Role of Activity in Determining Properties of the Neuromuscular System in Crustaceans

Abstract

SYNOPSIS. Crustacean muscle fibers, like those of higher vertebrates, are diversified in physiology, morphology, and biochemical attributes. However, unlike motor units of mammals, those of crustaceans usually do not contain fibers of uniform type. Motor neuron activity acts as a unifying force for the motor units of mammalian muscles, but its role in determining properties of crustacean motor units is less well defined. In certain crustacean muscles, differential activity of sensory-motor systems is important for establishing muscle fiber properties during early development. In freshwater crayfish, neuromuscular junctions of a phasic motor neuron are altered physiologically and morphologically by chronic stimulation; the adapted junctions release less transmitter per impulse and are more fatigue-resistant than naive junctions. The muscle fibers may also adapt to chronic stimulation, but less dramatically and at a slower rate. The adaptive responses of the neuromuscular junction can be achieved through manipulation of sensory input and with little increase in motor impulse activity. This suggests that altered protein synthesis is triggered centrally by synaptic input to the motor neuron. In general, present evidence suggests that long-term adaptation of neuromuscular junctions and muscle fibers of crustaceans can occur in response to altered activity in the nervous system, in spite of the fact that certain muscle fiber properties appear to be genetically predetermined. Some aspects of matching between neuromuscular junction and muscle fiber appear to be determined in response to growth of the muscle fiber; other features are activity-dependent; and some may result from expression of inherent neuronal properties.