Computational Model of Synaptic Transmission at the Vestibular Hair Cell Calyx Synapse
Govindaraju, Aravind Chenrayan
Raphael, Robert M
Master of Science
"In the sensory neuroepithelia of the vestibular system, the organ which detects head orientation and acceleration, Type I sensory hair cells are enveloped by a cup-like process (calyx) of the afferent neuron and possess a characteristic low voltage activated potassium conductance (gKL) on their basolateral surface. The presence of the calyx creates a unique synapse morphology which is thought to limit the diffusion of ions and support two modes of neurotransmission between the hair cell and afferent neuron: Quantal (Q) – through the release of neurotransmitters and Non-Quantal (NQ) – through non-neurotransmitter mediated effects such as ephaptic coupling and potassium accumulation in the synaptic cleft. The importance and necessity of NQ transmission has been unclear. Direct experimental measurement of electric potentials and ion concentrations in the hair cell and afferent, let alone the synaptic cleft, is difficult. We have developed a computational model to probe the dynamic behavior of the Vestibular Hair Cell Calyx (VHCC) synapse and understand the role of non-quantal transmission. The VHCC model uses expressions for K+ and Na+ electro-diffusion in the cleft, Hodgkin-Huxley-like ion currents based on whole-cell recordings, stochastic vesicle release, and the cable equation to calculate potentials in the hair cell, cleft, afferent calyx and afferent fiber. Model simulations suggest that ephaptic coupling at the VHCC synapse is active at all frequencies, does not exhibit high-pass behavior as previously thought and may be an indefatigable method of communication between the type I hair cell and calyx."