Efferent Pathways Flashcards
carrying something away from a central organ or part
efferent
what does efferent refer to in the CNS
refers to neurons carrying impulses away from the CNS which means away from the brain, or brainstem or spinal cord
There are ______ connects to both the auditory and vestibular peripheral sensory organs
efferent
what is an efferent connection in the middle ear?
Efferent motor control over the stapedius and tensor tympani muscles
What are the efferent pathways called?
descending auditory pathways
centrifugal pathways
what are the two sections of the efferent pathway?
rostral - descending fibers from the cortex to the lower auditory centers
caudal - aka olivocochlear bundle/Rasmussen’s bundle, from the superior olivary complex out to the cochlea
what are the divisions of the OCB?
crossed olivocochlear bundle (COCB
uncrossed (UCOCB)
A1 is rostral to the superior olivary complex.
true
Where does the rostral division of efferent pathway originate and descend?
originates in Auditory cortex
descends to mgb and IC (fibers here may give rise to other efferent fibers)
what do the centrifugal pathways allow
higher auditory system areas like the auditory cortex to modify the activity, sensory processing and focus attention on stimuli at lower levels
how many efferent neurons are in the OCB (caudal efferent system)
about 1600
besides the crossed and uncrossed classification of caudal efferent system, how else are they categorized
by where in the superior olivary complex the fibers arise. The lateral tract fibers originate from the cells near the lateral superior olive and will be referred to as the Lateral OCB and the medial tract fibers originate from cells in near the medial superior olive and will be referred to as the medial OCB
uncrossed fibers from LSO that terminate on the dendrites underneath the IHCs
Lateral OCB
fibers that are mainly crossed going directly to the contralateral OHCs
Medial OCB
What do efferent fibers do?
bringing action potential from cortex
they control and modulate what organ of corti is doing
Where do OCBs (efferent fibers) project
to the cochlea through the internal auditory canal as part of the VESTIBULAR NERVE
What fibers attach to the OHC?
type II afferent
MOC efferent
what fibers attach directly and indirectly to IHC
Type 1 afferent
LOC efferent attaches to afferent fiber
What role do MOC fibers mainly play?
inhibitory role by suppressing the contractility of the OHCs and thus reducing the biological mechanical amplifier function played by the OHCs
What are the functions of OCB
Exert control over synaptic transmission and neural signals
Mainly inhibitory role
Reduced neural output from the cochlea and auditory nerve during stimulation of the MOC (or crossed OCB –mostly MOC fibers)
However stimulation of some fibers resulted in lower thresholds for fibers in the cochlear nucleus, making them more easily stimulated (excitatory role)
What test measures have shown the effects of the inhibitory role of the olivocochlear efferent connections
OAEs, tuning curves, and ABRs
Does stimulating the OCB change tuning curves?
yes
The result is that the TC in not as sensitive at the characteristic frequency shown by the pointy time (see the small image as an example), and the TC may be less sharp during OCB stimulation (specifically medial OCB stimulation).
What helps ot hear in noise?
MOC bundle can trigger outer hair cell motility. The changes can enhance or dampen the basilar membrane movement, with effects on how the auditory system handles the presence of masking noise. When recording ABR wave I, in the presence of noise which would trigger the efferent pathways it was found that the best condition was with ipsi and contra noise. The studies suggest that the OCB may function to assist with hearing in noise.
what do OAEs test for?
outer hair cells movement
What happens when contralateral noise is introduced during the measure of OAEs
the amplitude of the OAEs decreases, subtly but consistently
The noise in the contralateral ear, triggers the COCB and the efferent signals influence the OHC motility, thus changing the OAE.
what is the main neurotransmitter of the efferent auditory system?
acetylcholine (gaba is also present but not the main)
VOR
Vestibular system sending efferent signals to control the eye muscles
VSR
signals to control muscles of the body, especially the antigravity muscles
What are the efferent vestibular connections
Efferent projections to the hair cells
Originating in the reticular formation, lateral to abducens nucleus
Projecting bilaterally in the vestibular division of CN VIII
Terminating directly or indirectly (afferent nerve endings) on vestibular hair cells
Theory—compensation (modulation) for self-generated head movements vs other, but this may not be true in humans
Role unknown
why do we have modulation?
hearing in noise
system gets overwhelmed and sharpens things and reduces that background noise (modulates)
does so by enhancing or dampening the basilar membrane movement
what is the efferent auditory system NT?
acetylcholine
