Efferent Synaptic Animation
The following interactive animation illustrates some of the various synaptic mechanisms governing the responses of vestibular afferents to hair cell and/or efferent stimulation. Each mechanism can be viewed by adjusting the magnification bar and dragging the dashed circle within the hair cell diagram located in the upper right corner. Neural and synaptic structures contained within the dashed circle are then shown in more detail in the large circular window located in the upper left corner.
Structures seen within the viewing window, including transmitters, receptors, and ion channels, can be identified by clicking once with the mouse cursor. The resulting names of each structure will then appear in the rectangular panel in the lower left corner. Simulated sharp-electrode recordings of action potentials from a bouton and calyx afferent are shown separately in the scrolling panel located in lower right corner. Controls for starting, stopping, or adjusting the scrolling speed can be found just underneath the scrolling panel. The sound may be turned on by clicking the speaker icon to the right of each trace. Hair cells and/or efferents can be stimulated by pressing the
run stimulus and/or
zap buttons, respectively.
Vestibular hair cells tonically release packets of the neurotransmitter glutamate onto their respective afferents. These packets of glutamate are stored in synaptic vesicles which are tethered to the synaptic ribbon. The synaptic ribbon is thought to position these synaptic vesicles at presynaptic release sites just adjacent to afferent terminals. Upon its release, glutamate predominantly activates AMPA receptors whose cation influx briefly depolarizes the afferent thereby generating action potentials. During a vestibular stimulus, hair bundles are deflected, and depending on the direction of the deflection, either depolarize or hyperpolarize hair cells. This results in more or less glutamate release, and more or fewer action potentials, respectively. Bundle deflections can be simulated by clicking the
run stimulus button. Deflections toward the tallest stereocilia of the hair bundle will increase action potential firing whereas deflections toward the shortest will decrease it. Increasing the time scale for the scrolling window is recommended.
As the hair cell diagram indicates, efferent fibers synapse on the base of type II hair cells, on bouton afferents synapsing on type II hair cells, and on the outer face of calyx afferents which enclose type I hair cells. The predominant efferent neurotransmitter is acetylcholine (ACh) whose postsynaptic effects on hair cells and afferents will be determined by which ACh receptors are activated. Action potentials in bouton afferents are inhibited during efferent stimulation whereas calyx afferents are excited.
The inhibition of bouton afferents is a result of the activation of α9/10 nicotinic ACh receptors (α9/10nAChRs) found on type II hair cells. The influx of calcium through α9/10nAChRs activates small-conductance, calcium-dependent potassium channels (SK) which then hyperpolarize the hair cell, turn off glutamate release, and thus inhibit action potentials in the afferent. Excitation of calyx afferents results from the activation of another class of nicotinic ACh receptors (nACHRs) which are pharmacologically distinct from α9/10nAChRs. These nAChRs directly depolarize the afferent resulting in more action potentials. Similar nAChRs are also found on bouton afferents but their activation is typically masked by the inhibitory effects associated with the coactivation of α9/10nAChRs type II hair cells.