Anatomy of the Eye

Anatomy of the Eyeball

  • Eyes are photoreceptors specialized to detect light.
  • Vision requires forming a recognizable image by focusing light on the retina.
  • Light creates a chemical reaction on the retina, leading to a nerve signal.
  • The brain interprets this nerve signal as sight.

Tissue Layers of the Eyeball

  • The eyeball is composed of three tissue layers:
    • Fibrous
    • Vascular
    • Neural
  • These layers form the wall of the eye, house optical components for light admission and focus, and contain neural components for light absorption and nerve signal generation.

Fibrous Layer

  • Composed of the sclera and cornea.
    • Sclera: A tough, protective layer, the "white of the eyes."
    • Cornea: A transparent cover over the anterior central region that admits light into the eye.

Vascular Layer

  • Consists of the iris, ciliary body, and choroid.
    • Iris: Contains pigment cells with varying amounts of melanin (more melanin = darker eyes, less melanin = lighter eyes). An adjustable structure that admits light through the pupil.
    • Ciliary Body: A thick ring of smooth muscle that encircles and supports the iris and lens, adjusting lens shape for focusing.
    • Choroid: A deeply pigmented layer rich in blood vessels that nourishes the retina.

Neural Layer

  • Consists of the retina and the beginning of the optic nerve.
    • Retina: A thin, transparent membrane lining the posterior two-thirds of the eye.
    • Optic Nerve: A bundle of nerve fibers that carries visual information from the retina to the brain.

Optical Components

  • Transparent elements that admit light rays, bend (refract) them, and focus images on the retina.
    • Cornea: Admits light into the eye.
    • Aqueous Humor: Watery fluid secreted by the ciliary body, filling the space between the cornea and lens.
    • Lens: Suspended behind the iris, focusing light onto the retina.
    • Vitreous Body: Transparent gel that fills the space behind the lens, maintaining the shape of the eyeball and holding the retina in place.

Other Features

  • Macula Lutea and Fovea Centralis
    • Patch of cells directly posterior to the lens. The fovea centralis is a pit in the center.
    • The area that produces the most finely detailed visual images.
  • Optic Disc
    • Area of the retina where the optic nerve originates, where all nerve axons converge.
    • Contains no sensory cells, forming the blind spot, which we don’t notice because the brain "fills in" the space.

Forming the Visual Image

  • Begins when light rays enter the eye and focus on the retina, producing a tiny, inverted image.
  • Pupil Dilation and Constriction: Control how much light enters via pupillary dilators and pupillary constrictors (photo pupillary reflexes).
    • Constrictors: Innervated by the parasympathetic nervous system when light intensity increases or when focus is shifted to a relatively close object.
    • Dilators: Innervated by the sympathetic nervous system when light intensity decreases, when focus is shifted to a relatively distant object, or when the body is in an aroused state.

Image Formation

  • Image formation depends on refraction (bending of light).
    • Light slows down when it hits a dense, flat surface.
    • Light bends when it hits a curved angle.
    • Most refraction and focusing happen at the cornea.
    • The lens contributes additional refraction and fine-tunes the focusing process.
    • The cornea’s curvature is fixed, whereas the lens is adjustable.

Focus

  • Eyes are in a relaxed state (emmetropia) when looking at objects greater than ~20 feet away.
    • Light rays are nearly parallel, pupils are relatively dilated, and the lens is relatively thin.
  • Looking at something closer than ~20 feet requires more effort because light rays diverge more.
  • Eyes must do three things to compensate for light divergence:
    • Convergence:
      • Eyes must turn medially so both eyes aim at the object (fixation point).
    • Accommodation:
      • Lenses thicken at the center, increasing refraction power to focus light on the retina.
    • Pupillary Constriction:
      • The pupil narrows to screen out divergent light rays that cannot be effectively focused.
      • Near point is the closest an object can come and still be in focus, dependent on the flexibility of the lens.

Generating the Visual Nerve Signal

  • Sensory transduction of light into nerve signals occurs in the retina.
  • Retina's Cellular Layout:
    • The back of the eye has a dark pigment epithelium that absorbs excess light.
    • Next is a layer of photoreceptor cells called rods and cones.
    • Rods and cones are packed with visual pigments that absorb light and begin the process of sensory transduction.
      • Rhodopsin in rods.
      • Photopsin in cones.
  • Rods and cones are connected to a thick layer of bipolar neurons.
  • Bipolar cells are connected either directly or indirectly to a layer of neurons called ganglion cells.
    • Ganglion cells are the innermost cells of the retina.
    • Their axons converge at the optic disc and form the optic nerve.
  • Rod Cells: Responsible for vision in dim light, contain rhodopsin (made of opsin protein + retinal, a vitamin A derivative).
  • Cone Cells: Responsible for color vision, contain photopsin.
    • Three variations of photopsin that absorb different wavelengths of light.

Rods: Dark vs. Light

  • In the dark, unstimulated rods continuously release an inhibitory neurotransmitter; this inhibits bipolar neurons, so no signal is sent to ganglion cells.
  • When light enters the eye, rhodopsin absorbs it and retinal changes shape and breaks apart, causing the rod to stop sending inhibitory neurotransmitters to bipolar cells.
  • Bipolar cells stimulate ganglion cells, and action potentials travel out the optic nerve to be perceived by the brain.
  • Cones function similarly but are less sensitive and only work in high light intensity.

Other Cells Involved

  • A substantial amount of information processing happens at the retina before a signal is sent to the brain.
  • Other retinal cells detect boundaries of objects, movement, changes in light intensity, etc.

Retina

  • Contains about 130 million rods and 6.5 million cones.
  • Rods:
    • Only contain rhodopsin, respond to light in only one way, and cannot distinguish color, providing only black and white vision in low light.
    • Become "bleached" in bright lights and do not respond at all.
  • Cones:
    • Three varieties of photopsins that respond to different wavelengths of light and are responsible for daylight vision and color vision.

Rods vs. Cones

  • Far fewer cones however, each cone has its own bipolar and ganglion cell - "private line" to the brain.
    • Each optic nerve axon that goes to the brain represents a distinct area of the retina, creating a fine-grain high resolution image.
  • Rods converge and multiple are needed to stimulate a single bipolar cell.
    • Each optic nerve axon represents a broader region of the retina and has a lower resolution image.
    • Fovea centralis produces the most finely detailed visual images and is made entirely of cones.

Color Vision

  • Three varieties of photopsin in cones:
    • Short-wavelength (S) cones: Absorb the violet part of the visible light spectrum.
    • Medium-wavelength (M) cones: Absorb the green part of the visible light spectrum.
    • Long-wavelength (L) cones: Absorb the yellow-green part of visible light-- only cones whose absorption extends to red wavelengths.
  • The brain distinguishes colors based on a mixture of signals from these three types of cones.

Visual Projection Pathway

  • Two optic nerves enter the cranial cavity, converge, and form an X called the optic chiasm.
  • At the optic chiasm, half of the nerve axons cross over to opposite sides of the brain, called decussation.
    • Right cerebral hemisphere receives input from the medial side of the left eye and the lateral side of the right eye.
    • Left cerebral hemisphere receives input from the medial side of the right eye.
  • Each cerebral hemisphere visually monitors the side of the body that has the most motor control over it.

Visual Projection Pathway Continued

  • Posterior to the optic chiasm, most optic nerves synapse at the thalamus.
  • New nerves begin here and travel to the primary visual cortex of the occipital lobe, where we become aware of the visual stimuli.
  • Neurons of the primary visual cortex communicate with neurons of the visual association area which integrates visual input with memories and enables us to identify and interpret what we see in the moment, and plays a role in forming new short or long-term visual memories.
  • A few optic neurons go to the midbrain nuclei to controls accommodation of the lens and the photo pupillary reflex, and controls the muscles that move the eye (ex: tracking something moving in our peripheral vision).

Issues with Vision

  • Astigmatism: Cornea or lens is not spherical, reduces the ability to focus light rays entering from different planes.
  • Hyperopia: Farsightedness, close objects look blurry because the shape of the eye causes light to be focused behind the retina rather than on it.
  • Myopia: Nearsightedness, far away objects look blurry because the shape of the eye causes light to be focused in front of the retina rather than on it.
  • Presbyopia: Reduced ability to flex lens and focus on close objects, common as you age.