MD110 Anatomy & Physiology

Chapter 9 – The Senses

Lecture Notes

I. Classification of sense organs

A. General sense organs

1. Often exist as individual cells or receptor units

2. Widely distributed throughout the body

B. Special sense organs

1. Large and complex organs

2. Localized grouping of specialized receptors

C. Classification by presence or absence of covering capsule

1. Encapsulated

2. Unencapsulated

D. Classification by type of stimuli required to activate receptors

1. Photoreceptors (light)

2. Chemoreceptors (chemicals)

3. Pain receptors (injury)

4. Thermoreceptors (temperature change)

5. Mechanoreceptors (movement or deformation of capsule)

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

II. Coverting a stimulus into a sensation

A. All sense organs have common functional characteristics

1. All are able to detect a particular stimulus

2. A stimulus is converted into a nerve impulse

3. A nerve impulse is perceived as a sensation in the central nervous system

4. The sensation is interpreted by the mind

5. Interpretation of sensation is a learned process

B. Sensations present the mind with a picture of the environment

C. Sensations may have strong affective (emotional) associations

III. General sense organs

A. Distribution is widespread; single cell receptors are common

B. Examples of general sense organs:

1. Free nerve endings – pain and crude touch

2. Meissner’s corpuscle – fine touch and vibration

3. Ruffini’s corpuscle – touch and pressure

4. Pacinian’s corpuscle – pressure and vibration

5. Krause’s end bulbs – touch

6. Golgi tendon receptors – proprioception

7. Muscle spindles – proprioception

IV. Special Sense organs

A. The Eye

1. Layers of the eyeball

a Sclera – tough outer coat; “white” of the eye; cornea is transparent part of sclera over iris.

b Choriod – 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.

c Retina – innermost layer of the eye; contains rods (receptors for night vision) and cones (receptors for color vision and day vision.)

2. Conjunctiva – mucous membrane covering the front surface of the sclera and lining the eyelid.

3. Lens – transparent body behind the pupil; focuses light rays on the retina.

4. Eye fluids

a Aqueous humor – in the anterior cavity in front of the lense

b Vitreous humor – in the posterior cavity behind the lense

5. Visual pathway

a Innermost layer of retina contains rods and cones

b Impulse travels from the rods and cones through the bipolar and ganglionic layers of the retina

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

d Visual interpretation occurs in the visual cortex of the cerebrum (occipital lobe)

B. Visual disorders

1. Refraction disorders

a Myopia – nearsightedness; is caused by elongation of the eyeball

b Hyperopia – farsightedness; is caused by shortening of the eyeball

c Astigmatism is distortion caused by an irregularity of the cornea or lense

d Conjuctivitis – inflammation of the conjunctiva – can interfere with refraction

(1) Trachoma – chronic chlamydial infection

(2) Acute bacterial conjunctivitis -- a highly contagious infection that produces a discharge of mucous pus

(3) Conjuctivitis can be caused by allergies

e Strabismus – improper alignment of eyes (crossed eyes)

(1) Eyes can converge (cross) or diverge

(2) If not corrected, can cause blindness

2. Disorders of the retina

a Retinal detachment can be a complication of aging, eye tumors, or head trauma

b Diabetic retinopathy – damage to retina from hemorrhages and growth of abnormal vessels associated with diabetes mellitus

c Glaucoma – intraocular pressure that decreases blood flow in retina and thus causes retinal degeneration.

d Nyctalopia (night blindness) or the inability to see in dim light is caused by retinal degeneration of lack of vitamin A

e Macular degeneration – progressive degeneration of central part of retina; leading cause of permanent blindness in the elderly

f Red-green color blindness is an x-linked genetic condition in which certain colors are not seen properly; it is caused by an abnormality in the cones’ photopigments

3. Disorders of the visual pathway

a Degeneration of the optic nerve resulting from diabetes, glaucoma, and other causes can impair vision

b Scotoma is the loss of only the central visual field when certain nerve pathways are damaged

c Stroke (CVA) can damage visual processing centers; example is acquired color blindness

C. The Ear

1. The ear functions in hearing and in equilibrium and balance

2. Receptors for hearing and equilibrium are mechanoreceptors

3. Divisions of the ear

a External ear

(1) Auricle (pinna)

(2) External auditory canal

(a) Curving canal 2.5 cm (1 inch) long

(b) Contains ceruminous glands

b Middle ear

(1) Houses ear ossicles – malleus, incus, and stapes

(2) Ends in the oval window

(3) The Eustachian tube connects the middle ear to the throat

(4) Inflammation of the middle ear is called otitis media

c Inner ear

(1) Bony labyrinth filled with perilymph

(2) Subdivided into the vestibule, semicircular canals, and cochlea

(3) Membranous labyrinth filled with endolymph

(4) The receptors for balance in the semi-circular canals are called cristae ampullaris

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

D. Hearing disorders

1. Conduction impairment

a Can be caused by blockage of the external or middle ear (for example, cerumen, and tumors)

b Otosclerosis – inherited bone disorder involving irregularity of the staps; it first appears as tinnitus (ringing) and then progresses to hearing loss

c Otitis – ear inflammation caused by infection; can produce swelling and fluids that block sound conduction

2. Nerve impairment

a Presbycusis – progressive nerve deafness associated with aging

b Progressive nerve deafness can also result from chronic exposure to loud noise

c Meniere’s disease – chronic inner ear disease characterized by tinnitus, nerve deafness, and vertigo

E. The taste receptors

1. Receptors are chemoreceptors called taste buds

2. Cranial nerves VII and IX carry gustatory impulses

3. Only four kinds of taste sensations

a Sweet

b Sour

c Bitter

d Salty

4. Gustatory and olfactory senses work together

F. The smell receptors

1. Receptors for fibers of olfactory of cranial nerve I line in olfactory mucosa of nasal cavity

2. Olfactory receptors are extremely sensitive by easily fatigued

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