Human Eye - Comprehensive Study Notes

Receptors and the Human Eye

Detect changes in the environment by means of receptors.
Impulses move from receptors along two main pathways: to the CNS via sensory neurons, and from the CNS to effectors via motor neurons.
General receptors: modified dendrites distributed throughout the body (e.g., skin).
Special sensory receptors: complex sensory organs (eyes and ears) and epithelial structures (e.g., taste buds).

Conjunctiva: mucous membrane covering the sclera and inner eyelids.
Pain receptors present in conjunctiva; mucous production contributes to lubrication.
Sclera: inelastic connective tissue providing internal protection and attachment points for muscles; maintains eyeball shape.
Cornea: continuation of sclera; convex and clear; allows light to pass and contributes to converging refraction.
Choroid: layer containing pigments and blood vessels; pigment absorbs excessive light; blood vessels supply nutrients and O₂.

Ciliary body: extension of the choroid containing ciliary muscles; controls curvature of the lens during accommodation.
Iris: continuation of the choroid; circular, pigmented curtain with a central hole (pupil) controlling the amount of light entering.
Photoreceptors: modified neurons sensitive to light.
- Rods: located on the edge of the retina; responsible for black-and-white vision and peripheral vision.
- Cones: located in the yellow spot (fovea) of the retina; detect red, blue, and green; enable color perception and sharp vision.

Lens: round, biconvex, flexible and transparent; located just behind the iris; attached to the ciliary body by ligaments; no blood vessels; changes shape to focus light on the retina.

Retina contains photoreceptors and layers that process light into neural signals.
Key internal components include: retina, choroid, sclera, vitreous body, and optic nerve.
Important zones: fovea (yellow spot) for sharp central vision; blind spot (optic disc) where the optic nerve exits; ora serrata; limbus.

Anterior cavity: filled with aqueous humour; provides nutrients and oxygen, carries away wastes, helps maintain intraocular pressure.
Posterior cavity: filled with vitreous humour; helps maintain intraocular pressure, holds retina against the choroid, and prevents the eyeball from collapsing.

Stages
- Pathway of light and image formation
- Stimulation of photoreceptors
- Pathway and destination of nerve impulses

Light enters the eye through the following sequence:
- Cornea → Aqueous humour → Pupil → Lens (biconvex) → Vitreous humour.
Light travels through the neural layers of the retina to stimulate photoreceptors.
Each layer has a different density, causing refraction (bending) of light at the cornea (entrance) and at the lens (adjustable focus).

The cornea provides most refraction, but its shape is constant and cannot change refractive power.
The lens can change shape to adjust the amount of refraction for fine, sharp focusing.
Light rays converge on the yellow spot (fovea) to form an image.
The image formed on the retina is smaller than the object and is inverted and reversed.

In the retina, rods and cones are stimulated by light; photopigments are broken down by light, generating an electrical impulse.
Nerve impulses travel along two layers of neurons; the axons of ganglion cells form the optic nerve, which leaves the eye carrying impulses to the cerebral cortex.
Impulses are interpreted as vision in the occipital lobe of the cerebral cortex.

Visual information from each eye is processed through a pathway that involves:
- Optic nerve
- Optic chiasma (where fibers cross)
- Lateral geniculate nucleus (LGN) of the thalamus
- Primary visual cortex (occipital lobe)
The left visual field is processed by the right side of the brain and vice versa, through the crossing at the optic chiasm.

Field of vision of the left and right eye overlaps.
Each eye receives a slightly different view.
The brain combines these views to form a single three-dimensional image, enabling depth and distance perception (stereopsis).

Accommodation produces a finely-focused image on the retina by changing the shape of the lens via the ciliary muscles.
More convex lens → more light rays bent; more flat lens → fewer light rays bent.
Occurs when viewing objects closer than 6 metres.
Mechanism (when viewing near objects):
- Circular ciliary muscles contract, pulling the ciliary body toward the pupil.
- Tension on the suspensory ligaments (zonules) decreases, allowing the lens to bulge.
- Light rays bend more, shortening the focal length, so the image focuses clearly on the retina.

1) Myopia (Short-sightedness)
- Far objects are blurred; near objects are clear.
- Caused by an elongated eyeball or an overly convex cornea or lens; image forms in front of the retina.
- Corrected with concave lenses (diverges light rays).
2) Hypermetropia (Long-sightedness / Hyperopia)
- Near objects are blurred; distant objects are clear.
- Caused by a shortened eyeball or a flatter cornea or lens; image forms behind the retina.
- Corrected with convex lenses (converge light rays).
3) Astigmatism
- Irregularly shaped cornea (corneal astigmatism) or lens (lenticular astigmatism).
- Causes light to focus at two points on the retina rather than one, leading to blurred or distorted vision and headaches.
- Usually hereditary; can also result from injury or thinning of the cornea.
4) Cataracts
- Lens is composed mainly of water and proteins; with age, proteins clump, clouding the lens.
- Clouding blocks light from reaching the retina, reducing vision.
- Corrected by cataract surgery (removal/replacement of the cloudy lens).

Vision is essential for daily function; defects affect reading, driving, and coordination.
Accommodation decreases with age (presbyopia), often requiring reading glasses or multifocal lenses.
Cataracts are common in aging and can be effectively treated with surgery to restore vision.
Binocular vision and depth perception are crucial for activities like driving and sports; impairment can affect safety and performance.
Tear production via the lacrimal apparatus (gland, puncta, canaliculi, sac, duct) maintains ocular surface moisture and comfort.
The eye’s protection system includes the sclera and choroid (pigments and vasculature) that safeguard internal structures from damage and excessive light.

Lacrimal gland, lacrimal ducts, lacrimal puncta, lacrimal canaliculi, lacrimal sac, lacrimal duct
Conjunctiva, sclera, cornea, choroid, ciliary body, iris, pupil, lens
Retina, photoreceptors (rods and cones), fovea, blind spot (optic disc)
Anterior cavity, aqueous humour; Posterior cavity, vitreous humour
Optic nerve, optic chiasm, LGN, primary visual cortex
Accommodation, presbyopia, myopia, hypermetropia, astigmatism, cataracts