Objectives
Classify sense organs as special or general and
explain the basic differences between the two
groups.
Describe how a stimulus is converted into a
sensation.
Discuss the general sense organs and their
functions.
Classification of Sense Organs
1. General sense organs
A. Individual cells or receptor units
B. Widely distributed throughout the body
2. Special sense organs
A. Large and complex
B. Localized grouping of specialized
receptors
3. Classification by presence or absence of
covering capsule
A. Encapsulated
B. Unencapsulated (“free” or “naked”)
4. Classification by type of stimuli required to
activate receptors
A. Photoreceptors (light)
B. Chemoreceptors (chemicals)
C. Pain receptors (injury)
D. Thermoreceptors (temperature change)
E. Mechanoreceptors (movement or
deforming of capsule)
F. Proprioreceptors (portion of body parts
or changes in muscle length or tension)
Converting a Stimulus Into a
Sensation
All sense organs have common functional
characteristics
1. All are able to detect a particular stimuli
2. A stimulus is converted into a nerve impulse
3. A nerve impulse is perceived as a sensation in
the CNS
General Sense Organs
1. Distribution is widespread
2. Single-cell receptors are common
3. Examples:
A. Free nerve endings – pain and crude
touch; skin and epithelial layers
B. Meissner’s corpuscles – fine touch and
vibration; skin, numerous in fingertips and lips
C. Ruffini’s corpuscles – touch and pressure;
skin and subcutaneous tissue of fingers
D. Pacinian corpuscles – pressure and
vibration; around joints, in mammary glands
and external genitalia of both sexes
E. Krause’s end bulbs – touch; skin, mucosa of lips and
eyelids, external genitals
F. Golgi tendon receptors – proprioreception
(sense of muscle tension); near junction of
muscles and tendons
G. Muscle spindles - proprioreception (sense of
muscle length); skeletal muscles
General Sense Receptors
Objectives
1. List the major senses.
2. List the special sense organs and its
receptor.
3. Describe the structure of the eye and the
functions of its components.
4. Describe the structure of the ear and the
functions of its components.
Major Senses
Sight
Hearing
Taste
Smell
Touch
Special Sense Organs
Eye
Rods and Cones
Photoreceptor
Vision
The Eye
Three primary layers of the eyeball
1. Sclera – tough, outermost, white layer that
surrounds and protects the eye
Its front surface (cornea) is transparent to let
light enter the eye.
2. Choroid – middle layer; contains iris (colored
portion of the eye)
The hole in the center of the iris is the pupil.
Light enters the pupil and the size of the pupil is
regulated by the iris.
The lens lies directly behind the pupil and is held
in place by ciliary muscles. It focuses images.
3.
Retina – innermost layer that contains rods
(dim light, night vision) and cones (color, day
vision)
Cones – less numerous than rods; densely
concentrated in fovea (small depression in macula
lutea, found near center of retina)
Aqueous humor and vitreous humor are fluids that fill
the hollow inside the eyeball, thus giving it shape.
Ear
1. Organ of Corti
Mechanoreceptor; hearing
2. Cristae ampullares
Mechanoreceptor; balance
Nose
Olfactory cells
Chemoreceptor
Smell
Taste buds
Gustatory cells
Chemoreceptor
Taste