audition learning objectives - michael Flashcards
1: be able to describe the characteristics of sound
a) Sound is pressure waves generated by vibrating air molecules
i) Amplitude corresponds to loudness
(1) Sound intensity (loudness) is expressed in a logarithmic scale (units = decibels)
(a) dB = 20 log10[P/Po]
ii) Frequency corresponds to pitch.
iii) Complexity corresponds to timbre
(1) Usually described qualitatively
b) Sound waves propagate as spherical “shells”
c) Human hearing range: 20Hz – 20kHz (peak sensitivity 2-3kHz – most speech is this loud)
2: 2) Be able to explain the mechanical advantages of the outer and middle ear.
a) The auricle (pinna) helps collect sound, while the external acoustic meatus (tube to tympanic) selectively “boosts” sound pressure 30-100 fold, especially around 3kHz (human speech)
b) Sound waves transition from air to fluid in the middle ear; this should result in reflection of 99% of sound waves. However, the middle ear boosts sound pressure ~200 fold by two mechanisms:
i) Energy from relatively larger tympanic membrane focused onto the smaller oval window
ii) The mechanical advantage of the lever action of the three ossicles: malleus, incus, stapes
3) Be able to describe the components of the attenuation reflex and how it occurs.
a) Two small muscles of the middle ear:
i) Tensor tympani, innervated by trigeminal nerve
ii) Stapedius, innervated by facial nerve
b) Contract during loud noises, stiffen the ossicles, and reduce the amount of sound pressure transmitted to the cochlea.
i) Damage to these muscles (as in Bell’s Palsy) results in hyperacusis, an extra sensitivity to moderate or even low intensity sounds (occurs in ~8% of the population)
4) Be able to describe the structural features of the cochlea.
a) The cochlear duct (scala media) separates cochlea into three compartments: scala vestibuli, scala media, scala typani
i) The scala media contains endolymph (high K solution) produced by stria vascularis (secretory epithelium)
ii) The scala vestibuli and scala tympani are connected at the far end by the helicotrema (small opening). They both contain perilymph (140 Na, 5 K, 3 Ca).
5) Be able to describe what is meant by tonotopy.
a) Tonotopy: systematic representation of sound frequency (along the cochlea)
6) Be able to explain how auditory transduction occurs in the Organ of Corti.
a) Organ of Corti: consists of hair cells, support cells, and the tectorial membrane (gelatinous membrane – the stereocilia are stuck in this).
i) The stereocilia of hair cells push up against the tectorial membrane. The kinocilium (tallest stereocilia; true cilia) disappears shortly after birth in humans.
b) If stereocilia move in one direction, the K channels close and the cell hyperpolarizes. If the stereocilia move in the other direction, the K channels open and the cell depolarizes, causing neurotransmitter release
i) When K channels open in the stereocilia, K ions flow into the cell (driving force is 125mV). Two reasons:
(1) Chemical gradient due to higher K concentration in endolymph than in interior of cell (80mV)
(2) Electrical gradient due to membrane potential (-45mV)
7) Be able to explain how hair cells function.
a) The inner hair cells are connected to 90% of the afferent fibers sending information from cochlea (a single fiber innervates a single hair cell). The outer hair cells are targets of efferent supply from the superior olivary complex. Activation of this efferent pathway leads to ACh release on to the outer hair cells and leads to dampened response to loud sound (less response in afferent fibers)
8) Be able to describe the properties of the basilar membrane that help distinguish between sounds.
More vibration in the base during high frequencies (stiffer), and more vibration at the apex during lower frequencies (more flexible).
9) Be able to explain what electromotility is, where it occurs, and the role it plays in the cochlear amplifier.
a) Electromotility: the outer hair cells shorten in response to depolarization; this results in more basilar membrane movement and inner hair cell displacement. The outer hair cells lengthen in response to hyperpolarization.
i) The voltage-sensitive motor protein in the outer hair cells is prestin – resides in the plasma membrane.
b) The change in the extent of basilar membrane movement is called the cochlear amplifier. The cochlear amplifier can:
i) Protect the cochlea from damage by loud sounds
ii) Selectively alter the range of membrane movement to dampen background noise and enhance specific frequencies
10) Be able to describe the neural mechanisms that are used localize sounds.
a) Spiral ganglion → CNVIII → ventral cochlear nuclei → superior olivary nucleus → inferior colliculus → medial geniculate of thalamus → temporal lobe (superior temporal gyrus)
i) Superior olivary nucleus: where the signals are used for sound localization
b) Two brainstem mechanisms for sound localization:
i) Time delay in the ears (below 3kHz)
(1) The medial superior olive neurons are coincidence detectors. A medial superior olive neuron is maximally stimulated when information from the two ears reach the neuron at the same time. Each neuron has dendrites of different lengths going to each ear. If the cell has a longer dendrite going to the right ear, it will be maximally stimulated when the sound is coming from the right.
ii) Intensity differences in each ear (above 3kHz)
(1) Lateral superior olive neurons are stimulated by sound in the ipsilateral ear, but inhibited by sound coming in from the contralateral ear via inhibitory interneurons in the medial nucleus of the trapezoid body (MNTB).
c) For low frequency sounds that are continuous, the difference between the times a particular point in the phase of the sound wave reaches each ear can also be used to localize the source of the sound
d) Asymmetry of the shape of the ears allows you to distinguish between sounds coming from directly in front of you and directly behind you
11) Be able to name cortical structures involved in sound perception.
a) Spiral ganglion → CNVIII → ventral cochlear nuclei → superior olivary nucleus → inferior colliculus → medial geniculate of thalamus → temporal lobe (superior temporal gyrus)
i) The medial geniculate is where you first see neurons with a pronounced selectivity for combinations of frequencies and specific time differences between the two frequencies.
b) Spiral ganglion → CNVIII → ventral cochlear nuclei → superior olivary nucleus → inferior colliculus → medial geniculate of thalamus → temporal lobe (superior temporal gyrus)
i) Physical landmarks: Heschl’s gyrus, superior temporal gyrus, Sylvian fissure
ii) Primary auditory cortex: columnar organization with all cells in a vertical column have the same best frequency
iii) Secondary auditory cortex (“belt areas”): contain neurons that are sensitive to specific combinations of sounds used in vocalizations
iv) Wernicke’s area: posterior to primary cortex, important for understanding speech (also receives visual information)
c) Dorsal and ventral streams of auditory processing:
i) Dorsal stream: location of sound
(1) Superior frontal gyrus and superior parietal cortex
ii) Ventral stream: pitch of sound
(1) Primary auditory cortex and inferior frontal gyrus
12) Be able to explain how elements of sound are involved in speech and language.
a) Speech is composed of sounds (phonemes) like consonants or vowels (“b” or “e”) and sound groups (lexemes) like “th” or “st” or a short word like “we”
b) Broca’s area: produces speech; a lesion in this area is referred to as Broca’s aphasia (cannot speak)
c) Wernicke’s area: understands speech; a lesion in this area is referred to as Wernicke’s aphasia (speech does not make sense)
d) Arcuate fasciculus: connects Wernicke’s area to Broca’s area
e) Supramarginal gyrus: matches incoming sounds with meaningful phonemes
f) Angular gyrus: matches incoming written language information with meaningful phonemes
g) Speech comprehension require s both auditory and visual cues
i) The McGurk effect: when watching a face saying a different syllable than the one we are hearing, our minds can be tricked. Link: bit.ly/cn2cpK (about 3 minutes long)
13) Be able to name the general classes of auditory disorders and describe a test that can be used to distinguish between them.
a) Types of deafness:
i) Conduction deafness: disturbance in the conduction of sound from the outer ear to the cochlea
ii) Sensorineural deafness: loss of hair cells or neurons in auditory nerve
iii) Acquired hearing loss: trauma, infection, ototoxic drugs, old age
iv) Inherited hearing loss: genetic
b) Rinne test: strike tuning fork, place base of tuning fork on mastoid bone of the patient, when the patient can no longer hear the fork, immediately move the fork to just outside the patient’s ear
i) If the patient has conduction deafness, the patient will hear the fork on his or her mastoid bone as long or longer than he or she hears the fork next to the ear
ii) If the patient has sensorineural deafness, the patient will hear the fork longer next to his or her ear (but less than normal)
14) Be able to describe specific types of auditory disorders.
a) Presbycusis: late onset hearing impairment; occurs with old age; usually due to loss of hair cells; high-frequency hearing loss
b) Hyperacusis: reduced tolerance to ordinary environmental sounds
c) Auditory agnosia: cannot verbalize meaning of nonverbal sound
d) Congenital amusia (tone deafness): inability to accurately detect and mimic changes in pitch
e) Tinnitus: perception of sounds in absence of stimulus
f) Acoustic neuroma: slow growing tumors of Schwann cell origin; more common in elderly; can result in hearing loss or tinnitus
g) Ménière’s disease: progressive hearing loss due to excess fluid buildup in endolymphatic sac
