Figure A.3 Skeletal Muscle Comparison. Us refers to vertebrate skeletal muscle. Each muscle fiber receives one synapse from one neuron. Synaptic stimulation results in both a PSP and an AP in the muscle fiber. Them refers to crustacean skeletal muscle. Each muscle fiber can receive multiterminal input (more than one synapse from a single neuron), polyneuronal input (synapses from more than one neuron), and inhibitory input (shown in red; inhibitory synapses may be directly on the muscle or may presynaptically modulate excitatory synapses). In the superficial flexor, synaptic stimulation causes only PSPs; the muscle fiber does not fire an AP.