Special Senses

Special Senses

Introduction

• The four traditional special senses are taste, smell, hearing, and vision

• Equilibrium is the body's ability to maintain balance

• Special sense receptors are found in specialized organs (eyes, ears, tongue, nasal passages), while general receptors are distributed throughout the body

The Eye and Vision

External and Accessory Structures

• Lacrimal Apparatus: Consists of lacrimal gland and tear ducts; tears clean and moisten the eye's surface, deliver oxygen and nutrients to the conjunctiva

• Eyelids and Eyelashes: Protect the eye from foreign bodies and block light

• Conjunctiva: Membrane that lines the inner surface of the eyelid and anterior surface of the eye

• Extrinsic Muscles: Controlled by cranial nerves including trochlear nerve (IV), abducent nerve (VI), and oculomotor nerve (III)

Internal Structures

• Sclera and Cornea: Cornea is a transparent extension of the sclera that allows light to enter the eye

• Choroid: Vascular layer of tissue that supplies oxygen and nutrients to the retina and sclera

• Retina: Thin layer of light-sensitive cells

• Rods: Located in the periphery of retina, active in dim light, responsible for night vision, cannot detect color

• Cones: Concentrated in the center of retina, responsible for color vision

• Lens: Regulates the amount of light entering the eye and focuses it on the retina

• Pupil: Opening that controls the amount of light entering the eye

• Optic Nerve: Cranial nerve II, transmits signals to the brain

• Optic Disc: Site where nerve fibers leave the retina

• Blind Spot: Area where the optic nerve leaves the eye and there are no photoreceptors

• Aqueous Humor: Fluid that fills the space between the cornea and lens

• Vitreous Humor: Gel-like substance that fills the space behind the lens

Vision Processes

• Refraction: Bending of light rays as they pass through the curved edge of cornea

• Accommodation: Ability of the lens to change shape, allowing it to focus on objects at varying distances

• Visual Field: Entire area a person can see when focusing on a central point

• Optic Chiasma: Where visual information from each eye partially crosses over

• Visual Reflexes:

• Convergence: Lining up the visual axis of each eye towards an object

• Photopupillary Reflex: When pupils constrict when exposed to bright light

• Accommodation Pupillary Reflex: Involves convergence, pupil size changes, and eye lens shape changes when focusing on near or distant objects

Visual Problems

• Myopia (Nearsightedness): Light rays focus in front of the retina instead of directly on it, so distant objects appear blurry

• Hyperopia (Farsightedness): Light rays focus behind the retina, objects up close appear blurry

• Astigmatism: Irregularity in the curvature of the cornea, causing light to be focused unevenly

• Presbyopia: Age-related condition affecting the eye's ability to focus on close objects

The Ear: Hearing and Balance

Anatomy of the Ear

• Outer (External) Ear:

• Pinna (Auricle): Funnels sound into the auditory canal

• External Auditory Canal: Channel from outside to eardrum

• Cerumen (Ear Wax): Waterproofs canal and protects it

• Tympanic Membrane (Eardrum): Vibrates in response to sound waves

• Oval Window: Beginning of inner ear, holds the stapes

• Round Window: Opening in middle ear that allows pressure to be released

• Middle Ear:

• Ossicles: Three small bones - malleus, incus, stapes

• Auditory Tube (Eustachian Tube or Pharyngotympanic Tube): Connects middle ear to nasopharynx

• Inner (Internal) Ear:

• Bony Labyrinth: Fluid-filled passages inside temporal bone

• Cochlea: Snail-like structure containing structures for hearing

• Vestibule: Marks entrance of labyrinth, contains organs for sense of balance

• Semicircular Canals: Contain structures for balance

Mechanisms of Equilibrium

• Static Equilibrium: Maintenance of equilibrium when stationary

• Dynamic Equilibrium: Body's ability to maintain balance while moving or rotating, includes sense of acceleration and positioning of head

• Vestibular Apparatus: Organs in ear that help maintain balance and sense head position and movement

• Vestibulo-cochlear Nerve: Cranial nerve VIII, responsible for equilibrium

Mechanisms of Hearing

• Organ of Corti: Hearing sense organ, consists of epithelium, hair cells, tectorial membrane, and nerve fibers

• Hair Cells: Receptors for hearing

• Basilar Membrane: Floor of the cochlear duct

• Cochlear Nerve: Part of cranial nerve VIII

Chemical Senses: Smell and Taste

Olfaction (Smell)

• Receptors: Chemoreceptors that respond to chemicals

• Olfactory Receptor Cells: Neurons with long cilia known as olfactory hairs

• Process: Chemicals must be dissolved in mucus for detection by chemoreceptors

• Olfactory Nerve: Cranial nerve I, responsible for sense of smell

Gustation (Taste)

• Receptors: Gustatory cells with long microvilli that protrude through taste pores

• Process: Stimulated by chemicals dissolved in saliva

• Basic Taste Sensations:

• Sweet

• Sour

• Bitter

• Salty

• Umami

General Sensory Receptors

• Nociceptors: Pain receptors

• Chemoreceptors: Detect chemicals for smell and taste

• Mechanoreceptors: Detect pressure, touch, and vibration