The Special SensesCHAPTER 15

Unit Objectives

Classify Sense organs as special or general and explain the basic differences between the two groups.

Discuss how a stimulus is converted into a sensation.

Discuss the general sense organs and their functions.

List the major senses.

Describe the structure of the eye and the functions of its components.

Discuss the anatomy of the ear and its sensory function in hearing and equilibrium.

Discuss the chemical receptors and their functions.

Classification of Sense Organs

General Sense Organs: Exist as individual cells or receptor units Widely distributed throughout the body

Special Sense Organs Large & Complex Localized grouping of specialized

receptors Presence or absence of covering

Encapsulated Unencapsulated

General Sense Organs

Free Nerve Endings Location: Skin, Epithelial & mucosa

General Senses: Pain, crude touch, temperature, itch, tickle

General Sense Organs

Encapsulated Nerve Endings

Meissner’s Corpuscles

Location: Skin, fingertips & lips

General Senses: Fine touch and low-frequency vibration

Ruffini’s Corpuscles

Location: Skin & subcutaneous tissue of fingers

Touch & Pressure

Pacinian Corpuscles

Around joints, in mammary glands & external genitals of both sexes

General Senses: pressure & high frequency vibration

General Sense OrgansEncapsulated Nerve EndingsCont

Krause’s end-bulbs Location : Skin dermal; mucosa of lips,

eyelids, & external genitals General Sense: Touch & cold

Golgi Tendon Receptors Location: Near Junction of Tendons & Muscles General Sense: Muscle tension

(proprioception: individual perception) Muscle spindles

Skeletal muscles General Sense: Muscle length (proprioception)

Special Sense Organs

Eye Specific Receptor: Rods & Cones Type of Receptor: Photoreceptor Sense: Vision

Ear Specific Receptor: Organ of Corti &

Cristae ampullares Type of Receptor: Mechanoreceptor Sense: Hearing & Balance

Special Sense Organs

Nose Specific Receptor: Olfactory cells

Type of Receptor: Chemoreceptor

Sense: Smell

Taste Buds Specific Receptor: Gustatory cells

Type of Receptor: Chemoreceptor

Sense: Taste

Type of Stimuli Defined

Photoreceptors (light)

Chemoreceptors (chemicals)

Pain receptors (injury)

Thermoreceptors (temperature change)

Mechanoreceptors (movement or deforming of capsule)

Proprioceptors (position of body parts or changes in muscle length or tension)

Converting Stimulus to Sensation

All sense organs have 3 common functional characteristics: Can detect a particular stimulus

Stimulus is converted into a nerve impulse

Nerve impulses is perceived as a sensation

The Eye

Layers of eyeball

Sclera: tough outer coat; white of eye; cornea is the transparent part of sclera over iris

Choroid: pigmented vascular layer prevents scattering of light; front part of this layer is made of ciliary muscle & iris, the colored part of the eye; the pupil is the hole in the center of the iris; contraction of iris muscle dilates or constricts pupil

Retina: inner most layer of the eye; contains rods (receptors for night vision) & cones (reception for day & color vision)

The Eye

Conjuctiva: mucous membrane covering the front surface of the sclera and lining the eyelid

Lens: transparent body behind the pupil; focuses light rays on the retina

humor

humor

Suspensory ligaments

Optic Disk

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Eye Fluids

Aqueous humor: in the anterior chamber in front of the lens

Vitreous humor: in the posterior chamber behind the lens

Visual pathway

Innermost layer of retina contains rods & cones

Impulse travels from the rods and cones through the layers of the retina

Nerve impulse leaves the eye through the optic nerve; the point of exit is free of receptors and is therefore called a blind spot

Visual interpretation occurs in the visual cortex of the cerebrum

The Ear

Functions in hearing, equilibrium, & balance Receptors for hearing & equilibrium:

mechanoreceptors

Divisions of Ear

External Auricle (pinna)

External auditory canal

Curves approx. 2.5 cm in length

Contains ceruminous glands (produce a waxy substance)

Ends at tympanic membrane

Divisions of Ear

Middle Ear Ear ossicles: malleus, incus, & stapes

Ends in the oval window

Eustacian tube connects the middle ear to the throat

Inflammation called otitis media

Divisions of Ear

Inner Ear Bony labyrinth filled with perilymph

Subdivided into the vestibule, semicircular canals, and cochlea

Membranous labyrinth filled with endolymph

Receptors for balance in the semi-circular canals: cristae ampullairs

Specialized hair cells on the organ of Corti respond when bent by the movement of surrounding endolymph set in motion by sound waves

VestibuleCochlea Pinna External Auditory Meatus

MalleusIncus

Stapes

Tympanic Membrane

Oval Window

Vestibulocochlear nerve

Eustachian Tube

Semi-Circular Canals

Taste Receptors

Chemoreceptors called taste buds

Cranial nerves carry gustatory (respond to chemicals dissolved in saliva) impulses

4 primary taste sensations

Sweet

Sour

Bitter

Salty

Gustatory & Olfactory senses work together

Smell Receptors

Receptors for fibers of olfactory/cranial nerve lie in olfactory mucosa of nasal cavity

Olfactory receptors are extremely sensitive but easily fatigued

Odor-causing chemicals initiate a nervous signal that is interpreted as a specific odor by the brain