Special Senses: Structure and Function in Vision, Hearing, and Equilibrium

Special Senses Lab

Objectives

• Locate and identify accessory structures associated with vision on anatomical models.

• Identify major structures of the human eye and ear on models.

• Identify major structures of the sheep eye and cow ear on models.

• Identify major structures of the cochlea on models.

Introduction to Special Senses

The special senses include vision, hearing, taste, smell, and touch, as well as equilibrium (balance). These senses are classified as either general senses or special senses. Special senses involve complex sensory organs and specialized receptor cells.

• Vision: Involves the detection of light and color by photoreceptors in the retina.

• Hearing: Involves the detection of sound waves by mechanoreceptors in the ear.

• Equilibrium: Involves the detection of head position and movement by receptors in the inner ear.

• Taste and Smell: Involve chemoreceptors that respond to chemical stimuli.

Eye Anatomy

Accessory Structures and Layers

The eye consists of several layers and accessory structures that protect and support its function.

• Conjunctiva: A transparent mucous membrane lining the eyelid and covering the sclera. Produces lubricating mucus.

• Lacrimal gland: Produces tears to wash the surface of the eye.

• Extrinsic eye muscles: Control eye movement; include superior/inferior oblique and rectus muscles.

Layers of the Eye

• Fibrous tunic: Outermost layer, includes the sclera (white of the eye) and cornea (transparent anterior portion).

• Vascular tunic (uvea): Middle layer, includes the choroid (pigmented, vascular), ciliary body (controls lens shape), iris (colored part, controls pupil size), and pupil (opening for light entry).

• Retina: Innermost layer, contains photoreceptors (rods and cones) and processes visual information.

Eye Cavities and Fluids

• Anterior cavity: Filled with aqueous humor, maintains intraocular pressure.

• Posterior cavity: Filled with vitreous humor, supports the retina.

Major Structures and Functions

• Optic nerve (cranial nerve II): Transmits visual signals from the retina to the brain.

• Fovea centralis: Area of highest visual acuity in the retina.

• Lens: Focuses light onto the retina; changes shape for near/far vision.

Sheep Eye Comparison

• Sclera: Tough, white outer coat.

• Cornea: Transparent, covers anterior eye.

• Iris: Suspended between cornea and lens; color varies by species.

• Pupil: Opening in the iris; shape varies (round in humans, oval in sheep).

• Lens: Flexible, focuses light.

• Vitreous humor: Fills cavity behind lens.

• Retina: Contains photoreceptors; connects to optic nerve.

Ear Anatomy

Outer Ear

• Pinna (auricle): Visible part of the ear; collects sound waves.

• External auditory canal (meatus): Tube through which sound travels to the eardrum.

• Tympanic membrane (eardrum): Vibrates in response to sound waves; separates outer and middle ear.

Middle Ear

• Ossicles: Three small bones (malleus, incus, stapes) transmit vibrations from the eardrum to the inner ear.

• Malleus (hammer): Attaches to the eardrum.

• Incus (anvil): Passes vibrations from malleus to stapes.

• Stapes (stirrup): Passes vibrations to the oval window of the cochlea.

Auditory Tube

• Auditory (Eustachian) tube: Connects middle ear to the pharynx; equalizes pressure.

Inner Ear

Bony Labyrinth

• Vestibule: Involved in detecting acceleration and deceleration of the head.

• Cochlea: Spiral-shaped, fluid-filled organ responsible for hearing.

• Semicircular canals: Detect rotational movements of the head.

Membranous Labyrinth

• Semicircular ducts: Located within semicircular canals; detect rotational movement.

• Cochlear duct (scala media): Contains receptors for hearing (organ of Corti).

• Organ of Corti: Contains hair cells that detect sound vibrations.

Key Structures and Functions Table

Structure	Location	Function
Conjunctiva	Eye surface	Lubricates and protects eye
Lacrimal gland	Above eye	Produces tears
Sclera	Outer eye	Protects and shapes eye
Cornea	Front of eye	Refracts light
Iris	Middle eye	Controls pupil size
Lens	Behind iris	Focuses light
Retina	Inner eye	Detects light, sends signals
Pinna	Outer ear	Collects sound
Tympanic membrane	Between outer/middle ear	Transmits sound vibrations
Ossicles	Middle ear	Amplify sound
Cochlea	Inner ear	Detects sound
Semicircular canals	Inner ear	Detects head movement

Equations and Scientific Principles

• Refraction of Light: The bending of light as it passes through different media (e.g., cornea, lens) is governed by Snell's Law: $n_1 \sin \theta_1 = n_2 \sin \theta_2$

• Sound Transmission: The speed of sound in air is given by: $v = \sqrt{\gamma R T / M}$ where $v$ is speed, $\gamma$ is adiabatic index, $R$ is gas constant, $T$ is temperature, and $M$ is molar mass.

Examples and Applications

• Clinical Application: Damage to the cochlea can result in sensorineural hearing loss; corrective devices include cochlear implants.

• Vision Correction: Glasses and contact lenses correct refractive errors by adjusting the path of light entering the eye.

Additional info: The notes above expand on the brief points in the original handout, providing definitions, functions, and examples for each structure. The table summarizes the main structures for quick review.