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Physiology Exam 4, Lecture 7
Auditory System
49
Physiology
Professional
11/08/2011

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Term
What structures are in the external ear and what are their functions?
Definition
The pinna and external auditory meatus, to gather and conduct sound waves to the tympanic membrane
Term
What separates the external and middle ear?
Definition
The tympanic membrane
Term
What are the three bones in the middle ear? What is their function
Definition
The malleus, incus, and stapes

Their function is to amplify sound (20 times) so that the liquid inner ear can hear it
Term
What connects the middle and inner ear?
Definition
The oval window and the round window
Term
What connects the middle ear with the pharnyx?
Definition
The eustachian tube
Term
What is the vestibular apparatus?
Definition
Important in equilibrium, includes the semicircular canals, utricle, and saccule
Term
What nerve carries afferents for hearing and equilibrium?
Definition
The vestibulochlear nerve (cranial nerve 8)
Term
What in a sound wave determines volume?
Definition
Amplitude
Term
What in a sound wave determines pitch?
Definition
Frequency
Term
What is the point at the end of the cochlea spiral?
Definition
The helicotreme
Term
What seperates the scala vestibuli and the scala media?
Definition
The vestibular membrane
Term
What seperates the scala media and the scala tympani?
Definition
The basilar membrane
Term
What membrane is on the organ of corti?
Definition
The tectorial membrane
Term
What is another name for the scala media?
Definition
The cochlear duct
Term
Where is the perilymph fluid?
Definition
Scala vestibule and scala tympani (connected at the helicotrema)
Term
Where is the endolymph fluid?
Definition
The scala media
Term
What are the concentrations in the endolymph?
Definition
High potassium, low sodium
Term
What are hair cells embedded into?
Definition
The basilar membrane and (if outer hair cells) the tectorial membrane
Term
What is the result of bending toward the tallest cilia?
Definition
More K+ channels open, depolarization
Term
What is the result of bending away from the tallest cilia?
Definition
Elastic fibers go slack, K+ channels close, and hyperpolarization
Term
What does the brain stem do with auditory input?
Definition
Uses the information for alertness and arousal
Term
What does the medial geniculate nucleus of the thalamus do with auditory info?
Definition
Sorts and relays the information to the higher brain
Term
What specifically in the cerebrum detects sound quality?
Definition
The primary auditory cortex
Term
What in the thalamus sorts and relays information to the cerebrum?
Definition
The medial geniculate nucleus of the thalamus
Term
What kind of deafness is conductive deafness?
Definition
Sound wave disruption through external and middle ear

(ear wax, fluid, perforation of tympanic membrane, otosclerosis)
Term
What kind of deafness is sensorineural deafness?
Definition
The sound waves reach the inner ear, but are not converted into electrical signals

(noise, drugs (ototoxicity), presbycusis, tumor)
Term
What is the function of the tympanic membrane
Definition
Conducts sound waves to the middle ear, and converts sound energy to mechanical displacements.
Term
Describe the attenuation (tympanic) reflex
Definition
Two muscles in the inner ear (the stapedius and the tympani) are activated during loud sounds to diminish the movement of middle ear bones, reducing damage to hair cells

Doesn't work for sudden or high pitched sounds

Also can help filter out ambient low tones in an ambient environment
Term
Describe the transmission of sound from the middle to inner ear
Definition
Piston-like action of the stapes on the oval window converts a mechanical displacement to a hydraulic displacement (movement of perilymph).

Movement of fluid pushes against the round window and displaces the basilar membrane, ultimately resulting in action potentials
Term
Where do high frequencies displace the basilar membrane?
Definition
The base (stiff and narrow)
Term
Where do low frequencies displace the basilar membrane?
Definition
The apex (floppy and wide)
Term
Define tonotopic organization, as it has to do with the basilar membrane
Definition
Different frequencies are coded at different places along the basilar membrane. For example, a hair cell located at the site on the basilar membrane that moves best to a frequency of 4000 Hz will respond best (more depolarization and hyperpolarization) to 4000 Hz.
Term
What immediately causes neurotransmitter release from hair cells?
Definition
The amount of Ca++ that enters the cell is proportional to the degree of depolarization. Ca++ causes the neurotransmitter, glutamate, to be released onto the sensory neurons that innervate the hair cell.
Term
Describe the sensory neurons for sound
Definition
Consist of bipolar neurons with their cell bodies in the spiral ganglion (located within the bony part of the cochlea) and their peripheral processes forming synapses with the hair cells.

The central processes comprise the cochlear portion of the vestibulocochlear nerve
Term
Which hair cells (inner or outer) transmit more auditory information?
Definition
Inner hair cells

95% of neurons innervate inner hair cells, 5% outer hair cells

Outer hair cells act to amplify the sound signal by enhancing the movement of the basilar membrane
Term
Describe "characteristic frequency"
Definition
Auditory neurons have a characteristic frequency at which they respond with the greatest intensity; thus, neurons are "tuned" to different frequencies.

The characteristic frequency of a neuron depends on the hair cells that the neuron innervates.
Term
How is frequency neuronally coded? How is loudness?
Definition
Frequency of sound is mainly encoded by which neurons are excited.

Loudness is coded by the frequency of action potentials and number of neurons activated by the sound.
Term
Describe the path of auditory projections past the cochlear nuclei
Definition
All auditory projections past the cochlear nuclei are bilateral. This means that a person will not become
deaf as a result of a brain lesion above the level of the cochlear nuclei.
Term
Describe the auditory pathway for (horizontal) sound
Definition
ventral cochlear nucleus → superior olivary nucleus → inferior colliculus (via the lateral lemniscus) → medial geniculate nucleus → primary auditory cortex
Term
Where is the primary nucleus where horizontal sound localization occurs?
Definition
Superior olivary nucleus
Term
Describe how horizontal sound is localized
Definition
Because of the arrangement of neurons in the superior olivary nucleus, there are neurons that discharge in response to the particular delay between stimuli reaching each ear.
Term
How is vertical sound localized?
Definition
The ear somehow uses a comparison of direct sound vs. reflected sound to determine relative altitude of
sound. Because of the shape of the pinna, sounds arriving from different vertical levels will vary in both the direct and reflected pathways.
Term
Where is the primary auditory complex located?
Definition
The superior portion of the temporal lobe
Term
What is the function of the secondary auditory complex?
Definition
Secondary auditory cortex, surrounding the primary auditory cortex, processes input from the primary cortex and provides interpretation of complex sounds.
Term
Define Wernicke's area
Definition
An area partly in the secondary auditory cortex, important for understanding speech
Term
Describe what lesions in the central pathways would cause
Definition
With injuries up to the cochlear nuclei, there will be degrees of deafness on the ipsilateral side. Past the cochlear nuclei, deafness does not occur because the pathways are bilateral. Some lesions do produce difficulties in localizing sounds, however.
Term
Define tinnitus
Definition
Ringing in the ears, or perception of sound without a stimulus. This can be caused by a number of CNS deficits, not just auditory problems.
Term
How do hearing aids work?
Definition
With an intact auditory system but with inadequate numbers of hair cells, hearing aids simply amplify the incoming sounds.
Term
How do cochlear implants work?
Definition
If hair cells are dead, a cochlear implant has a series of electrodes that directly stimulate the cochlear nerve.
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