Untitled Quiz

Eye Anatomy and Function

The eye is housed in a bony orbital cavity surrounded by a cushion of fat for protection.
The palpebral fissure is the elliptical opening between the eyelids.
The lower lid margin is the area between the cornea and sclera.
Tarsal plates are structures inside the eyelids that contain meibomian glands, which secrete an oily lubricant.
The conjunctiva is a transparent protective layer covering the eye.
The cornea is a transparent structure covering the iris and pupil, which protects these structures from external elements.
The lacrimal apparatus produces tears that lubricate and irrigate the eye.

Extraocular Muscles

Six extraocular muscles connect the eyeball to its orbit, allowing for both straight and rotary movement.
These muscles always maintain parallel axes, ensuring conjugate movement, where both eyes move together.
The four rectus muscles are the superior, inferior, lateral, and medial rectus muscles.
The two oblique muscles are the superior and inferior oblique muscles.
Parallel axes are crucial for binocular vision, allowing the brain to perceive a single image.
The movement of these muscles is controlled by three cranial nerves:
- Cranial nerve VI (abducens nerve) innervates the lateral rectus muscle, responsible for abduction (moving the eye outward).
- Cranial nerve IV (trochlear nerve) innervates the superior oblique muscle.
- Cranial nerve III (oculomotor nerve) innervates the superior, inferior, and medial rectus muscles, as well as the inferior oblique muscle.

Internal Anatomy of the Eye

The eye is composed of three concentric coats:
- Outer Fibrous Layer:
  - Sclera: The tough, protective, white outer layer of the eye.
  - Cornea: Transparent structure covering the iris and pupil, responsible for refracting light rays for proper focus on the retina.
  - Corneal Reflex: A blink reflex triggered by contact with a wisp of cotton, involving the trigeminal nerve (cranial nerve V) for afferent sensation and the facial nerve (cranial nerve VII) for efferent stimulation.
- Middle Vascular Layer:
  - Choroid: A darkly pigmented layer that prevents internal light reflection and is heavily vascularized to provide blood to the retina.
  - Iris: Functions as a diaphragm, controlling the size of the pupil opening at its center.
    - Muscle fibers in the iris contract the pupil in bright light for near vision and dilate the pupil in dim light for far vision.
  - Pupil: The round, regular opening in the center of the iris.
    - Constriction of the pupil is mediated by the parasympathetic nervous system through cranial nerve III.
    - Dilation of the pupil and elevation of the eyelid are controlled by the sympathetic nervous system.
  - Lens: A biconvex disc located behind the pupil.
    - This transparent structure acts as a refracting medium, ensuring focus on the retina.
  - Anterior and Posterior Chambers: Filled with clear, watery aqueous humor produced by the ciliary body.
    - This fluid provides nutrients and drains metabolic wastes, contributing to intraocular pressure regulation.
- Inner Layer:
  - Retina: The light-sensitive layer of the eye, containing photoreceptor cells that convert light waves into nerve impulses.
    - Using an ophthalmoscope, you can visualize the optic disc, retinal vessels, background, and macula.
  - Optic Disc: The point where nerve fibers from the retina converge to form the optic nerve.
    - Located on the nasal side of the retina, with distinctive color, shape, and margins.
  - Retinal Vessels: Typically include paired arteries and veins extending to each quadrant of the retina.
  - Macula: Located on the temporal side of the fundus.
    - This pigmented area receives and transduces light from the center of the visual field.

Visual Pathways and Visual Fields

Light rays are refracted through the transparent structures of the eye (cornea, aqueous humor, lens, and vitreous humor) before reaching the retina.
Once light reaches the retina, it stimulates photoreceptor cells (rods and cones).
This stimulation triggers a series of neural events that convert light signals into nerve impulses.
These impulses travel through the optic nerve, optic chiasm, optic tract, lateral geniculate body, optic radiation, and finally reach the visual cortex in the occipital lobe of the brain.
The visual field refers to the area of space that an individual can see with both eyes fixed straight ahead.
Each eye's visual field is divided into a temporal (outer) and nasal (inner) half.
The visual fields partially overlap, creating binocular vision that provides depth perception.
Damage to a specific area of the visual pathway can result in corresponding visual field defects.