Comparison of perinatal and adult multi-innervation in human laryngeal muscle fibers.

The innervation of human laryngeal myofibers appears distinct from that of skeletal myofibers, because some of them exhibit multiple neuromuscular junctions. We attempted to understand the significance of the multi-innervation phenomenon by comparing intrinsic laryngeal muscles obtained from autopsies of a fetus, a stillbirth, and a 7-month-old infant to muscles from adults. In longitudinal sections (40 to 60 microm thick) the cholinesterase sites and the nerve terminals were stained simultaneously for light microscopy. Multi-innervation reached no more than 23% at 7 months, and this percentage decreased notably with age. Until 7 months, end plates were innervated either by single or by multiple axons (unineuronal and polyneuronal innervation), whereas in adults, the pattern was exclusively unineuronal. Moreover, the structure of neuromuscular junctions in the young was limited to 1 spot of acetylcholinesterase, while in adults the end plate zones increased in size and complex axon terminals were observed. These observations are consistent with a delayed maturation of human laryngeal innervation compared to that of skeletal muscles.