transparent, gelatinous mass that fills the space between the lens and the retina; presses the retina in place.

Front 13

Autonomic innervation of the eye

Back 13

Innervates iris and ciliary body
Both parasympathetic and sympathetic

Front 14

Parasympathetic innervation of eye

Back 14

Parasympathetic preganglionic fibers arise in the Edinger-Westphal nucleus and project to the ciliary ganglion

Front 15

Sympathetic innervation of eye

Back 15

Sympathetic preganglionic fibers arise in T1 spine and project to superior cervical ganglion

Front 16

Edinger-Westphal nucleus

Back 16

Located in rostal midbrain (upper midbrain).

Front 17

Pupillary light reflex

Back 17

Parasympathetic response. Some light intensity signals from the optic nerve terminate in the pretectal area, which in turn projects to the EW nucleus, which then stimulates parasympathetic preganglionic fibers

Front 18

Pretectum

Back 18

Midbrain structure involved in visual function

Front 19

Parasympathetic activity of the eye

Back 19

Causes contraction of the circular muscle, constricting the pupil (miosis). Mediated by release of Ach onto a muscarinic receptor.

Front 20

Sympathetic activity of the eye

Back 20

Not part of pupillary reflex, causes contraction of the radial/dilator muscles (mydriasis)

Front 21

What stimulates accommodation?

Back 21

Stimulated by chromatic aberration, convergence of eyes, differences in focus in fovea vs edges of retina. Near vision directed by stimulation from parasym. fibers in EW nucleus.

Front 22

Accommodation

Back 22

Contraction of ciliary body reduces suspensory ligaments tension and lens rounds up.
Object nearby - tension is low, lens thickened, rounds up
Distant object - tension is high, lens flattened

Front 23

Chromatic aberration

Back 23

A type of distortion in which there is a failure of a lens to focus all colors to the same convergence point, occurs b/c lenses have a diff refractive index for diff wavelengths of light.

Front 24

Organization of retina

Back 24

Outer layer to inner layer:
Pigments cells, rods and cones, amacrine cells, horizontal cells, bipolar cells, ganglion cells.

Front 25

Pigment cells

Back 25

Epithelial cells which prevent light scatter, phagocytose discs from rods, and recycle all-trans-retinal to 11-cis-retinal.

Front 26

Rods and cones

Back 26

Photoreceptors which transduce light to MEMBRANE potentials.

Front 27

Amacrine cells

Back 27

Regulate bipolar cells; provide most synaptic input to ganglion cells.

Front 28

Horizontal cells

Back 28

Mediate lateral inhibition of visual fields (see discussion in later part of this presentation).

Front 29

Bipolar cells

Back 29

Transmit ELECTRICAL potentials from photoreceptors to ganglion cells.

Front 30

Ganglion cells

Back 30

Send visual signals, in the form of ACTION potentials, to the brain. Note that all other visual signals up to this point are graded changes in membrane potential. This is possible b/c within the retina itself, electrical signals do not have great distances to travel.

Front 31

Rods

Back 31

Periodically shed renew and shed their discs; much higher rhodopsin content, and, thus, sensitivity to light.

Front 32

Cones

Back 32

Relatively low rhodopsin content; each cell requires several hundred photons to be activated; responsible for color vision by variable expression of one of three different photopigments.

Front 33

Rhodopsin

Back 33

Made up of opsin protein (GPCR) and retinal ligand
In rods - embedded across double-membrane discs w/n the outer segment cytoplasm.
In cones - are actually infoldings of the outer segment cell membrane.

Front 34

Different types of rhodopsin

Back 34

3 types of photopigments (red, green and blue) can be expressed in cones.
People w/colorblindness have a mutation where they only express 2 photopigments

Front 35

Distribution of photoreceptors

Back 35

Cones are highly concentrated in the fovea (visual sharpness & acuity), rods are distributed throughout the peripheral retina (sensitive to low light conditions)

Front 36

What is the best way to see a dim object?

Back 36

Peripheral vision, where you use more rods than cones b/c they are more sensitive in low light and better for night vision.

Front 37

Rods convergence

Back 37

Rod cells have a much greater degree of convergence onto bipolar cells. 15-30 rods per 1 bipolar cell=high convergence. This results in a much larger receptive field for contacting bipolar cells (more sensitive) and lower visual acuity (not as sharp).

Front 38

Cones convergence

Back 38

1:1 ratio = low convergence, low sensitivity, small receptive field but high acuity

Front 39

Retinal

Back 39

Part of rhodopsin (along with opsin protein-GPCR) that responds to light by changing from cis to trans conformation.

Front 40

Photoisomerization

Back 40

The process of light converting retinal from cis to trans form. Once in trans form, the retinal causes the opsin protein to undergo a conformational change, which activates the coupled G protein and phototransduction occurs.

Front 41

Returning retinal to cis form

Back 41

Once in trans form, retinal has to go through many steps to get back to cis form, is taken up by pigment cells and transformed back to cis form. This process requires vitamin A. A vitamin A deficiency will lead to night blindness.

Front 42

Phototransduction steps

Back 42

1. Rhodopsin is isomerized by light and opsin GPCR undergoes conformation change.
2. Transducin G protein senses this shift and is activated.
3. Transducin alpha subunit stimulates phosphodiesterase (PDE) embedded in disc membrane.
4. PDE hydrolyzes cyclic GMP to GMP.
5. Less cAMP is available to open cell membrane Na+ channels.
6. Cell is hyperpolarized and releases less neurotransmitter onto bipolar cells.

Front 43

Phototransduction in dark conditions

Back 43

cGMP levels higher, Na+ channels open, cell depolarized and releasing neurotransmitter

Front 44

Bipolar cell postsynaptic potentials

Back 44

Photoreceptors only release glutamate onto bipolar cells, hyperpolarization or depolarization can both occur, it depends on the type of glutamate receptor expressed on the bipolar cell postsynaptic membrane.

Front 45

On-center bipolar and ganglion cells

Back 45

Fire when light is shone in center of receptive field

Front 46

Off-center bipolar and ganglion cells

Back 46

Fire when center of receptive field is dark.

Front 47

Contrast

Back 47

On-center and off-center cells make it possible to perceive contrasting light levels in the visual field. Contrast information makes up a huge proportion of what we see. Color requires encoding of much less information.

Front 48

Lateral inhibition

Back 48

Horizontal cells antagonize the effects of the direct pathway (central cone to ganglion), release the inhibitory neurotransmitter, GABA, which causes center cone to release even more glutamate, which hyperpolarizes the bipolar cell even more, leading to less glutamate release onto ganglion cell=hyperpolarization of ganglion cell

Front 49

Central visual pathway

Back 49

Ganglion cell axons form optic nerve, some cross at optic chasm.
Collateral branches end in hypothalamus and midbrain (pretectum and superior colliculus).
Optic nerve fibers synapse in LGN of thalamus and neurons of the LGN project to visual cortex.

Front 50

Hypothalamus

Back 50

Receives visual info, needed for photo-entrainment of light dark cycles (circadian rhythms)

Front 51

Pretectum

Back 51

Receives visual info for pupillary reflex and accommodation.

Front 52

Superior colliculus

Back 52

Receives visual info for orienting movements of head and eyes.

Front 53

Lateral geniculate nucleus

Back 53

Neurons project into the visual cortex

Front 54

Retinal fields

Back 54

Left peripheral visual field sensed by left nasal retina; right peripheral visual field sensed by right nasal retina. Binocular (central )visual field sensed by both temporal retinas.

Front 55

Optic chiasm

Back 55

Decussation of axons from nasal retinas at the optic chiasm; right visual field represented in the left visual cortex and vice versa. Understand what this means for lesions of the pathway.

Front 56

Hemianopia

Back 56

Loss of vision in half of the visual field of one or both eyes.

Front 57

Macular sparing

Back 57

Caused by a lesion in the geniculocalcarine tract, missing part of vision, but area of macula is spared and can still see.

Front 58

Primary visual cortex (V1)

Back 58

Tissue has striped appearance, often V1 called the ‘striate cortex’; find neurons tuned to very basic components of a visual scene, such as line orientation (see figure).

Front 59

Areas outside of V1

Back 59

Note that visual areas just outside the visual cortex (extrastriate) are involved in sensing movement (MT area), attending to objects, and processing color (V4). Neural cells in extrastriate areas are typically responsive to visual information from both eyes.