2.2.2 vision at the eye

Which clear, dome-shaped outer layer of the eye provides protection and performs the initial refraction of incoming light?

The cornea.

What is the functional role of the iris and its two opposing muscle groups in visual regulation?

The iris regulates pupil diameter using the pupillary sphincter muscle (contraction) and pupillary dilator muscle (dilation).

What is the function of the lens (biconvex shape)?

The lens focuses light onto the retina, adjusting its shape for distance and near vision, called accommodation.

What is the ciliary body’s structure and function?

Contains ciliary muscle attached to the lens via zonule fibres and produces and reabsorbs aqueous humor.

In the anterior portion of the eye, what fluid-filled compartments are there?

The anterior chamber lies between the cornea and iris; the posterior chamber, between the iris and lens. Both contain aqueous humor.

In the posterior portion of the eye, what is the fluid-filled compartment?

Vitreous humor.

What is aqueous humor made of?

98-99% water, 1-2% nutrients.

What is vitreous humor made of?

Gel-like substance containing 98% water and collagen, 2% hyaluronic acid and salts.

What is the retina?

Inner neural layer containing photoreceptors.

What is the choroid?

A vascular layer of the eye between the retina and sclera that provides nutrients and oxygen to the outer layers of the retina.

What is the sclera?

Tough, white protective outer layer.

What is the macula?

Yellowish, blood-vessel-free central area of the retina.

What is the forvea?

The fovea is a small depression in the macula that contains a high concentration of cones.

What is the optic disk?

The point on the retina where the optic nerve fibres exit the eye, creating a blind spot.

What is the visible spectrum for people?

Between 400 nm and 700 nm.

How are colour, brightness and saturation manipulated?

Wavelength determines colour, amplitude determines brightness, and purity corresponds to saturation.

What physical mechanism allows corneal refraction to occur when light strikes the eye?

Light slows down and bends as it transitions from air into the fluid aqueous humor, focusing rays toward the retina.

What is accommodation?

Process of lens adjustment to focus objects at varying distances.

How is accommodation achieved for distant objects?

The ciliary body relaxes, zonule fibres tighten, and the lens flattens.

How is accommodation achieved for near objects?

The ciliary body contracts, zonule fibres loosen, and the lens becomes thicker.

What are two refractive errors?

Myopia and hyperopia.

What occurs with myopia?

The eyeball is too long, and incoming light rays converge in front of the retina, blurring distant vision.

What occurs with hyperopia?

The eyeball is too short, so incoming light rays converge behind the retina, blurring near vision.

What is presbyopia?

Age-related reduction in accommodation flexibility due to lens crystallisation and weakening of ciliary muscles.

What is the function of the pigmented epithelium?

Absorb light that passes through the retina, preventing scattering and providing nutrients to photoreceptors.

What is the function of photoreceptors?

Generate graded potentials in response to light absorption.

What is the function of bipolar cells?

Transport graded signals from photoreceptors to ganglion cells.

What is the function of ganglion cells?

Generate action potentials that transmit signals to the brain.

What is the function of horizontal cells?

Interneurons that modulate synapses between photoreceptors and bipolar cells.

What is the function of amacrine cells?

Interneurons that modulate synapses between bipolar and ganglion cells.

What primary neurotransmitter do photoreceptors release in the dark to communicate with bipolar cells?

Glutamate.

How does light exposure uniquely change the electrical state of a photoreceptor compared to typical neurons?

Light causes the photoreceptor to become less active by hyperpolarizing its membrane, leading to decreased neurotransmitter release.

What is the step-by-step biochemical sequence of the phototransduction cascade?

Light activates rhodopsin, which activates transducin; transducin activates phosphodiesterase (PDE); PDE breaks down cGMP; the drop in cGMP closes $\text{Na}^+/\text{Ca}^{2+}$ channels; the cell hyperpolarizes and decreases glutamate release.

What role does phosphodiesterase (PDE) play during the phototransduction cascade?

It breaks down (hydrolyzes) cyclic GMP (cGMP), leading to a reduction in intracellular cGMP levels.

What mechanism causes $\text{Na}^+/\text{Ca}^{2+}$ channels to close when light hits a photoreceptor?

The reduction in intracellular cGMP, which is required to bind and hold the channels open in the dark.

What type of glutamate receptors do ON bipolar cells use to detect increases in light?

G-protein coupled receptors (GPCRs).

What type of glutamate receptors do OFF bipolar cells use to detect decreases in light?

Ionotropic glutamate receptors.

How does an increase in light lead to activation of the ON bipolar cell pathway?

The photoreceptor hyperpolarizes and decreases glutamate release, which removes inhibition and causes ON bipolar cells to depolarize.

How does a decrease in light lead to activation of the OFF bipolar cell pathway?

The photoreceptor depolarizes and increases glutamate release, which binds ionotropic receptors and causes OFF bipolar cells to depolarize.