Chapter 15: The Special Senses – Structure and Function

The Special Senses

The special senses include smell, taste, hearing, equilibrium, and sight. These senses rely on specialized organs and receptors to detect specific environmental stimuli and transmit information to the brain for interpretation.

Smell (Olfaction)

Olfaction is the sense of smell, allowing detection of airborne chemicals.

• Proper Name: Olfaction

• Type of Receptors: Chemoreceptors (specifically, olfactory receptor cells)

• How is Odor Detected? Odorant molecules dissolve in the mucus of the nasal cavity and bind to receptors on the cilia of olfactory receptor cells, initiating a signal transduction pathway that generates an action potential.

• Structure of Olfactory Epithelium: Located in the roof of the nasal cavity, it contains olfactory receptor cells, supporting cells, and basal cells (stem cells for regeneration).

• Cranial Nerve: Olfactory nerve (Cranial Nerve I) transmits signals to the olfactory bulb and then to the brain.

Taste (Gustation)

Taste is the sense that detects dissolved chemicals in the oral cavity.

• Structure and Function of a Taste Bud: Taste buds are clusters of gustatory receptor cells located mainly on the tongue's papillae. Each taste bud contains supporting cells, gustatory cells (with microvilli), and basal cells.

• 5 Basic Taste Sensations:

  • Sweet

  • Sour

  • Salty

  • Bitter

  • Umami (savory)

Hearing (Auditory Sense)

Hearing involves the detection of sound waves and their conversion into electrical signals by the ear.

• Structure of the Ear:

  • External Ear: Auricle (pinna), external auditory canal, tympanic membrane (eardrum)

  • Middle Ear: Ossicles (malleus, incus, stapes), auditory (Eustachian) tube

  • Inner Ear: Cochlea, vestibule, semicircular canals

• What is Sound? Properties of Sound Waves: Sound is a mechanical vibration transmitted through a medium. Key properties include frequency (pitch), amplitude (loudness), and wavelength.

• Transmission of Sound to the Cochlea:

  1. Sound waves enter the external auditory canal.

  2. Vibrate the tympanic membrane.

  3. Ossicles amplify and transmit vibrations to the oval window.

  4. Vibrations create pressure waves in the perilymph of the cochlea.

  5. Basilar membrane vibrates, stimulating hair cells in the organ of Corti.

• Structure of the Cochlea: A spiral-shaped organ divided into three chambers: scala vestibuli, scala media (cochlear duct), and scala tympani. The organ of Corti (sensory epithelium) sits on the basilar membrane.

• Detection of Sound and Receptor Potential: Movement of the basilar membrane bends hair cells, opening ion channels and generating a receptor (local) potential, which leads to action potentials in the cochlear nerve.

• Disorders:

  • Conduction Deafness: Impaired transmission of sound to the inner ear (e.g., due to earwax, otosclerosis).

  • Sensorineural Deafness: Damage to hair cells or auditory nerve (e.g., due to loud noise, aging).

Dynamic and Static Equilibrium

The vestibular system detects head position and movement, maintaining balance.

• Structure of the Vestibule and Semicircular Ducts: The vestibule contains the utricle and saccule; semicircular ducts are oriented in three planes.

• Position Detection by Utricle and Saccule: Hair cells embedded in a gelatinous otolithic membrane detect linear acceleration and gravity. Otoliths (calcium carbonate crystals) add mass and inertia.

• Semicircular Ducts and Rotational Movement: Each duct contains an ampulla with a crista ampullaris (hair cells in a cupula). Rotational movement of the head displaces endolymph, bending hair cells.

• Disorders: Motion Sickness results from conflicting sensory input regarding movement.

Sight (Vision)

Vision is the sense that detects light and interprets images.

• Structure of the Eye – 3 Layers:

  • Fibrous Layer: Sclera (white of the eye) and cornea (transparent front part)

  • Vascular Layer: Choroid, ciliary body, and iris

  • Neural Layer: Retina

• Structure of the Retina: Contains photoreceptor cells (rods and cones), bipolar cells, and ganglion cells. The macula lutea and fovea centralis are regions of high visual acuity.

• Cells in Visual Pathway: Photoreceptors → bipolar cells → ganglion cells (axons form the optic nerve)

• Photoreceptive Cells: Rods (dim light, black and white vision) and cones (color vision, sharpness)

• Fluids in the Eye: Aqueous humor (anterior segment), vitreous humor (posterior segment). Glaucoma is increased intraocular pressure due to impaired drainage of aqueous humor.

• Characteristics of Light: Light is electromagnetic radiation. Key properties include wavelength (color), intensity (brightness), and refraction (bending of light).

• Refractive Errors:

  • Emmetropic: Normal vision; image focused on the retina.

  • Presbyopia: Age-related loss of lens elasticity; difficulty focusing on close objects.

  • Hyperopia: Farsightedness; image focused behind the retina.

  • Myopia: Nearsightedness; image focused in front of the retina.

• Stereoscopic Vision: Overlapping visual fields from both eyes allow depth perception.

Example Table: Comparison of Photoreceptors

Feature	Rods	Cones
Number	~120 million	~6 million
Location	Peripheral retina	Central retina (fovea)
Function	Dim light, black & white vision	Bright light, color vision
Visual Acuity	Low	High

Additional info: Academic context and definitions have been added to expand on the original study guide points for clarity and completeness.