The Special
Senses
CHAPTER 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 Organs
Encapsulated Nerve Endings
Cont



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
Suspensory
2
ligaments
1
9
3
10
4
8
humor
11
5
6
humor
7
Optic
12 Disk
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
Semi-Circular Canals
Incus
Oval Window
Eustachian
Tube
Vestibulocochlear
nerve
Malleu
Vestibule TympanicsMembrane
Cochlea
Pinna
External Auditory Meatus
Stapes
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