1. Chapter 9 The Senses

2. CLASSIFICATION OF SENSE ORGANS
General sense organs (Table 9-1)
Often exist as individual cells or receptor units
Widely distributed throughout the body
Special sense organs (Table 9-2)
Large and complex organs
Localized grouping of specialized receptors

3. CLASSIFICATION OF SENSE ORGANS
Classification by presence or absence of covering capsule
Encapsulated
Unencapsulated (“free” or “naked”)
Classification by type of stimuli required to activate receptors
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)

4. CONVERTING A STIMULUS INTO A SENSATION
All sense organs have common functional characteristics
All are able to detect a particular stimulus
A stimulus is converted into a nerve impulse
A nerve impulse is perceived as a sensation in the central nervous system

5. GENERAL SENSE ORGANS (Table 9-1)
Distribution is widespread; single-cell receptors are common
Examples (Figure 9-1, Table 9-1)
Free nerve endings—pain, temperature, and crude touch
Meissner’s corpuscles—fine touch and vibration
Ruffini’s corpuscles—touch and pressure
Pacinian corpuscles—pressure and vibration
Krause’s end-bulbs—touch
Golgi tendon receptors—proprioception
Muscle spindles—proprioception

7. SPECIAL SENSE ORGANS The eye (Figure 9-2) Layers of eyeball
Sclera—tough outer coat; “white” of eye; cornea is transparent part of sclera over iris
Choroid—pigmented vascular layer prevents scattering of light; front part of this layer made of ciliary muscle and 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 (Figure 9-4)—innermost layer of the eye; contains rods (receptors for night vision) and cones (receptors for day vision and color vision)
Conjunctiva—mucous membrane covering the front surface of the sclera and also lines the eyelid; kept moist by tears found in the lacrimal gland
Lens—transparent body behind the pupil; focuses light rays on the retina

10. SPECIAL SENSE ORGANS The eye (cont.) Eye fluids Visual pathway
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 and cones
Impulse travels from the rods and cones through the bipolar and ganglionic layers of retina (Figure 9-4)
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

11. SPECIAL SENSE ORGANS The ear
The ear functions in hearing and in equilibrium and balance—receptors called mechanoreceptors
Divisions of the ear (Figure 9-5)
External ear
Auricle (pinna)
External auditory canal
Curving canal 2.5 cm (1 inch) in length
Contains ceruminous glands
Ends at the tympanic membrane