Afferent Synaptic Mechanisms

This chapter will focus on the first synapses in the auditory system, which in all vertebrate animals are located directly on the cell bodies of the mechanosensory hair cells. The presynaptic cells do not fire action potentials. Instead, deflections of the hair cell's stereocilia cause changes in the membrane potential of <1mV for barely perceptible sounds. Voltage-gated presynaptic calcium channels respond to these minute changes, allowing calcium to flow into the cell, where it modulates the release of excitatory neurotransmitter (glutamate) onto the postsynaptic endings of the primary afferent neurons. The resulting excitatory postsynaptic currents trigger action potentials that travel to the brain via the vestibulocochlear (eighth cranial) nerve. Thus, the afferent synapse transforms auditory information in the form of graded changes in the hair cell's membrane potential into spike trains in afferent neurons.