Sensory systems

Chapter 16

Ear anatomy

• 3 major areas of ear:1) outer ear

- pinna – directs sound waves toward auditory canal

- external auditory canal – produces earwax that prevents foreign particles from entering

- tympanic membrane (eardrum) – vibrates in response to sound waves and sends signal to middle ear

- separates outer and middle ear

Ear anatomy

2) middle ear- oval window- round window- Eustachian tube – links middle ear to throat

- opens in response to swallowing or yawning- equalizes pressure in middle ear cavity with

air pressure- important for proper vibration of

eardrum- ossicles – transmit vibration of eardrum to oval

window- malleus, incus, and stapes

Ear anatomy

3) inner ear or labyrinth

- bony labyrinth

- vestibule

- cochlea

- semicircular canals

- membranous labyrinth – sacs and ducts within the labyrinth

Inner ear

• Vestibule – central cavity of labyrinth

- 2 membranous sacs:

- saccule

- utricle

- important in maintaining balance

- respond to movement of head and change in gravity

Inner ear

• Semicircular canals

- anterior and posterior at right angle

- lateral lies horizontally

- semicircular duct – communication with utricle

- contains ampulla at one end

- maintain balance

Inner ear

• Cochlea (snail)

- cochlear duct

- organ of Corti – receptor region for hearing

- spiral lamina – divides cochlea into 3 regions

- scala vestibuli – next to oval window

- filled with perilymph

- scala media – cochlear duct

- fsAAilled with endolymph

- scala tympani – next to round window

- filled with perilymph

Inner ear

• Cochlea

- basilar membrane in cochlear duct

- supports organ of Corti

Properties of sound

• Sound is vibration that creates pressure disturbance- propagated by compression of molecules - speed determined by medium- sound wave is produced by a series of molecule compressions and rarefactions (low pressure areas)

- moves outward in all directions- energy transferred in direction of sound wave

and as the wave energy declines, sound disappears

Properties of sound

• Sine wave can be used to infer frequency and amplitude- distance between 2 consecutive crests is wavelength- wavelength constant for a particular tone

• Frequency – number of sine waves that pass a given point in a given time- smaller wavelengths have higher frequency- human hearing is 20 – 20,000 Hz

- best between 1400 and 1500 Hz- perceive frequency as pitch

- increased frequency means higher pitch

Properties of sound

• Amplitude – height of sound waves

- corresponds to intensity

- pressure differences between compressed and rarefied areas

• Loudness – person’s interpretation of sound intensity

Sound transmission in ear

• Signal for sound must reach the auditory cortex before sound can occur

• Steps involved in sound transmission:

1) sound enters auditory canal and hits tympanic membrane (eardrum)

2) tympanic membrane vibrates at the frequency of the sound wave

3) vibration is passed to ossicles and sound is amplified

4) ossicle activity causes oval window to vibrate and fluid in cochlea to move in waves

Sound transmission in ear

5) Basilar membrane swings up and down and round window swings in and out

6) Basilar membrane vibrates

- if vibration occurs near the oval window then high frequency

- if vibration occurs near the cochlear apex then low frequency

- difference due to size of fibers

Sound transmission in ear

6) Movement of basilar membrane causes hair cells in organ of Corti to bend- if hair bends toward tallest hair, then get depolarization

- neurotransmitter release- cochlear fibers transmit impulses to brain

for auditory interpretation- if hair cells bend away from tallest hair, then hyperpolarization

From ear to brain

7) Impulses from organ of Corti travel to cochlear nuclei of the medulla

8) From medulla impulses travel to superior olivary nucleus then to the inferior colliculus

9) From inferior colliculus impulses go to medial geniculate bodies in the thalamus and then to auditory cortex- inferior colliculus also sends impulses to superior colliculus for reflex action

Hearing disorders• Conduction deafness – sound cannot reach fluids of inner

ear- earwax buildup- ruptured eardrum- middle ear inflammation (ear infection)

• Sensorineural deafness – damage to auditory structures in brain- loss of hearing receptors

- age- sudden explosive loud noise- prolonged exposure to high-intensity sounds

Hearing disorders

• Tinnitus – ringing or clicking sound in ears

- cochlear nerve degeneration

- middle or inner ear inflammation

- side effect from some medications