6.8 Neurophysiology of Hearing Flashcards
What is the auditory system made up of
Miniature acoustical detectors where there is transduction of vibrations.
What are sounds
Audible variations in air pressure
How is loudness of sound quantified
The amplitude of a sound wave
How is pitch of sound quantified
The frequency of the sound wave
Describe the phase difference between sound waves
Phase is the position of a wave at a given time in the cycle, so a difference of phase between two waves (sounds) corresponds to a difference in time. Phase difference is used to locate sounds in space
What are the 3 structures of the ear
inner ear, middle ear and outer ear
How is sound amplified in the outer ear
In external/outer ear, the auditory meatus boosts the sound pressure into the tympanic membrane, causing amplification in the frequency range of 2-5 kHz (very important for speech perception)
How is sound amplified in the middle ear
The force hitting the oval window is greater than the force hitting the tympanic membrane. Also, the tympanic membrane has a larger surface area than the oval window, so the same amount of force is applied to a smaller surface area. These both cause an increase in pressure across the inner ear boundary.
What converts sound waves into neural signals
The cochlea in the inner ear
How are sound waves transmitted to the basilar membrane in the cochlea
There are 3 fluid filled chambers in the cochlea; scala media, scala tympani and scala vestibuli. The sound waves are transmitted through the fluid and transmitted into the middle of the cochlea, all the way to the basilar membrane.
Describe what occurs at the basilar membrane of the cochlea
The basilar membrane is in between the oval window and round window. It contains two ends (the base and apex). The shape of the basilar membrane allows for frequency tuning. The membrane is narrow and stiff at the base to transmit high frequency and wide and flexible at the apex to transmit low frequency. The sound bends the basilar membrane at the base and then the wave moves towards the apex.
What is tonotopy
The organisation of sound frequencies along an axis, seen in the basilar membrane of the cochlea and all brainstem and central auditory centres
Describe what happens at the organs of corti of the cochlea
There are hair cells that are sensory receptors that convert vibrations from the basilar membranes into neural signals through bending of the cilia. The hair cells have rapid responses for accurate localisation of the sound source and rapid adaptation to constant stimuli.
Describe the mechanism of the mechanoelectrical transduction of sound waves at the hair cells
-the movement of the cilia either open or close K+ channels (depending on the direction), either causing depolarisation or hyperpolarisation, changing levels of glutamate neural transmission onto spiral ganglion neurons
-the louder the sound, the more the cilia bend, hence more action potentials are generated
Describe the main auditory pathway
-hair cells at organs of corti
-spiral ganglion neurons
-auditory nerve
-cochlear nuclei in brainstem
-superior olivary complex
-inferior colliculus
medial geniculate nucleus
-thalamus
-auditory cortex
Describe the superior olivary nucleus
The superior olivary nucleus in the brainstem receives projections from both sides to allow for integration from both ears. Allows for sound localisation
How is sound localised at low frequencies
The interaural time delay (difference between sound waves reaching the closer ear, and the further ear) enables us to locate the source of the sound in the horizontal plane. With continuous tone at low frequencies, the brain will compare the time at which the same phase of the sound wave reaches each ear.
How is sound localised at high frequencies
At high frequencies the interaural time delay is very small, however the sound can be localised by the interaural intensity difference (due to the sound shadow). This information is processed by the lateral superior olive and the medial nucleus of the trapezoid body
Describe the mechanisms of interaural time delays
Medial superior olive neurons are coincident detectors, they respond maximally when excitatory signals arrive simultaneously (bilateral excitatory inputs from cochlear nuclei). When a sound reaches one ear first, its action potential will be transmitted towards the medial superior olive before that of the other ear. As the action potentials converge on an medial superior olive neuron that will give the strongest response for when their arrivals coincide
Describe the mechanism of interaural intensity difference
-if sound comes from the left side, a strong stimulus excites the left lateral superior olive which also inhibits the right lateral superior olive, the excitation from left side is greater than inhibition from right, resulting in net excitation to higher centers from left side - but inhibition from left side is greater than excitation from right side, resulting in no net excitation to higher centers from right side. This excitatory, inibitory interaction results in a net excitation by the lateral superior olive on the same side as the sound source
Describe the secondary auditory cortex
secondary auditory cortex (A2/belt area) receive more diffuse input from thalamus (MGN) and therefore are less precise in their tonotopy. Important role in sound localisation and analysis of complex sounds - it is pecialized for processing of harmonic, melodic and rhythmic patterns
Which has more tonotopic organisation, A1 or A2
A1
What is the tertiary auditory cortex
tertiary auditory cortex for integration of everything into the overall experience of music and speech
What are the 2 processing streams in the auditory cortex
there are 2 processing streams in the auditory cortex:
-dorsal pathway: where, space related features
-ventral pathways: what, identification and recognition of sounds
