Regional Differentiation of Neuromuscular Synapses in a Lobster Receptor Muscle

Properties of the synapses and muscle fibres of the distal accessory flexor muscle (DAFM) were examined in the first and second walking legs of the lobster, Homarus americanus. Stimulation of the single excitor axon produces large amplitude, poorly facilitating excitatory postsynaptic potentials (EPSPs) in the distally located fibres and small amplitude, highly facilitating EPSPs in the proximally located fibres. The input resistances (R_in) of the muscle fibres were correlated with EPSP properties such that small amplitude, highly facilitating EPSPs occurred in fibres with low R_in and large amplitude, poorly facilitating EPSPs occurred in fibres with higher R_in. All muscle fibres were similar for other membrane electrical properties. Regression analyses however show a minor contribution of R_in to the size of intracellularly recorded synaptic potentials and to their facilitation properties. Thus, differences in muscle membrane properties cannot explain the observed diversity in EPSPs. Instead EPSP diversity is based on differences in transmitter output at single synaptic foci: highly facilitating synapses with low quantal release occur only on proximally located muscle fibres and poorly facilitating synapses with high release occur only on distally located ones. Thus, the EPSP diversity from the single excitor axon to the lobster DAFM is largely presynaptic in origin.