Special Senses: The Eye and Vision – Anatomy and Physiology II Study Notes

Course Introduction and Structure

Course Overview

This course, BIO 112: Anatomy and Physiology 2, covers the structure and function of the human body, focusing on cells, tissues, organs, and organ systems. The course uses the Marieb Human Anatomy and Physiology textbook and laboratory manual as primary resources.

• Learning Outcomes:

  • Identify anatomical structures associated with cells, tissues, organs, and organ systems.

  • Describe mechanisms underlying normal function of these biological levels.

• Assessment: Includes quizzes, midterms, a concept map assignment, a cumulative final exam, and laboratory work.

Chapter 15: Special Senses

Overview of Special Senses

The special senses are distinct from general senses (touch, pain, temperature) and are mediated by specialized receptor cells localized in the head region. These include:

• Vision

• Taste

• Smell

• Hearing

• Equilibrium

All special senses use special sensory receptors that are distinct from the modified nerve endings of general receptors.

The Eye and Vision

General Features

• 70% of the body's sensory receptors are located in the eye.

• Half of the cerebral cortex is involved in visual processing.

• The eye consists of accessory structures and the eyeball itself.

Accessory Structures of the Eye

• Eyebrows: Protect the eyes from sweat and sunlight.

• Eyelashes: Trigger reflexive blinking.

• Eyelids (palpebrae): Protect and lubricate the eye.

• Conjunctiva: Transparent mucous membrane lining the eyelids and covering the sclera.

• Lacrimal apparatus: Produces and drains tears.

• Extrinsic eye muscles: Control eye movement.

Glands of the Eye

• Sebaceous glands: Lubricate the eyelid margins.

• Tarsal glands: Secrete oil to prevent eyelids from sticking together.

• Lacrimal glands: Produce tears for lubrication and protection.

Extrinsic Eye Muscles

• Rectus muscles: Move the eye up, down, and side-to-side.

• Oblique muscles: Allow for rotational movement of the eye.

Muscle	Action	Controlling Cranial Nerve
Lateral rectus	Moves eye laterally	VI (Abducens)
Medial rectus	Moves eye medially	III (Oculomotor)
Superior rectus	Elevates eye	III (Oculomotor)
Inferior rectus	Depresses eye	III (Oculomotor)
Superior oblique	Depresses eye and turns it laterally	IV (Trochlear)
Inferior oblique	Elevates eye and turns it laterally	III (Oculomotor)

Structure of the Eyeball

Layers of the Eyeball

• Fibrous layer: Outermost, dense avascular connective tissue.

  • Sclera: Protects and shapes the eyeball; anchors extrinsic muscles; continuous with dura mater posteriorly.

  • Cornea: Transparent, allows light entry and bends light; contains pain receptors for blinking and tearing reflexes.

• Vascular layer: Middle layer, provides blood supply.

  • Choroid: Supplies blood to all layers.

  • Ciliary body: Contains ciliary muscles that control lens shape; ciliary zonule (suspensory ligament) holds lens in place.

  • Iris: Colored part, controls pupil size and thus light entry.

• Inner layer (retina): Originates as an outpocketing of the brain; contains photoreceptors, neurons, and glial cells.

  • Pigmented layer: Absorbs light, prevents scattering, stores vitamin A.

  • Neural layer: Contains photoreceptors (rods and cones), bipolar cells, and ganglion cells.

Internal Chambers and Fluids

• Lens and ciliary zonule separate the eye into two segments:

• Posterior segment: Contains vitreous humor (transmits light, supports lens, holds retina in place).

• Anterior segment: Divided into anterior and posterior chambers by the iris; contains aqueous humor (formed by ciliary processes, drained continuously).

Clinical Note: Glaucoma

• Glaucoma: Blocked drainage of aqueous humor increases intraocular pressure, potentially damaging the optic nerve.

Lens Structure and Function

• Biconvex, transparent, flexible, avascular

• Changes shape to focus light on the retina

• Composed of:

  • Lens epithelium: Anterior cuboidal cells

  • Lens fibers: Filled with protein crystallin

• With age, lens becomes denser and less elastic (presbyopia)

Focusing Light on the Retina

• Light passes through cornea, aqueous humor, lens, vitreous humor, retina, and finally photoreceptors.

• Lens curvature adjusts for fine focusing:

  • Distant vision: Lens flattens

  • Close vision: Lens bulges

Refractive Errors and Age-Related Changes

• Myopia (nearsightedness): Eyeball too long; focal point in front of retina.

• Hyperopia (farsightedness): Eyeball too short; focal point behind retina.

• Astigmatism: Unequal curvature of cornea or lens.

• Presbyopia: Age-related loss of lens accommodation.

Retina: Structure and Function

Pigmented Layer

• Single-cell thick, next to choroid

• Absorbs light, prevents scattering, stores vitamin A

• Phagocytizes photoreceptor fragments

Neural Layer

• Contains photoreceptors (rods and cones), bipolar cells, ganglion cells

• Ganglion cell axons form the optic nerve

• Optic disc: Site where optic nerve exits; no photoreceptors (blind spot)

Photoreceptors: Rods and Cones

• Rods: Sensitive to dim light, peripheral vision, no color vision, low acuity

• Cones: Require bright light, color vision (red, green, blue), high acuity, concentrated in fovea centralis

Feature	Rods	Cones
Color vision	None (one pigment)	Yes (three pigments)
Sensitivity	High (dim light)	Low (bright light)
Acuity	Low (converging pathways)	High (one cone per ganglion cell in fovea)
Distribution	Peripheral retina	Central retina (fovea)

Information Processing in the Retina

• Photoreceptors and bipolar cells generate graded potentials only.

• Light hyperpolarizes photoreceptors, stopping glutamate release.

• Bipolar cells depolarize, release neurotransmitter to ganglion cells.

• If threshold is reached, ganglion cells fire action potentials along the optic nerve.

Visual Pathway to the Brain

• Axons of ganglion cells form the optic nerve.

• Medial fibers cross at the optic chiasma; continue as optic tracts.

• Most fibers synapse in the thalamus; thalamic neurons project to the primary visual cortex in the occipital lobe.

• Some ganglion cells project to the suprachiasmatic nucleus (circadian rhythms) and pretectal nucleus (pupillary reflexes).

Depth Perception

• Both eyes view the same image from slightly different angles.

• The visual cortex fuses these images, resulting in three-dimensional vision and depth perception.

• Requires input from both eyes (binocular vision).

Clinical Terms

• Age-related macular degeneration (ARMD): Degeneration of the macula lutea, leading to loss of central vision.

• Exophthalmos: Abnormal protrusion of the eyeball.

• Scotoma: Area of partial alteration in the field of vision (blind spot).

• Trachoma: Infectious disease causing roughening of the inner surface of the eyelids.

Additional info: For further study, students are encouraged to use the Mastering A&P online resources, including animations and videos related to Chapter 15 and the special senses.