Study Notes on The Human Eye

Arvaco Roxo
Vision lecture covering gross ocular anatomy, retina, visual brain regions, and related diseases
Shaylah McCool - November 24, 2025

List the main structures of the eye and their function.
Specify the different cell types of the retina.
Describe the process of phototransduction and compare synaptic signaling in the retina to that of other types.
Identify the retinal projection targets of the brain and differentiate between image-forming and non-image-forming vision.
Discuss various methods for clinically testing vision.
Aim: By the end of this chapter, you should be able to…

Cortical Territory: Vision occupies the largest cortical territory, comprising 25-30% of the human cortex.
High Resolution: Provides superb spatial detail encompassing patterns, shapes, motion, color, and depth.
Rapid Decision Making: Contributes to swift resolutions in decision-making scenarios.
Communication & Interaction: Facilitates communication, aiding in interpreting facial expressions and body language.
Memory & Learning: Vision is intrinsically linked to memory and learning processes.

Pupil: Controls the amount of light entering the eye.
Iris: Colored part of the eye, regulating the pupil size.
Lens: Responsible for focusing light onto the retina; contains 90% protein, the highest protein concentration in the human body.
Aqueous Humour: Fluid filling the front part of the eye.
Vitreous Humour: Gel-like substance filling the eye behind the lens.
Retina & Retinal Pigment Epithelium (RPE): Key components for vision.
Fovea: Area of high acuity vision exclusively containing cones.
Optic Nerve: Transmits visual information from the retina to the brain.

To observe the Blind Spot:
- Cover your left eye,
- Focus on a dot and move your face closer or farther until you notice the cross disappearing.

Mapping of visual fields by both left and right eyes.
Visual Fields:
- Left visual field processed by the right visual cortex, and vice versa.
Relevant structures include optic nerves, optic tracts, lateral geniculate nucleus of thalamus, and others.
Essential organizations process inputs from both eyes and send them to the respective visual areas in the cortex.

Rods:
- 100 million rods, responsible for scotopic (low-light) vision.
- Characteristics: Low light sensitivity and high convergence leading to low spatial resolution.
Cones:
- 4 million cones, responsible for photopic (color) vision.
- Characteristics: Higher light requirement, lower convergence (1:1 connections) providing high spatial resolution.

Foveal Vision Density:
- Cone peak: 180
- Rod peak: 160
Distribution across the retina varies, shown through various graphs and images.

Phototransduction in photoreceptors involves specific opsins that absorb light at different wavelengths (e.g.,
- Rod opsin absorbs around 496 nm.
- Cones have peak absorbances: B cones (blue) 419 nm, G cones (green) 531 nm, R cones (red) 559 nm.

Definition: The process where light is transformed into neural signals.
Key Elements:
- Rhodopsin and cone opsins function as G protein-coupled receptors.
- Sensitive due to the chromophore, also known as retinal.

Dark State:
- Resting membrane potential at -40 mV; this is known as the private current.
- Glutamate is released by photoreceptors.
Light State:
- Hyperpolarization occurs, resulting in a membrane potential of -70 mV;
- Glutamate release ceases.

Photoreceptors and bipolar cells utilize graded potentials (GPs) instead of action potentials for signaling.
GPs are localized changes in membrane potential that diminish over short distances due to passive current flow.
Temporal and spatial summation can trigger action potentials under the juiste conditions.

On and Off pathways:
- On pathway: Activated by light.
- Off pathway: Inhibited by light (light-off response).
Diversity observed in amacrine cell morphologies across species, affecting their functional roles.

About 40 types in mouse and 20-30 in primate species.
They tile the retina with functional classes such as ON, OFF, and ON-OFF.
Special roles include direction-selectivity and orientation-selectivity, supporting various visual tasks.

Horizontal and Amacrine Cells:
- Modulate signals within the retina to enhance visual processing.

Comprises microglia, astrocytes, oligodendrocytes, and Müller glia.
Müller glia span the entire thickness of the retina and are essential for:
- Nutrient transport and structural support
- Promoting vascularization (source of VEGF) and synaptic pruning.
- Debris clearance and pH regulation.

Include various subtypes of retinal ganglion cells (RGCs) projecting to areas involved in:
- Pupillary light reflex
- Circadian rhythms
- Visual-motor guidance
- Visual scene perception

Glaucoma: Progressive RGC degeneration, commonly linked to high intraocular pressure (IOP).
Age-Related Macular Degeneration (AMD): Degeneration of the macula causing central vision loss, having dry and wet forms.
Diabetic Retinopathy (DR): Damage to retinal microvasculature due to chronic hyperglycemia, resulting in hemorrhage and neovascularization.
Retinitis Pigmentosa (RP): Inherited condition leading to progressive loss of vision due to rod and cone degeneration.

Comprehensive resources exist for further examination of vision physiology and pathology, such as academic journals and dedicated websites.