Intrinsic noise at synapses between a wing hinge stretch receptor and flight motor neurons in the locust

Variability in postsynaptic potential (PSP) amplitude due to intrinsic nois e limits the reliability of communication between neurons. I measured PSP v ariability at synapses between a forewing stretch receptor and wing depress or motor neurons in locusts, a pathway that is important in the control of flying. The intrinsic noise in the stretch receptor output synapse was meas ured by subtracting the background noise, originating in other synaptic pat hways onto the motor neuron, from the variability in the amplitudes of PSPs evoked by the stretch receptor. Intrinsic synaptic noise caused successive PSPs to vary by 4-10 % in basalar and subalar flight motor neurons. Record ings from pairs of these wing depressor motor neurons showed that the amoun t of transmitter released varied independently between different output sit es from the stretch receptor. Histograms of excitatory postsynaptic potenti al amplitude were normal distributions that lacked separate peaks. I estima te that quantal amplitude is significantly less than 0.1 mV and that severa l hundred quanta are released for each presynaptic spike. This accords well with a previous estimate of the number of discrete anatomical synapses and would facilitate modulation of output from the stretch receptor.