eyes

Introduction

  • Greetings and attendance recording.

  • Plans to cover significant content on the day, particularly glaucoma and additional topics post-lecture if time permits.

Glaucoma Overview

  • Definition: A condition where aqueous humor fails to drain properly, leading to increased intraocular pressure (IOP) and potential vision loss.

  • Aqueous Humor Production: Generated by ciliary bodies, filling the posterior and anterior chambers of the eye.

  • Drainage Pathway: Aqueous humor exits through Schlemm's canal, at the angle formed by the iris and iris muscle.

Types of Glaucoma

  1. Wide Angle Glaucoma:

    • Angle between the iris and anterior chamber is obtuse, leading to less efficient drainage.

    • Intraocular pressure increases due to slow drainage.

  2. Closed Angle Glaucoma:

    • Medically classified as an emergency.

    • The angle is completely closed off, blocking drainage and causing a rapid increase in IOP, jeopardizing blood supply and risking permanent blindness.

Effects of Increased IOP

  • Pressure on the retina and optic nerve because the eye is a closed system.

  • The resultant damage can lead to irreversible vision loss.

Treatment of Wide Angle Glaucoma

  • Medications like beta-blockers and cholinergic agonists, which reduce the secretion of aqueous humor.

Eye Functionality

Pupil and Iris Mechanics

  • Muscle Types in Iris:

    • Radial Muscles (controlled by norepinephrine from the superior cervical ganglion).

    • Circular Muscles (respond to acetylcholine from cranial nerve III, the oculomotor nerve).

Pupil Dilation and Constriction

  • Dilation (Mydriasis): Occurs in dim light when norepinephrine activates alpha-1 receptors in radial muscles, leading to contraction and outward pulling of circular muscles.

  • Constriction (Miosis): Occurs in bright light with acetylcholine stimulating M3 receptors in circular muscles for contraction, reducing the pupil size.

Pharmacologic Implications

  • Mydriasis Medications: Use of M3 antagonists (e.g., tropicamide) to prevent constriction caused by acetylcholine, leading to dilation regardless of light.

  • Effects of Drugs: Alpha-1 agonists (e.g., cocaine) can cause mydriasis by continuously activating radial muscles while preventing circular muscle contraction.

Lens and Refraction

Lens Mechanics

  • Accommodation: Changes in lens shape for near and far vision.

    • Ciliary muscle contraction leads to a thicker lens for close vision; relaxation results in a thinner lens for distance.

Common Refractive Errors

  1. Myopia (Nearsightedness): Light focuses in front of the retina; corrected with concave lenses.

  2. Hyperopia (Farsightedness): Light focuses behind the retina; corrected with convex lenses.

Pathology

  • Cataracts: A progressive opacity of the lens due to debris buildup, often linked to age, diabetes, UV exposure, and smoking. Current treatments involve laser surgery.

Anatomy of the Retina

  • The retina serves as an extension of the optic nerve, with layers including photoreceptors (rods and cones), ganglion cells, and bipolar cells.

  • Photoreceptors:

    • Rods: Sensitive to low light and used for night vision; provide low-resolution vision (many rods converge onto a single ganglion cell).

    • Cones: Function in bright light for color vision, offering high-resolution (1:1 ratio with ganglion cells).

Phototransduction Process

  1. In Dark:

    • Rods with rhodopsin remain depolarized due to open sodium channels, releasing GABA.

  2. In Light:

    • Light converts 11-cis-retinal to 11-trans-retinal, activating transducin which inhibits phosphodiesterase, closing sodium channels, hyperpolarizing the rod, stopping GABA release.

Fovea Centralis

  • Contains only cones; responsible for sharp, high-resolution vision when detail is focused upon.

Color Vision

  • Trichromatic Vision: Based on three types of cones (Blue-S, Green-M, Red-L) with genetics determining expression and leading to color blindness in males more often than females.

Anatomy of the Ear

External and Middle Ear

  • Structures: Pinna (external ear), tympanic membrane, ossicles (malleus, incus, stapes).

  • Function: Converts sound waves into mechanical vibrations transmitted to the cochlea.

Cochlea and Sound Transduction

  • Structure: Spiral shape with three ducts (scala vestibula, cochlear duct, scala tympani).

  • Endolymph in Cochlear Duct: High in potassium; essential for hair cells in the organ of Corti to initiate auditory nerve signals by opening potassium channels when hair cells bend from sound vibrations.

Sound Processing

  • The organ of Corti hair cells are crucial for sound transduction. Vibration of surrounding fluid moves hair cells leading to depolarization and action potentials sent to the auditory cortex via the vestibulocochlear nerve.

Vestibular System and Balance

  • Utricle and Saccule: Detect static equilibrium and linear motion using calcium carbonate otoliths embedded in the otolithic membrane which bend hair cells in response to gravitational pull.

  • Semicircular Canals: Detect rotational movements, each canal aligned in different planes, activating hair cells to maintain balance and orientation, sending signals to the vestibulocochlear nerve.

Dizziness and Motion Sickness

  • Meniere's Disease: Excessive endolymph causing vertigo and tinnitus; drugs can disrupt potassium levels causing ototoxicity and balance disturbances.

Spinal Cord Tracts

Ascending Tracts

  1. Spinothalamic Tract:

    • Lateral for pain/temperature, ventral for crude touch/pressure.

    • Carries sensory information to the thalamus, then primary sensory cortex.

    • Both decussate in the spinal cord.

  2. Spinocerebellar Tract:

    • Coordinates muscle contraction to cerebellum; does not decussate for posterior path but does for anterior path.

  3. Dorsal Column-Medial Lemniscal Pathways:

    • Senses fine touch/pressure; decussates in the medulla before reaching the cortex.

Descending Tracts

  • Function: Transmit motor commands from the brain to muscles.

  1. Corticospinal Tract: Controls fine, skilled movements. Decussates in the medulla (lateral) and spinal cord (anterior).

  2. Extrapyramidal Tracts: Include rubrospinal (flexors), tectospinal (head movement response), vestibulospinal (balance), and reticulospinal (postural tone).

Conclusion

  • Key insights about eye and ear anatomy/function, glaucoma management, and auditory signaling are highlighted as well as pharmaceutical implications related to the discussed systems and conditions. For optimal understanding, students encouraged to clarify remaining questions and engage with study materials in further detail.