module 14: sensory pathways 1: vision (unfinished)

amplitude of light

• intensity of light

• eg large amplitude = bright light

wavelength of light

• colour of light

• the shorter the distance is between peaks = the more energetic the light is

  • becomes more blue as well (opposite = red)

visible light wavelength

380 (violet) -750 (red) nm

diopter (D)

• unit

• power of lens/ ability to bend light

lens

• crystalline protein structure = composed crystallin proteins which give the lens its transparency + ability to focus light

• absorbs, focuses and directs incoming light to the retina

• fine-tunes focus by changing shape

• 12 D

cornea

• transparent, curved front layer of the eye

• focuses light and protects the eye

• 42 D - bends (refracts) light the most

iris

• muscle

• controls size of the pupil - regulates light entry

retina

• layer at the back of the eye

• also a part of the brain/CNS

• contains photoreceptors (rods and cones)

optic nerve

• transmits visual info from the retina to the brain

  • lateral geniculate in the thalamus

  • visual vortex

accommodation (lens adjustment)

• distant objects = lens flattens to reduce refraction

• near objects = lens thickens to increase refraction

image inversion

• light passing through the lens produces inverted + reversed image on the retina

• the brain (visual cortex) reorientates the image correctly

myopia

• short-sightedness

• lens is too strong or the eye is too long => light focuses before it reaches retina (ie focuses too soon)

• correction = concave lens to spread out light before reaching the eye + into the retina

hyperopia

• far/long-sightedness

• lens too weak or eye too short => light focuses behind the retina

• correction = convex lens to converge light before it reaches the eye + into the retina

photoreceptors in the retina

• found at the back of the retina - makes light pass thru all cells types

rods:

• function in low light (night vision)

• highly sensitive

• don’t detect colour

• cylindrical shape

cones:

• function in bright light (day vision)

• detect colour (red, blue, green cones)

why cant see colour well at night

• rods used for night vision

• rods cant detect colour

• therefore little colour visible

Retina as Part of the Brain

• Retina is considered an extension of the central nervous system (CNS).

• Directly connected to the brain via the optic nerve.

Optic Pathway

Optic Nerve → Lateral Geniculate Nucleus (in thalamus) → Visual Cortex

Retinal Cell Layers (Information Pathway)

Photoreceptors (Rods & Cones) → Bipolar Cells → Retinal Ganglion Cells → Optic Nerve → Brain

Retinal Ganglion Cells

Convert light signals into action potentials and send them to the optic nerve to be processed in brain

neurotransmitter used by photorecetpors

glutamate (glutamatergic excitatory neurones)

Horizontal cells

• Found between photoreceptors and bipolar cells

• involved in adapting to light intensity and spatial + colour processing.

Amacrine Cells

• Found between bipolar cells and retinal ganglion cells

• help extract visual features such as motion direction and background light adaptation

• modulate circadian rhythm

Retinal Ganglion Cells

Neurons that receive input from photoreceptors and transmit action potentials to the brain via the optic nerve.

intrinsically photosensitive retinal ganglion cells (ipRGC)

• type of retinal ganglion cell

• Contain melanopsin so can detect and respond to light- help regulate circadian rhythms and pupil size

Bipolar Cells

• Intermediate neurons in the retina that transmit signals from photoreceptors to ganglion cells.

• types:

  • on bipolar cells = Depolarize when light is detected by photoreceptors

  • off bipolar cells = Hyperpolarize when light is detected by photoreceptors

  • type of bipolar cell it is is dependent on the type of glutamate receptor it has - determines the way in which it responds to glutamate

Retinal Pigmented Epithelium (RPE)

• pigmented layer at back of retina

• supports photoreceptors by regenerating retinaldehyde - maintains rod and cone function

• reduces oxidative stress of rods and cones

retinaldehyde

important cofactor in light-sensing proteins / opsins found in rods / cones

Tapetum Lucidum

• reflective layer behind RPE in some animals

• enhances night vision by reflecting light back through the retina.

Fovea

A small central region of the retina with a high concentration of cones for sharp vision.

Peripheral Retina

Contains more rods than cones

Visual Acuity

The sharpness of vision

Opsins

• Light-sensitive proteins in photoreceptors that, when bound to retinal, detect light

• maximises rods/cones sensitivity to light

retinal

• vitamin A derivative

• absorbs light and changes conformation => phototransduction occurs - allows us to see light

Rhodopsin

• opsin + retinal = rhodopsin

• The opsin in rods responsible for night vision

Cone Opsins

• opsin in cones responsible for colour vision

• 3 types:

  • detect red, blue, green

Melanopsin

• An opsin found in some retinal ganglion cells

• mainly involved in:

  • Regulating the biological clock (day-night cycle)

  • Pupil constriction in bright light.

Retinal

• Vitamin A Derivative

• absorbs light

• types:

  • 11-cis retinal

  • all-trans retinal (active form)

    • 11-cis becomes all-trans via a conformational change when light is absorbed

Dark Adaptation

The process by which rods adjust to low light conditions over time.

Cyclic GMP (cGMP)

A molecule that keeps sodium channels open in photoreceptors (in the dark)

Depolarisation in Darkness

• Photoreceptors remain depolarised in the dark due to open sodium channels

• presence of cGMP in photoreceptors keeps sodium ion channels open => depolarisation

• depolarisation leads to release of glutamate - photoreceptors are active

Hyperpolarisation in Light

• Light causes sodium channels in photoreceptors to close

• light causes cGMP to become GMP => sodium channels remain closed and so photoreceptors are hyperpolarised

• hyperpolarised state means glutamate is not released

phototransduction

• process by which light is converted into electrical signals in the retina

• When light hits rhodopsin, retinal changes from 11-cis retinal → all-trans retinal

• this activates opsin, triggering a G-protein signaling cascade: transducin → phosphodiesterase → cGMP breakdown

• leads to an electrical signal sent to the brain.

• After activation, retinal recycled back to 11-cis form to reset rhodopsin

Transducin

• A G-protein that activates phosphodiesterase when light is detected

• swaps GDP for GTP when activated, allowing activation of phosphodiesterase

Phosphodiesterase (PDE)

• Enzyme that breaks down cyclic GMP

• inhibited when 0 light absorbed

• active in the presence of light

trichromatic vision

• normal color vision

• ability to perceive colors by sensing three primary wavelengths of light: red, green, and blue in retina

• done by 3 cones in mammals

Monochromatic Vision

• Vision in animals with only one type of cone photoreceptor.

• common in aquatic animals

Dichromatic Vision

• Vision in animals with two types of cone photoreceptors

• eg dogs, cats, horses

Tetrachromatic Vision

Vision in some animals or rare humans with four cone types for enhanced color discrimination.

Mantis Shrimp Vision

Known for having up to 16 types of cone photoreceptors

Color Blindness

• A condition caused by mutations in opsin genes

• red and green opsin genes found on X chromosome, blue found on chromosome 7

  • means men more likely to be (red-green) colourblind cos only have 1 X chromosome - if mutation found on X chromosome then condition manifests

• women can be tetrachromatic

Ishihara Test

A color vision test using colored dot patterns to identify color blindness.

X-Linked Inheritance in Color Blindness

Red and green opsins are on the X chromosome

Lateral Geniculate Nucleus (LGN)

• key relay station in the visual pathway

• located in the thalamus

• receives visual information from the retina and relays it to the primary visual cortex (V1). 

Visual Cortex (V1)

main processing center for vision, located at the back of the brain (occipital lobe)

Optic Chiasm

The point where nasal retina signals cross to the opposite side of the brain

Orientation Selectivity in V1

• Neurons in the visual cortex respond to specific orientations of visual stimuli.

• The brain has specialized neurons in the visual cortex (V1) that respond to different line orientations.

• Some neurons detect vertical lines, others detect horizontal or diagonal lines.

• When a pattern rotates, different neurons activate, helping the brain break down shapes and recognize objects, letters, and patterns.