Morphological differences between excitatory and inhibitory nerve terminals in cockroach coxal muscles

Area of Science:

• Neuroscience
• Muscle Physiology
• Insect Morphology

Background:

• Neuromuscular junctions (NMJs) are critical for muscle function.
• Previous studies have physiologically characterized fast, slow, and inhibitory nerve fibers innervating cockroach leg muscles.
• The morphological basis for these distinct physiological roles at the NMJ remains unclear.

Purpose of the Study:

• To morphologically characterize the neuromuscular junctions in the coxal leg muscles of the cockroach, Periplaneta americana.
• To correlate observed morphological differences with previously established physiological classifications of nerve fibers (fast, slow, inhibitory).

Main Methods:

• Light microscopy was used to examine the ultrastructure of neuromuscular junctions.
• Axon terminals and their relationship with muscle fibers were analyzed.
• Synaptic vesicle morphology and the nature of the muscle fiber contact were documented.

Main Results:

• Two distinct morphological types of NMJs were identified: short contact (SC) and long contact (LC).
• SC-type NMJs feature axon terminals with numerous round, clear synaptic vesicles contacting sarcoplasmic extensions of the muscle fiber over a short distance.
• LC-type NMJs exhibit axon terminals with polymorphic synaptic vesicles directly contacting the muscle fiber over a longer region.

Conclusions:

• The SC type of neuromuscular junction is proposed to represent both fast and slow nerve terminals based on morphological and physiological correlations.
• The LC type of neuromuscular junction is suggested to be associated with inhibitory nerve terminals.
• This study provides a morphological framework for understanding the functional diversity of neuromuscular transmission in insects.