Lecture 2 The light sense 1 transduction

Q: What is transduction?

The process by which light energy is converted into neural activity by photoreceptors.

Q: What factors reduce the amount of light reaching the photoreceptors?

Reflection at optical interfaces

Pupil size

Optical density of ocular media

Spectral absorption by ocular tissues

Q: Approximately how much light is lost by reflection in the eye?

About 5% of incident light is lost through reflection.

Q: Why is the pupil important in vision?

Acts as the eye's entrance pupil

Limits light entering the eye

Retinal illuminance is proportional to pupil area

Q: What is the relationship between transmission and optical density?

Q: What is the main spectral absorption role of the cornea?

The cornea absorbs UVC radiation (<280 nm) and helps protect the eye from UV damage.

Q: Why is the crystalline lens important for spectral absorption?

Absorbs short wavelength (blue) light

Protects the retina from UV radiation

Absorbs approximately 295-400 nm

Pigmentation increases with age

Q: What is macular pigment and what does it do?

Located in the macula

Consists of lutein and zeaxanthin

Peak absorption ≈ 460 nm

Absorbs short wavelength light

Functions:

Protects photoreceptors

Reduces scatter

Reduces chromatic aberration

Q: Why is the retinal pigment epithelium important?

The RPE absorbs light that passes through the retina and prevents internal reflections that would degrade the retinal image.

Q: What are the four main parts of a photoreceptor?

Outer segment

Inner segment

Cell body

Synaptic terminal

Q: What is found in the outer segment?

Stacks of discs containing visual pigments:

Rhodopsin (rods)

Iodopsin/photopsins (cones)

Q: What are the two components of a photopigment?

Opsin

Protein component

Retinal

Vitamin A derivative

Chromophore component

Q: What happens in photoreceptors in darkness?

Na⁺ channels open

Dark current present

Cell is depolarised

Glutamate released continuously

Dark = Depolarised + Glutamate release

Q: What happens in photoreceptors when light is absorbed?

Light causes:

11-cis retinal

→ all-trans retinal

→ transducin activation

→ phosphodiesterase activation

→ cGMP decreases

→ Na⁺ channels close

→ photoreceptor hyperpolarises

→ glutamate release decreases

Q: What are the peak sensitivities of rod and cone photopigments?

Cones

S cone = 420 nm

M cone = 530 nm

L cone = 565 nm

Rods

Rhodopsin peak = 505-507 nm

Q: Compare Vλ and V'λ.

Vλ

Photopic sensitivity

Cone-mediated

Peak sensitivity = 555 nm

V'λ

Scotopic sensitivity

Rod-mediated

Peak sensitivity = 507 nm

Q: What is the Purkinje Shift?

As luminance decreases from photopic to mesopic/scotopic levels, peak visual sensitivity shifts from:

555 nm → 507 nm

This makes blue-green objects appear relatively brighter and red objects appear relatively darker under dim lighting.

Q: Explain transduction and spectral sensitivity.

Transduction is the conversion of light into neural activity.

Light reaching photoreceptors is reduced by reflection, pupil size, optical density and spectral absorption.

About 5% of light is lost by reflection.

The pupil controls retinal illuminance.

The cornea absorbs UVC radiation.

The lens absorbs UV and short-wavelength blue light and becomes increasingly yellow with age.

Macular pigment (lutein and zeaxanthin) absorbs around 460 nm and protects the retina.

The RPE absorbs unabsorbed light and reduces scatter.

Photoreceptors contain:

Outer segment

Inner segment

Cell body

Synaptic terminal

Visual pigment consists of opsin + retinal.

In darkness photoreceptors are depolarised and continuously release glutamate.

Light causes 11-cis retinal → all-trans retinal.

Transducin and phosphodiesterase activation reduce cGMP.

Na⁺ channels close.

Photoreceptors hyperpolarise.

Glutamate release decreases.

Rod pigment = rhodopsin (peak ≈ 505-507 nm).

Cone pigments:

S = 420 nm

M = 530 nm

L = 565 nm

Vλ describes photopic cone sensitivity (peak 555 nm).

V'λ describes scotopic rod sensitivity (peak 507 nm).

Purkinje Shift = sensitivity shifts from 555 nm to 507 nm as illumination decreases.