Auditory system Flashcards
What is auditory perception?
It is the ability to perceive sound by detecting vibrations.
Therefore, hearing is a mechanosensation
What is sound?
the alternation of compression and rarefaction of air. The distance between the pressure peaks is called wavelength.
Humans can detect sound in the frequency range of 20 to 20 000Hz (Hz= waves per second)
Describe the features of sound and their measures
frequency determines the pitch of the sound; loudness is the amplitude and is measured in decibels – dB
What is the ascending auditory pathway?
tympanic membrane –> ossicles–> fluid-filled choclea –> air compression –> mechanical vibrationsà electrical potentials
What is the role of the cochlea?
cochlea traduces sound waves into electrical potentials. The pressure wave initiates the motion of the basilar membrane and passes to the Organ of Corti (sitting on the basilar membrane)
What is the basilar membrane and its structure?
undamental function is that of frequency selectivity. Basilar membrane vibration is nonlinear. Each place of the membrane is more sensitive to a characteristic frequency, where it will vibrate strongly. As the characteristic frequency of a particular sound varies the response diminishes. The amplitude determines the extend of the vibration. Loud sounds will affect wider portion of the basilar membrane and will activate more neurons. The amplitude of a sound wave increases slowly and decreases abruptly.
What is frequency selection?
It is the Ability to distinguish sounds occurring simultaneously. It depends on ability of the basilar membrane to perform the frequency selection, therefore the basilar membrane separates different frequency components in complex sounds.
* Frequency determines WHERE the basilar membrane responds more intensively * Amplitude determines how WIDE is the portion of membrane responding to a sound
What is the organ of Corti?
it is a ribbon shaped epithelium, running lengthwise down the cochlea. Its hair cells transform fluid waves into nerve signals
What are the types of hair cells and their roles?
Inner hair cells - the mechanoreceptors for hearing. They transduce the vibration of sound into electrical activity (graded response)
Outer hair cells - motor structure
Describe how the hair cells transduce vibrations into sound?
- The hairs of the hair cell project into the tectorial membrane. Tectorial membrane does not move to the same degree of the basilar membrane - shearing motion on the hairs à change in potential (de- or iper-polarization) àauditory nerve
- towards the kinociliumà increment of firing rateàdepolarizationà excitatory effect
- away from the kinocilium->decrement of firing rate->perpolarizationà inhibitory effect
Describe the process of hair cell depolarisation
- Stereocilia move towards the kinocilium à mechanical opening of ion channel à positive ions (K+) enter into the cell à Voltage –depended ion channels open and other positive ions enter into the cell à DEPOLARISATION à Ion channels close à Positive ions are pumped outside the cell.
What is the spatial mapping of the auditory nerve?
Since each inner cell is responding to vibration at a single place of the basilar membrane -à each inner cells is responding best to a characteristic frequency;
Since each inner cell is innervated by a neuron (of the 30,000) of the auditory nerve à each neuron is responding to a characteristic frequency
Therefore the spatial mapping of frequency along the basilar membrane is now converted into a spatial mapping of frequency in the auditory nerve
Information about sound frequency is maintained at auditory nerve level
Describe the representation of sound in the brain (including the auditory cortex?
- From medial geniculate nuclei - IV layer primary auditory cortex
- From auditory cortex (V layer) - medial geniculate nuclei
- From auditory cortex (VI layer) - Inferior colliculi
- Primary auditory area comprises BA 41 and 42
- Basal end (towards the oval window) of the basilar membrane is represented most medially in the cortex; whereas apical end is represented most laterally in the cortex;
- A particular region of the auditory cortex responds to low frequencies, whereas another region responds to higher frequencies;
- The primary auditory cortex has a bilateral representations – lesions in the central pathway do not give raise to monaural deficits or compromise auditory perception dramatically
- The primary auditory cortex is organized in perpendicular columns (summation and suppression
- Summation columns neurons respond strongly to stimulation of either ear
- Suppression columns neurons are excited by stimulation od one ear but inhibited of stimulation of the other
- Summation and suppression columns are alternated and are the basis of ear dominance
What is the auditory fovea?
larger areas to process important sound stimuli (overrepresentation) e.g. speech stimuli are overrepresented and lateralized.
For example,
* Musical stimuli ate overrepresented on the right belt area (volume of the primary auditory cortex is significantly larger in musicians than in none musicians. The increase in the volume correlates with musical expertise. Deficits (e.g. tinnitus are associated with reduced volume of the primary auditory cortex). Different areas are involved in different processes of musical stimuli: Whatà more ventral areas; where à more dorsal areas * Emotional aspect of speech and prosody are more linked to the right hemisphere
What is the descending auditory pathway and its role?
- Auditory cortex –> (V layer) medial geniculate nucleus–>(VI layer) inferior colliculus—>cochlear nuclei (olivary-cochlear path)—> outer hair cells
- The function of this pathway is unclear
- May modulate sensitivity of the basilar membrane to low sound and sensitivity to frequency selectivity (e.g. attention to sound)
- Interaural time differences: a sound coming from the right will reach the right ear first.
- For low frequencies <3kHz
- Olivary complex (medial superior) - cells that are sensitive to the difference in the time arrival of the sound to the two years
- Interaural level difference: a sound coming from the right is more intense in the right ear
- Mainly for higher frequencies >3kHz
- Lateral superior olive (LSO) and medial nuclei of Trapezoid body (MNTB) – cells sensitive to differences in sound intensity
- Interaural time differences: a sound coming from the right will reach the right ear first.
