Week 9, Wednesday

We have Binocular vision

  • both eyes are in the front of our heads, to focus on only one set of objects on the same point in the retina

  • Allows for perception of depth and viewing objects in 3D (Stereopsis)

Convergence:

  • refers to medial movements of the 2 eyeballs so tat both are directed toward the object being viewed

  • I.e. tracking a pencil moving towards your eyes

  • The coordinated action of the extrinsic eye muscles

Special Senses-Vision-Signal Transduction pdf

Retina and Signal Transduction:

  • retina is the light-sensitive neural structure of eye

    • Has 2 receptors

      • Cone photoreceptor

        • Color vision in bright light

      • Rod photoreceptor

        • Night vision and dim light

  • Light must pass through neural elements to activate light sensitive rods and cones

  • Pigmented layer contains Melanin, a pigment that absorbs light and prevents scatter

Photoreceptor Structures:

  • Outer segments contain the actual receptors

    • Rods contain Rhodopsin receptors

      • AKA visual purple

    • Cones contain color-sensitive Photopsin receptors

      • 3 different photopsins sensitive to 3 different wavelengths of light

  • Inner segment contains Mitochondria

Rhodopsin:

  • the receptor that will be stimulated by light

  • A combination of Scotopsin (an opsin protein) and Retinal (pigment)

    • Retinal is in Cis configuration (11-cis-retinal)

    • Only cis configuration can bind with Scotopsin to form active rhodopsin

  • These photoreceptors (rods and cones) are metabotropic receptors

    • Couple to G proteins called G transducin

Bleaching and Regeneration of photopigment:

  • bleaching= decomposition and degradation

  • A photon of light (stimulus) Causes isomerization of 11-cis-retinal to all trans retinal which will split away form Rhodopsin

  • 4 steps:

    • Step 1

      • has intermediaries

      • Light enters, isomerization of 11-cis-retinal To trans-Retinal

      • Metarhodopsin II is the activate form of rhodopsin

        • Vitamin A is a precursor of all-trans-retinal

        • Lack of Vitamin A causes a decrease in retinal, which results i decreased production of rhodopsin, and a lower sensitivity of retina to light (night blindness)

    • Step 2

      • Trans-retinal separates from opsin (bleaching) becoming colorless

    • Step 3

      • Retinal isomerase converts trans-retinal to cis-retinal

    • Step 4

      • Cis-retina binds to Opsin (regeneration) In the photoreceptor disc membrane

      • Becomes colored again

Rod Receptor Potential:

  • Resting membrane potential of eye is about -40mV (in total dark) because of an inward current carried by Na+ ions flowing into outer segment f rod through cGMP-gated sodium channels

    • cGMP is always produced in the dark

    • In the dark, the eye is depolarized

  • There is also an outward current carried by K+ ions occurring in the inner segment of the rod

  • When Rhodopsin splits (after activation by light) it causes hyperpolarization of rods to about -70mV by decreasing Na+ permeability of outer segment

    • cGMP gets broken down

    • In light, the eye gets Hyperpolarized

  • The greater the intensity of light, the greater the electronegativity

Signal Transduction in Rods and Cones:

  • In total darkness:

    • No light = No metarhodopsin II

    • Photopigment not split

    • Guanylyl cyclase is highly-active

      • Has high levels of cGMP

    • cGMP-gated channels activate

      • Inflow of Na+ (dark current)

    • Depolarization

    • Calcium influx and exocytosis of neurotransmitters RMP= -40mV

Mechanism of how Light Decreases Sodium Conductance:

  • In the Presence of light:

    • Light activated rhodopsin (metarhodopsin II) activates a G protein called Transducin

      • This happens in either dim or bright light

    • Transducin activates an enzyme (cGMP phosphodiesterase) which breaks Down cGMP

    • cGMP levels decrease

    • Sodium channels close (decreasing sodium conductance)

    • Closure of sodium channels causes photoreceptors to hyperpolarize to -70mV

In Darkness:

  • Darkness acts as an inhibitor

  • Rod and cone photoreceptors release glutamate which triggers the IPSPs that hyperpolarize bipolar cells

In Light:

  • Hyperpolarizing receptor potentials are generated (~-70mV)

  • In rods (dim light)

    • results in decrease of glutamate release that partially turn off neurotransmitter release

  • In cones (bright light)

    • Glutamate release is almost shut down completely

  • Light excites bipolar cells by turning off the release of an inhibitory neurotransmitter

    • The excited bipolar cells subsequently stimulate the ganglion cells to form action potentials in their axons

Transmission of signals towards ganglion cells is also by electrotonic conduction

  • allows graded response proportional to light intensity

  • The higher the intensity of light, the faster the electrotonic conduction

Adaptation in eye:

  • significance

    • Provides the eyes the ability to change its sensitivity

    • Large range of adaptation

  • Light adaptation

    • Occurs when we stay in bright light for a few minutes

    • It occurs very fast

    • Cone pigments regenerate fast

      • Photopigment degrade

        • Forms all trans retinal all the way to Vitamin A (retinol)

      • Pupils constrict

      • neural adaptation

        • Transmission of information is faster through bipolar and ganglion cells by electrotonic conduction

  • Dark adaptation

    • Takes place when one moves form a lighted area into a dark room

    • It is slower because rod pigments regenerate slowly

    • Pupils dilate

Rods and Cones (summary):

  • Rods

    • Dim light

    • High sensitivity

      • Specialized for night vision

    • High amplification

      • Single photon detection

    • Slow response

    • More sensitive to scattered Light

    • Low acuity

      • Highly convergent retinal pathways

      • Absent in Fovea Centralis

    • Achromatic

      • One type of Rod pigment

    • No color vision

  • Cones

    • Bright light

    • Lower sensitivity

      • Specialized for day vision

    • Less amplification

    • Fast response

    • More sensitive to direct axial rays

    • High acuity

      • Less convergent retinal pathways

      • Concentrated in central fovea

    • Chromatic

      • 3 types of cones, each with a different pigment that is sensitive to a different wavelength of visible spectrum

    • Can see color vision

The Fovea:

  • at center of the retina

  • Center of fovea, called central fovea (or fovea centralis) contains only cones

  • This is the area of greatest visual acuity

    • Sharpest vision and we see more details when light rays fall here

  • At central fovea

    • No rods

    • Ratio of cones to bipolar cells to ganglion cells is 1:1:1

  • May explain high degree of visual acuity in central retina


Special Senses- Vision- Retina pdf

Signal Transmission in the Retina:

  • rods and cones secrete neurotransmitters (glutamate - in the dark)

  • Transmission of signals towards ganglion cells is mostly by electrotonic conduction which allows graded response proportional to light intensity

  • Only ganglion cells generate action potentials

  • 2 types of bipolar cells

    • Inhibitory

    • Excitatory

  • In Fovea

    • 1 cone synapses directly to 1 bipolar cell, directly to 1 ganglion cell

  • In Peripheral

    • 3 rods can synapse with 1 bipolar cell to Amacrine cells to ganglion cells

    • 1 Horizontal cell synapses with rods and cones, and run perpendicular to all other cells here

    • 1 cone synapses with either 1 horizontal cell or bipolar cells to amacrine cells to ganglion cells

Lateral Inhibition:

  • enhances visual contrast

    • Horizontal cells connect laterally between rods and cones and bipolar cells

      • Always inhibitory

    • Prevents lateral spread of light excitation or retina by inhibiting adjacent areas to excitation

    • Contrast is enhanced with excitatory center and inhibitory surround

Function of Amacrine Cells and Ganglion cells:

  • Amacrine cells

    • ~30 different types

    • They are thought to be interneurons that aid in the beginning of visual signal analysis

  • Ganglion Cells

    • Different types

    • In humans

      • P cells

        • AKA Parvocellular cells

        • Like X cells in animals

        • Receives input from at least one cone, may be responsible for color transmission

        • Fast transmission

      • M cells

        • AKA Magnocellular

        • Like Y cells

        • Detect changes in visual fields (movement), so eyes move towards visual stimuli

        • Fast response

    • Excitation of Ganglion Cells:

      • These are the 1st cells in the retina that transmit actual action potentials

      • Their axons make up the optic nerve

      • Many excited by changes in light intensity

      • Respond to contrast borders, this is the way the pattern of the scene is transmitted to brain

The Optic Disc:

  • Blind spot

  • The point where ganglion cell axons exit the eye to form the optic nerve

  • Entry and exit point for retinal blood vessels

  • Creates a blind spot since there are no rods or cones (no receptors)

Retina- Color Vision:

  • color vision results from activation of cones

  • There are 3 types of cones:

    • Blue

    • Red

    • Green

  • Each cone is receptive to a particular wavelength of light

  • Equal stimulation of all 3 cones is interpreted as white color

  • Color blindness

    • Can occur due to a deficiency of lack of a particular type of cone

      • Deuteranopia

        • Green cone issue

      • Protanopia

        • Red cone issue

      • Tritanopia

        • Blue one issue

    • Genetic disorder are passed on through X chromosome and so almost exclusively occurs in male genders