Sensory Physiology 5
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What is the stimulus of hearing
How fast does sounds travel though air and though water?
- Air: 767 mph (340 m/s)
- Water: 1,500m/s
What are the variables in sound?
- 1. Frequency (pitch) - the number of compressed air patches
- Hertz - # of cycles/second
- 2. Intensity (loudness) - the difference in pressure between compressed air patches (amplitude)
What is the human frequency range?
20 (low) - 20,000 (high) Hz
What is the decibel?
- The unit used to express amplitude (loudness)
- Named after Alexander Graham Bell
- The decibel scale is logarithmic
What is the audibility curve?
Indicates how sensitivity changes across the frequencies that we can hear by plotting the threshold for hearing versus frequency
We are most sensitive at frequencies between 2,000 and 4,000 Hz. Which are also the frequencies at which human speech operates
What are the three major divisions of the auditory system?
- 1. Outer ear
- 2. Middle ear
- 3. Inner ear
Outer ear consists of?
- Auricle (Pinna) - movable in some animals
- Auditory Canal (external auditory meatus)
Middle Ear contains
- Tymponic membrane (ear drum)
- Ossicles (three small bones)
The auditory canal
- 3cm long
- protects the ear drum
- enhances of intensities of sound by means of resonance
A mechanism that enhances the intensity of certain frequencies because of the reflection of sound waves in a closed tube
What is the frequency that is most intensified?
The resonance frequency, it is determined by the length of the tube. These frequencies are between 2000 and 5000 Hz in humans
The middle ear is an ______
it contains three ossicles, they are
- air filled cavity
- Malleus (hammer)
- Incus (anvil)
- Stapes (stirrup)
Auditory tube (eustachian tube) is an
- airway between the middle ear and the pharynx
- it is important in changing air pressure
Why are the ossicles necessary?
- 1. Because sound vibrations are ineffective for moving fluid
- 2. The ossicles amplify the force exerted against the oval window
- 3. The ossicles convert air pressure changes into mechanical pressure.
The inner info
The cochlea is spiral-shaped - a coiled tube that looks like a snail (2 and 3/4 turns)
If you unroll the tube it is about 3.5 cm long and 2mm in diameter
- At the beginning of the tube, there are two windows with membranes
- 1. the oval window - what the stapes contacts
- 2. The round window
The three chambers in the cochlea are
- 1. Scala Vestibuli - filled with perilymph
- Reissner's membrane seperates
- 2. Scala Media - filled with endolymph
- Basilar membrane seperates
- 3. Scala Tympani - filled with perilymph
The basilar membrane is key! What does it contain?
The Organ of Corti, where the hair cells are
The tectorial membrane hangs over the organ of corti
The helicotrema is
a hole between the scala vestibuli and scala tympani
The basilar membrane
the base is ______ and the apex is ______
- thin and the mebrane is stiff (where high frequencies are encoded)
- wide and the membrane is less rigid (where low frequencies are encoded)
Organ of Corti info
- It lies on the basilar membrane
- It contains hair cells
- Hair cells look hairy - they have about 100 stereocilia on their heads
- The tips of the hair cells contact the tectorial membrane
- Movement of the hairs cells against the tectorial cells afainst the tectorial membrane changes the activity of the hair cells
- Hair cells depolarize when the stereocilia bend!
What are the two types of hair cells?
- Inner hair cells
- Outer hair cells
Inner hair cells
Auditory receptors cells in the inner ear that are primarily responsible for auditory transduction and the perception of pitch
Outer hair cells
- Auditory receptors cells in the inner ear that amplify the response of the inner hair cells
- It has motile response
What is motile response
A response to sound of the outer hair cells in which the cells move. The cells tilt and get slightly longer, which amplifies basilar membrane vibration and therefor amplifies the response of the inner hair cells
How do signals get out of the cochlea?
- Hair cells synapse with spiral ganglion cells
- The cell bodies of these neurons are located in the spiral ganglion
- The acons of these neurons form the auditory portion of the Vestibulocochlear nerve (Cranial Nerve VIII)
- Therefore, spiral ganglion cells are similar to retinal ganglion cells (the output neurons)
The primary auditory cortex - superior temporal gyrus info
- A tonotopic map can also be identified in auditory cortex
- Low frequencies are represented anteriorly in auditory cortex and high frequencies are represented posteriorly
- Unlike the visual system, destruction of uditory cortex in one hemisphere does not result in the loss off hearing on one side
- - because inpu from both ears goes to each hemisphere
- It does result in a loss of the ability to localize sound in the opposire hemifield
- However, most hearin loss is due to the death (with age) or destruction (due to loud noise) of hair cells
- Unfortunately, hair cells do not regenerate
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