Encyclopedic Notes on Physiology of Vision and Ocular Mechanics

Tears Formation, Composition, and Circulation

• Composition of Tears

  • Tears are primarily composed of water.
  • They contain specific solutes, including various salts, mucus, and lysozyme.
  • Lysozyme is an enzyme specialized in killing certain types of bacteria to protect the ocular surface.

• Fluid Dynamics and Circulation

  • The majority of the fluid produced by the lacrimal glands evaporates directly from the surface of the eye.
  • Excess tear fluid that does not evaporate is collected in the medial corner of the eye.
  • The collection structures are known as the lacrimal canaliculi.

• Secretory Regulation

  • The secretion of aqueous fluid by the lacrimal gland is a physiological response to autonomic stimulation.
  • Both the parasympathetic and sympathetic nervous systems play a role in this stimulation.

Functional Anatomy and Autonomic Innervation of the Eye

• Internal Eye Structures (Sagittal Section)

  • Key components include the Ciliary body, the lens, the iris, and the cornea.
  • The pupil is controlled by two specific muscles:
    • Dilator (Radial) papillae muscles.
    • Sphincter (Circular) papillae muscles.

• Parasympathetic Innervation

  • Origin/Pathway: Originates in the Edinger-Westphal nucleus, travels via the 3rd Cranial Nerve (Oculomotor) to the ciliary ganglion, and enters the eyeball via the ciliary nerves.
  • Target Actions:
    • Ciliary muscle: Excitation of this muscle controls the focusing mechanism of the eye lens.
    • Sphincter of the iris: Excitation causes the pupil to constrict (miosis).

• Sympathetic Innervation

  • Origin/Pathway: Originates at the $1^{ST}$ thoracic segment of the spinal cord and passes through the superior cervical ganglion.
  • Target Actions:
    • Radial fibers of the iris: Excitation causes the pupil to dilate.
    • Extraocular muscles: Innervates several extraocular muscles.
  • Clinical Correlation: Interference with this pathway can result in Horner’s syndrome.

The Eye as a Sensory Organ

• Sensory Components

  • Stimulus: Light photons.
  • Receptors: Rods and cones located in the retina.
  • Pathway: Visual signals travel through the retina, the optic nerve, and the optic tracts.
  • Center: The primary processing center is the visual cortex located in the occipital lobe.

• Primary Functions of the Eye

  • Refraction/Optics: Focusing light onto the receptive surface.
  • Photoreception: The eye receives light and generates action potentials.
  • Signal Processing: Action potentials are transferred to the visual cortex for interpretation, resulting in the perception of vision.
  • Vision Categories: Includes Far and near vision (accommodation) and Color vision.

Optics and Refractive Power of the Eye

• Camera Analogy

  • The eye is optically equivalent to a photographic camera.
  • Lens system: Equivalent to the camera lens.
  • Pupil: Serves as a variable aperture system.
  • Retina: Corresponds to the photographic film.

• Refractive Interfaces

  • The eye's lens system consists of four distinct refractive interfaces:
    1. The interface between air and the anterior surface of the cornea.
    2. The interface between the posterior surface of the cornea and the aqueous humor.
    3. The interface between the aqueous humor and the anterior surface of the lens.
    4. The interface between the posterior surface of the lens and the vitreous humor.

• Refractive Indices ($n$)

  • Air: $1$
  • Cornea: $1.38$
  • Aqueous humor: $1.33$
  • Crystalline lens: $1.40$
  • Vitreous humor: $1.34$

• Measurement of Refractive Power

  • Refractive power is measured in "Diopters."
  • The refractive power of a convex lens is calculated as the reciprocal of its focal length in meters.
  • Formula: $\text{Lens power (diopter)} = \frac{1}{\text{focal length}}$
  • Total refractive power of the eye: Approximately $60$ diopters.
  • Standard lens refractive power: Approximately $20$ diopters.

• Image Formation

  • The lens system focuses an image directly on the retina.
  • The image on the retina is both inverted and reversed with respect to the object.
  • The brain perceives objects in an upright position because it is trained to interpret an inverted retinal image as normal.

Mechanism of Accommodation and the Near Reflex

• Process of Accommodation

  • Accommodation is the mechanism used to focus the lens system for high visual acuity.
  • It results from the contraction or relaxation of the ciliary muscle.
  • Contraction: Increases the refractive power of the lens. Contraction of the ciliary muscle can add up to $34$ diopters to the lens's refractive power.

• Factors Affecting Accommodation

  • The state of the ciliary muscle.
  • The state of the zonules (suspensory ligaments).
  • The elasticity of the lens.

• The Near Vision Reflex (Triad)

  • Engaging in near vision elicits three simultaneous responses:
    1. Convergence (eyes turn inward).
    2. Miosis (pupillary constriction).
    3. Lens accommodation (increased lens curvature).

• Neurological Control and Pathway

  • Visual signals for accommodation are analyzed in specific brain regions:
    • Brodmann’s cortical areas $18$ and $19$.
    • Pretectal area in the brain stem.
    • Edinger-Westphal nucleus.
    • Parasympathetic nerve fibers.
    • Ciliary muscle (execution of miosis and lens change).

Photoreceptors: Rods vs. Cones

Phototransduction in Rods

• Current Flow in Rods

  • Involves cGMP-gated channels.
  • Characterized by potassium ($K^+$) selective channels and sodium ($Na^+$) channels using ATP for maintenance.

• Steps in Photoreception (Biochemical Cascade)

  1. Light Activation: Light hits $11\text{-cis retinal}$, converting it to $\text{all-trans retinal}$, leading to the formation of Metarhodopsin II.
  2. Second Messenger Activation: Metarhodopsin II activates Transducin (a G-protein), which in turn activates phosphodiesterase.
  3. cGMP Reduction: Phosphodiesterase converts cyclic GMP (cGMP) into $5' \text{ GMP}$. Decreased levels of cGMP lead to the closure of $Na^+$ channels.
  4. Membrane Potential: The closure of sodium channels causes hyperpolarization of the photoreceptor membrane.
  5. Glutamate Release: Hyperpolarization leads to a decreased release of the neurotransmitter glutamate.

• Post-Synaptic Response

  • Ionotropic Receptor Response: Decreased glutamate results in a decreased excitatory response, leading to hyperpolarization (inhibition) of bipolar and horizontal cells.
  • Metabotropic Receptor Response: Decreased glutamate results in a decreased inhibitory response, leading to depolarization (excitation) of bipolar and horizontal cells.

Light and Dark Adaptation

• Light Adaptation (In Bright Light)

  • Large portions of photochemicals in both rods and cones are reduced to retinal and opsins.
  • Retinal is converted into Vitamin A.
  • Result: Concentrations of photosensitive chemicals are reduced, and the eye's sensitivity to light correspondingly decreases.

• Dark Adaptation (In Darkness)

  • Retinal and opsins are converted back into light-sensitive pigments.
  • Vitamin A is converted back into retinal to replenish pigments.
  • The final limit of adaptation is determined by the amount of opsin available to combine with retinal.
  • Note: A deficiency in Vitamin A can lead to Nyctalopia (night blindness).

Color Vision and Visual Pathway

• Color Interpretation

  • Color vision is determined by the degree of stimulation of different color-sensitive cones.
  • Cones respond to monochromatic lights of four primary colors: blue, green, yellow, and orange.

• The Visual Pathway Structure

  • Layered structure of the retina.
  • The Optic Pathway: Progression from the retina through the optic nerve and tracts.
  • Visual Cortex: Includes Primary versus secondary visual cortex.
  • Macular Representation: Special coverage in the cortex for the macula and fovea (central vision).
  • Clinical Considerations: Potential for interruption of the optic/visual pathway at various points.