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Flashcards in Module 10: Special Senses Deck (313)
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1

• Form of mechanical energy
• Waves of particle displacement
*Longitudinal vibrations of molecules in alternating phases of compression and rarefaction
• Produced by pressure changes (sound pressure) which is picked up by the ear and translated into audible sound

Sound

2

PHYSICAL CHARACTERISTICS OF SOUND:

1. Frequency
2. Intensity
3. Phase

3

• Determines pitch
• Expressed in Hertz (Hz)
- Number of cycles per second

Frequency

4

Audible range, Greatest sensitivity and Speech

Audible range: 20 – 20,000 Hz
Greatest sensitivity: 1000 – 4000 Hz
Speech: 300 – 3500 Hz

5

• frequency at which a mass vibrates with the least amount of external force

Resonant frequency

6

• Determines loudness
• Expressed in decibels (dB)
• Decidbel: unit of sound
• expressed in terms of the logarithm of their intensity
• a 10 fold increase in energy is 1 bel
• 0.1 bel is a decibel
• 1 decibel is an increase in sound energy of 1.26 times

Intensity

7

• Prolonged exposure to how many db SPL can cause deafness?

more than 80 dB SPL

8

Sound and dB SPL

• Jet plane, Gunshot blast: 140 dB SPL
• Automobile horn: 120
• Motor cycle engine: 100
• Average factory: 80-90
• Noisy restaurant, busy traffic, shouting: 80
• Conversational speech: 65
• Quiet office: 40
• Soft whisper: 30

9

dB SPL

• approx 140 dB SPL: threshold for pain
• approx 120 dB SPL: damage to cochlear hair cells
• approx 110 dB SPL: threshold for discomfort
• long exposue to >90 dB SPL may harm the hearing
• >80 dB SPL: "loud sound"

10

The Tympanic Membrane and the Ossicular System

• Tympanic membrane functions to transmit vibrations in the air to the cochlea
• Amplifies the signal because the area of the tympanic membrane is 17 times larger than the oval window
• Tympanic membrane connected to the ossicles
- malleus
- incus
- stapes

11

• can be damaged by loud sound (120 dB) or some drugs especially those for treatment of Tuberculosis (Pyrazinamide, Ethambutol)

Tympanic Membrane

12

• equalizes the pressure between the ear and the atmosphere around us

Eustachian Tube

13

• two muscles attach to the ossicles
- stapedius
- tensor tympani
• a loud noise initiates reflex contraction after 40 - 80 milliseconds
• attenuates vibration going to cochlea
• serves to protect cochlea and damps low frequency sounds i.e., your own voice

Attenuation of Sound by Muscle Contraction
(Attenuation Reflex)

14

• smallest muscle in our body; it contracts if a loud noise is initiated

Stapedius

15

Sound Conduction to the Cochlea

1. Bone conduction - Plays a role only in transmission of extremely loud sound

2. Air (ossicular) conduction - Main pathway for normal hearing; most common form of hearing

16

Amplification of Sound Pressure:

1. Sound collection
2. Impedance matching

17

• Resonator
• Cause minor increase in sound intensity
- By 10 – 20 dB between 2000 – 5500 Hz
- EAC resonant frequency: ~3000 Hz

The External Ear

18

• Displacement of TM and ossicular chain varies with frequency and intensity
- Most efficient: 500 – 3000 Hz

The Middle Ear

19

• increase sound pressure from Tympanic Membrane to oval window: 10 – 35 dB (22-fold)

• the louder the sound, the lower the ossicular displacement (the movement of the ossicular bones get lower when the sound is louder.
1. Area disparity between the resonating TM and stapes footplate
2. Lever action of the ossicular chain

• Requires equal pressure between the atmosphere and the middle ear cavity
- Maintained by the periodic opening of the eustachian tubes

Impedance Matching

20

• system of three coiled tubes separated by membranes into the scala tympani, scala media, scala vestibuli
• sound waves cause back and forth movement of the tympanic membrane which moves the stapes back and forth
• this causes displacement of fluid in the cochlea and induces vibration in the basilar membrane

Cochlea

21

• contains about 30,000 fibers which project from the bony center of the cochlea, the modiolus
• fibers are stiff reed-like structures fixed to the modiolus and embedded in the loose basilar membrane
• because they are stiff and free at one end they can vibrate like a musical reed
• the length of the fibers increase and the diameter of the fibers decrease from base to the helicotrema, overall stiffness decreases 100 X, high freq. resonance occurs near base, low near apex

Basement Membrane

22

Displacement of fluid in the cochlea depends on the frequency

High frequency - will displace fluid at the base
Middle frequency - will displace fluid in the middle
Lower frequency - will displaced fluid towards the end near the helicotrema

23

2 Types of Fluid inside the Cochlea

• Endolymph - fluid in the middle; near the organ of Corti
• Perilymph - fluid inside the scala vestibuli and scala tympani

24

• similar to CSF (high sodium, low potassium)
• Voltage: 0 mV

Perilymph

25

• similar to ICF (low sodium, high potassium)
• Voltage: +80 mV

Endolymph

26

• receptor organ that generates nerve impulses
• lies on the surface of the basilar membrane, contains rows of cells with stereocilia called hair cells
• the tectorial membrane lies above the stereocilia of the hair cells
• movement of the basilar membrane causes the stereocilia of the hair cells to shear back and forth against the tectorial membrane

Organ of Corti

27

Sensory Transduction of Sound

1. Hair cell depolarization
2. Voltage-gated Ca+ channels open
3. Release of neurotransmitter (glutamate/aspartate?)
4. Generation of action potentials in the afferent cochlear nerve fibers

28

Nerve Impulse Origination

• The stereocilia, when bent in one direction cause the hair cells to depolarize, and when bent in the opposite direction hyperpolarize.
- this is what begins the neural transduction of the auditory signal

• Auditory signals are transmitted by the inner hair cells.
- 3-4 X as many outer hair cells than inner hair cells
- outer hair cells may control the sensitivity of the inner hair cells for different sound pitches

29

Determination of Sound Frequency and Amplitude

• Place principle determines the frequency of sound perceived.
- Different frequencies of sound will cause the basilar membrane to oscillate at different positions.
- Position along the basilar membrane where hair cells are being stimulated determines the pitch of the sound being perceived.

• Amplitude is determined by how much the basilar membrane is displaced.

30

Central Auditory Pathway

fibers enter dorsal and ventral cochlear nuclei of the medulla —> 2nd order neurons project through trapezoid body to the contralateral superior olivary nucleus —> some fibers pass through the ipsilateral olivary nucleus —> from superior olivary nucleus to via lateral lemniscus —> from inferior colliculus to medial geniculate —> from medial geniculate to auditory cortex