Auditory System Flashcards
Inner hair cells
Receptive cells in cochlea responsible for converting vibration into a nerve signal (most important)
Outer hair cells
Act as a cochlear amplifier.
Regulate vibration of tectorial membrane and help us detect the signal correctly
High frequencies produces vibration of
Base of the cochlea
Low frequencies produces vibration of
Tip of the cochlea
Mechanisms of frequency coding
Basilar membrane vibration
Electrical impulse (HC) to auditory nerve
Electrical impulse (HC) to auditory nerve - relationships
APs in VIII CN maintain 1:1 relationship: 1 AP per vibration up to certain frequencies —> + 5 kHz, AP gets sent every 2 - 3 vibrations (1:n relationship)
Important for perceiving low frequencies (fundam freq and harmonics) —> music, different notes
Auditory neurons - location
Soma in spiral ganglion (middle of the cochlea) - ascend through the nerve - brainstem
In the cochlea, HC in Organ of Corti synapse w/ auditory neurons
Types of auditory neurons
Type I neurons = synapse w/ 1 single inner HC each ( +precise detection of sound)
Type II neurons = synapse w/ many outer HC (- precision in the message as it picks up many sources)
General characteristics of the auditory pathway
Tonotopic organization (pitch freq represent at any point)
Multiple commissure (connections between both sides of pathway)
Parallel processing (sub pathway for each type of info that converge in inf colliculus)
Descending projections (every higher projection controls lower projections)
Cochlear nuclei - parts, location
Ventral cochlear nuclei (VCN)
Dorsal cochlear nuclei (DCN)
Located in the junction between medulla and pons
VII + VIII CN enter/exit brainstem -> pontocerebellar angle
Cell types in cochlear nuclei
Many types
VIII enters the nucleus = axon diverges and synapse w/ several types of cells —> each cell type receive specific type of info
From 1 input (1 nerve fibre), we need parallel processing of many types of info —> nucleus = major relay point (1st)
- Temporal processing = synapse in a region with specific cells
- Frequency processing = synapse in structures related to frequency recognition
Auditory nerve characterization
Peristimulus histogram —> time curve = measures discharges along time
Running curve —> how frequency specific a neuron is (they loose selectivity as the sound intensity increases)
Rate-intensity curve —> how a neuron responds to changes in the intensity of sound
Superior olivary complex - location, function
After the cochlear nucleus
Localizing sound
Superior olivary complex - nuclei
Medial olivary nucleus
Lateral olivary nucleus
Medial nuclei of the trapezoid body
Periolivary regions
1st areas originating descending pathways: the olivocochlear path
Olivocochlear path
Project to the cochlea itself and act on HC.
Important for the action of outer HC (contract) -> regulated by superior olivary complex
Mechanisms for lateral localization of sound
Lat sup olive and med nucleus of the trapezoid body
Med sup olive
Lateral superior olive and medial nucleus of the trapezoid body
Intensity differences between ears (level detector)
Useful for high pitch sounds
Best developed in humans
Medial superior olive
Detects phase / time differences between areas) —> can only be done when different parts of sound wave are contacting each ear at = time ↴
Useful for low pitch sounds (where wavelength is broad = each wave can be in contact w/ 2 ears at = point in time)
Differences in intensities are useful for
High pitched sounds
Phasic differences (differences in time) are useful for
Low pitch
Lateral localization for LOW-PITCHED sounds
Neurons in med sup olive receive R + L axons of different lengths. Different pairs of neurons.
- 1st pair (lt = rt): sound at = time —> sound is reaching 2 ears at the = time, in = manner
- 2nd pair (lt < rt): sound at = time —> sound is reaching 1 side (L in this case) later than at the R side
- 3rd pair (lt «< rt), even + time to reach the other ear
Lateral localization for HIGH-PITCHED sounds
For intensity: from one side we excite, from the other the signal is inhibited. Say we are on the R side:
- Ipsilateral neuron reaches lat sup olive nucleus (LSO)
- Contralateral neuron, from the L ear, reaches the med nucleus of trapezoid body (NMTB)
Both nuclei are interconnected.
If signal coming from 1 side is stronger than the other, the other will be inhibited.
Neuron coming from ipsilateral side (right) is firing at higher rates (it’s perceiving + intensity) -> LSO activated on the R side, NMTB inhibited = sound coming from R.
In L side, opposite will happen (contralat neuron coming from R is conveying + APs) —> NMTB on L activated and L LSO inhibited.
Descending auditory pathways
Olivocochlear bundle
Cortico-thalamic projections (to medial geniculate body)
Cortico-collicular projections (to inferior colliculus)
Descending projections from the Inferior colliculus (to cochlear nuclei & sup olivary complex)
Nuclei of lateral lemniscus
Ventral nucleus of LL: involved in temporal processing of sound, only receives input from contralateral ear
Dorsal nucleus LL: unknown function, sends efferences to ipsilateral and contralateral inferior colliculi
Inferior colliculus
Point where all the auditory pathway converges
Inferior colliculus parts
Central nucleus
- frequency processing
- laminae —> each processes a specific frequency (tonotopic organization)
- functional groups within each lamina
Cortices
- surrounds the nucleus
- more integrative function
- receive afferences from central nucleus
Superior colliculus
Motor structure that integrates sensory inputs from different pathways to give off efferents.
No exactly part of auditory pathway, but related
Superior colliculus function
Orient responses (gaze) towards images or sound.
Respond to different stimuli by moving our head.
Auditory pathways final stages
After converging at inf colliculus, fibres are sent to thalamus —> medial geniculate body (ventral + dorsomed regions)
Ventral MGN
Tonotopic distribution
Fibres coming from the central nucleus of the IC
More frequency specific (narrow tuning curves)
Projects to the primary auditory cortex
Dorsal MGN
Fibres coming from the cortices of the inferior colliculus
Less frequency specific
Go to cortex and 2º auditory areas (small associative areas)
Auditory Cortex (temporal lobe)
Primary auditory cortex (area 41)
Secondary auditory cortex = area 42
Secondary auditory cortex
Concerned with memory and sound classification
Multimodal integration areas, auditory not specific responses may be present.
Primary auditory cortex
Tonotopic organization w/ binaural (from both sides) responses
Auditory cortices from 2 sides are highly connected and communicated through corpus callosum (contralat connections)
Cortical, thalamic and collicular projections
