Lab Manual 6: activity 2 reading

Activity 2: Investigating Special Senses & Ocular Physiology

• Focus on how special senses work; differ from general senses which use simpler receptor mechanisms.

• Special senses require dedicated organs to house sensory receptors.

• Dissection of a mammalian eye will provide practical understanding of these concepts.

Structures of the Eye

• The eyeball consists of three main layers:

  • Outer Fibrous Layer: Support structure including sclera and cornea.

  • Middle Vascular Layer: Contains iris, ciliary body, and choroid.

  • Inner Nervous Layer: Houses the retina and photoreceptors.

• Layers organized from outer to inner, similar to peeling a fruit.

• Key function: transmission and transduction of light.

Outer Fibrous Layer

• Composed of:

  • Sclera: Tough, whitish membrane providing structure; continuous with dura mater of the brain.

    • Large sclera:cornea ratio increases visual acuity in bright light.

    • Acts as an anchoring site for extra-ocular muscles.

  • Cornea: Transparent structure allowing light entry; five epithelial layers that are renewable for healing.

    • Na+/K+ pumps maintain clarity by removing excess water.

    • Composed of microscopic fibers which maintain transparency; fibers run parallel and are not well hydrated.

    • Breakdown post-mortem leads to cloudy appearance due to water absorption.

Middle Vascular Layer

• Contains iris, ciliary body, and choroid, as well as two fluid-filled compartments.

Iris

• Regulates light entry by controlling the pupil size through:

  • Radial Smooth Muscles: Dilate pupil (sympathetic nervous system).

  • Circular Smooth Muscles: Constrict pupil upon contraction.

Lens

• Located behind the iris; soft and transparent.

• Changes shape to focus on close or distant objects:

  • Accommodation: Adjustments made via ciliary muscle contractions altering lens shape.

  • Relaxation of ciliary muscles flattens lens for distance vision.

  • Contraction of ciliary muscles rounds lens for proximity vision; relies on tension from zonular fibers.

Choroid

• Heavily pigmented, black structure lining the posterior meningeal layer; provides blood supply and minimizes light reflection and distortion.

• Contains dense blood vessels aiding nutrient delivery and waste removal.

• Albinism can impact choroid pigmentation and affect vision.

Inner Nervous Layer

• Composed of the retina:

  • Outer Layer (Retinal Pigmented Epithelium): Absorbs light, supports photoreceptors, and stores vitamin A.

  • Inner Layer: Contains photoreceptors responsible for light transduction:

    • Rods: Rod-shaped, facilitate black and white vision, sensitive to low light.

    • Cones: Thicker, cone-shaped, detect colors and details.

• Photoreceptors send signals to bipolar cells, which relay them to ganglion cells (generate action potentials).

• Optic Nerve: Formed by axons of ganglion cells; exits at the optic disc, which lacks photoreceptors (blind spot).

• Macula: Region with high density of photoreceptors, ensuring exceptional visual acuity.