Innervation of muscle in the cestode Hymenolepis microstoma

Abstract

The innervation of muscle in the cestode Hymenolepis microstoma was investigated by analysis of serial thin sections in the electron microscope. The myocytons, containing the nucleus and cell organelles, formed sarcoplasmic processes that contained the myofibers. Junctional complexes resembling gap junctions were found between sarcoplasmic processes. Sarcoplasmic extensions from the myofibers passed to the segmental ring commissures and formed sarconeural junctions. The presynaptic component of the sarconeural junctions was characterized by an axon terminal, or axon en passant, containing electron-lucent and occasional electron-dense synaptic vesicles, occasional mitochondria, and presynaptic dense projections. The presynaptic terminal was isolated from the postsynaptic terminal by a synaptic cleft of ~20 nm. The postsynaptic sarcoplasmic process, which often was wrapped around the nerve terminal, displayed a postsynaptic thickening. Single axon terminals were observed to form sarconeural junctions with up to four sarcoplasmic processes. Sarcoplasmic processes formed synaptic contacts with more than one axon, and sarcoplasmic processes formed more than one synaptic contact with a single axon, suggesting a polyneuronal multiterminal pattern of innervation. Serial synapses were found in conjunction with sarconeural junctions in the suckers. The functional significance of this type and pattern of innervation of muscle in H. microstoma is discussed.