Heterogeneous Ca²⁺ influx along the adult calyx of held: A structural and computational study

The calyx of Held is a morphologically complex nerve terminal containing hundreds to thousands of active zones. The calyx must support high rates of transient, sound-evoked vesicular release superimposed on a background of sustained release, due to the high spontaneous rates of some afferent fibers. One means of distributing vesicle release in space and time is to have heterogeneous release probabilities (P_r) at distinct active zones, which has been observed at several CNS synapses including the calyx of Held. P_r may be modulated by vesicle proximity to Ca²⁺ channels, by Ca²⁺ buffers, by changes in phosphorylation state of proteins involved in the release process, or by local variations in Ca²⁺ influx. In this study, we explore the idea that the complex geometry of the calyx also contributes to heterogeneous P_r by impeding equal propagation of action potentials through all calyx compartments. Given the difficulty of probing ion channel distribution and recording from adult calyces, we undertook a structural and modeling approach based on computerized reconstructions of calyces labeled in adult cats. We were thus able to manipulate placement of conductances and test their effects on Ca²⁺ concentration in all regions of the calyx following an evoked action potential in the calyceal axon. Our results indicate that with a non-uniform distribution of Na⁺ and K⁺ channels, action potentials do not propagate uniformly into the calyx, Ca²⁺ influx varies across different release sites, and latency for these events varies among calyx compartments. We suggest that the electrotonic structure of the calyx of Held, which our modeling efforts indicate is very sensitive to the axial resistivity of cytoplasm, may contribute to variations in release probability within the calyx.