11. VISION The Eye, Retina and Visual Transduction

Stimulus

An environmental change that causes a response in an organism.

Sensory Receptors

specialized neurons that receive sensory stimulus for vision - photoreceptors – detect light

Photoreceptors

Neurons in the eye that detect light; includes rods and cones.

Visual Sensation

detection of stimuli from the environment, in this case light

Visual Perception

The conscious experience and interpretation of sensory information by the brain. (stars as consellations)

Visual Transduction

The conversion of light energy into neural signals by photoreceptors.

Hue

The attribute of a color that enables it to be classified as red, blue, green, etc., based on its wavelength.

( wavelength (short wavelength –violet; long wavelength – red))

Brightness

The perceived intensity of a color, determined by the amplitude of light waves.

intensity (greater amplitude of light waves then brighter)

Saturation

The purity of a color, influenced by the mixture of different wavelengths.

purity of light (mixture of wavelengths; if mostly one  wavelength then color is pure or saturated)

Ganglion Cells

Neurons in the retina that receive input from bipolar cells and send signals to the brain via the optic nerve.

Receptive Field

The specific area of the visual field that a particular neuron responds to.

Accommodation

The process by which the eye's lens changes shape to focus on near or distant objects.

Strabismus

A condition where the eyes are misaligned or crossed, impacting vision.

Amblyopia

Also known as 'lazy eye', a vision development disorder.

Nystagmus

Involuntary eye movements, often resulting in jerky motion.

Optic Nerve

The nerve that transmits visual information from the retina to the brain.

Ciliary Muscles

Muscles that adjust the shape of the lens to facilitate focus.

Vitreous Humor

The clear, gel-like substance filling the eye that helps maintain its shape.

Photopigments

Molecules that absorb light and play a crucial role in phototransduction.

Rhodopsin

The photopigment found in rods that is sensitive to light.

Qualities or Dimensions of color perception 

hue, brightness, saturation

visual system of colors

processes by which the human eye perceives and interprets different colors through interaction of light with photopigments in the retina.

percives wave length of light waves

Electromagnetic radiation

380 – 760nm is visible spectrum.

visual system

visual sensation, visual perception, large portion of human brain devoted to visual analysis

eye anatomy

Sensory receptors, Sensory transduction, Receptor potential

Sensory transduction

detection of stimuli by sensory receptors alters the membrane potential of the cell - photoreceptor

Receptor potential

electrical changes in cell (photoreceptor) as a result of sensory transduction

Eyes are suspended in orbits by -?-extraocular muscles that move the eye through

-6- ; vergence, saccadic and pursuit movements 

Retina

a. Inner lining of the eye; has the photoreceptors

b. Image must be focused on retina for vision to occur

c. Part of the CNS connected to brain by optic nerve

Types of eye movements

Vergence movements 

i.   Cooperative

ii.  Keep both eyes fixed on one target 

Saccadic movements (microsaccades)

i.   Rapid and jerky (keep image moving over retina)

ii.  Occur when gaze shifts 

iii.  also occur during dreaming (REM sleep)

Pursuit movements 

i.   Maintain image on fovea (back of the eye)

ii.  Slower, controlled eye movements

Vision deficits in development

Strabismus 

  - Eyes misaligned or crossed eyes

Amblyopia 

  - Lazy eye

Nystagmus 

  - Jerky movement up/down or side to side

Retinal layers

-Photoreceptor layer; deepest (contains rods and cones)

-Bipolar cell layer

-Ganglion cell layer

Receptive Fields of Ganglion Cells

Three types initially found in the frog retina

i. ON cells: respond when retina is illuminated  

ii. OFF cells: respond when light turned off  

iii. ON/OFF cells: respond when light turns on and when turns off 

Receptive Fields of Ganglion Cells

Cat retina

Receptive field is a circular center surrounded by a ring

i. ON cells: respond when center is light, but ring is dark  

ii. OFF cells: respond when center is dark, but ring is light  

iii. ON/OFF cells: respond when light turns on and when it turns off 

 

conjunctiva

membranes covering muscles

sclera

opaque white of the eye

cornea

transparent

iris

pigmented muscles that open/close pupil (reflexive)

pupil

hole in iris through which light enters eye

lens

transparent layers that focus light

Ciliary muscles

i.  Change shape of lens to focus light on retina

ii. Results in accommodation  (focus of near/far objects)

Vitreous humor

clear, gelatinous liquid (gives eye volume)

Fovea/Macula 

(central visual axis)

 Cones (concentrated in the fovea) -

6 million; used for daytime/high acuity/color vision    - central or foveal vision 

Rods (located outside of the fovea) -

120 million;  used in dim lighting (night vision)  - peripheral vision

 Optic disk 

i.   Back of eye where ganglion cell axons leave eye to form optic nerve

ii.  Blind spot

Retinal circuitry

Ganglion cells

Bipolar cells

Horizontal cells

Amacrine cells  

Rods and Cones

Ganglion cells

(axons form the optic nerve; convey information to the brain)

Bipolar cells

convey information from photoreceptors to ganglion cells)

Horizontal cells

(provide input from surrounding photoreceptors/retina)

Amacrine cells

(provide input from surrounding photoreceptors/retina)

 Rods and Cones

(interact with light – site of sensory transduction)

Visual (Sensory) Transduction

process by which environmental energy, in this case light, is converted to a change in neuronal membrane potential

Photopigments (absorb light)

i.   Located in membrane of lamellae in outer segment of           the photoreceptors (see Figure)

ii.  Comprised of two molecules - opsin and retinal 

iii. In Rods is called Rhodopsin: 

Rhodopsin

a. Made of rod opsin plus retinal

b. Retinal synthesized from Vitamin A

In the dark the photoreceptors are

depolarized-

Light breaks Rhodopsin into its two components and the opsin closes Na⁺ channels and hyperpolarizes photoreceptor membrane (receptor potential).

Central and Peripheral Vision

1. Receptive field: visual field that an individual neuron “sees” 

2.   Central vision

a. Foveal vision

b. Very acute (sharpness)

c. Contains equal numbers of

    ganglion cells and cones

3.   Peripheral vision

a. Less precise

b. Many receptors converge on

    single ganglion cell

Circuitry in the retina

a. Photoreceptor connects to bipolar cells  

b. Bipolar cells = no action potentials (no axon)

c. Membrane potential controls release of glutamate  

-Depolarization increases release of glutamate  

- Hyperpolarization decreases release of glutamate  

d. Dark, photoreceptors constantly release glutamate (depolarized; Na⁺ channels are open)

e. Light splits rod/cone opsin, hyperpolarizes photoreceptor and         reduces release of neurotransmitter/glutamate (Na⁺ channels close)