Hearing, Taste, Smell

What is sound?

• Vibration of pressure in a medium 

• Created from the vibration of an object 

• Vibrating air causes the earbrumm to vibrate, which the brain interprets as a sound

What is sound wavelength?

• Sound wave: alternating zones of compression (molecules more compressed, pressure higher) and zones of refraction (molecules are further apart, lower pressure)

• Amplitude of a sound wave = maximum pressure difference between zones of compression and rarefaction 

• Loudness is directly related to amplitude

threshold of human hearing

baseline = 0dB

loudness of normal conversation

55-65 dB

loudness that  produces  damage to the ear

greater than or equal to 100 dB

loudness that causes pain

Greater than or equal to 120dB

 frequency range of the human ear

20 Hz → 20,000 Hz

infrared frequency

 below the human audible range (20Hz)

ultrasound frequency

greater than human audible range (20,000 Hz)

effect of a sound wave on the tympanic membrane

Sound wave enters external auditory canal and causes tympanic membrane (eardrum) at the end of the canal to vibrate

Sound transmission across cochlea (5 steps)

1. Tympanic membrane deflects

2. Middle ear bones move

3. Membrane in oval window moves

4. Basilar membrane moves

5. Membrane in round window moves

function of the Eustachian tube

Maintain pressure in inner ear at equilibrium with pressure outside

function of the ossicles

• Chain of small bones (malleus, incus, stapes). 

• They transmit sound energy to the membrane over the oval window at the cochlea’s entrance, amplifying the sound x15-20

effect of sound pressure waves on the basilar membrane

Pressure waves in the cochlear fluid make the basilar membrane vibrate, which bends the stereocilia on hair cells

regions of the membrane are most sensitive to higher frequencies

Base (near oval window)

regions of the membrane are most sensitive to lower frequencies

Apex (near helicotrema)

STEPS OF SOUND TRANSDUCTION

• When bundle bends in direction of tallest stereocilium, K channels open, K enters, cell depolarizes, and opens voltage-gated Ca channels and neurotransmitter glutamate released onto afferent neuron.

• When bundle bends away from tallest stereocilium, K channels close, K current stops, cell hyperpolarizes and neurotransmitter release stops

How many taste buds do humans have?

10,000

Where are taste buds located?

 Upper esophagus, palate, pharynx, and state papillae on tongue

How are odors discriminated?

• Each receptor cell has only one type of odorant receptor.

• The same odorant can activate more than one receptor type.

• The brain identifies an odorant by the combination of receptor cells it activates.

• Hypothesis: odors are detected and discriminated when they activate one or more different transduction pathways in parallel in olfactory neurons

structure of olfactory receptor cells