The formation of synapses in striated muscle during development

TLDR

The study examined synapse formation in developing striated muscles with focal (rat hemidiaphragm) or distributed (avian anterior latissimus dorsi) innervation using histological, ultrastructural, and electrophysiological methods. In the diaphragm, a single random synaptic contact forms on short myotubes, whereas in the avian ALD multiple contacts form on long myotubes spaced at least 170 µm apart; each initial contact later receives additional axons confined to the same sites, a process that ceases after four weeks, suggesting the first axon induces a refractory zone whose length is greater for focal than for distributed innervation. 1.

Abstract

1. A study has been made of the formation of synapses in developing striated muscles which receive either a focal (the rat hemidiaphragm) or a distributed (the avian anterior latissimus dorsi) innervation using histological, ultrastructural and electrophysiological techniques.2. In the developing diaphragm only a single synaptic contact was initially established at random along the length of the short (300 mum) myotubes by a single axon; in the developing ALD more than one synaptic contact could be established initially along the length of the long (2500 mum) myotubes by axons, but the distance between these was never less than 170 mum.3. Each synapse established by the initial axonal contact in either the diaphragm or the ALD subsequently received a multiple innervation from further exploring axons in the muscles, and all such additional innervation of muscle cells was constrained to the sites of the initial synaptic contacts; this multiple innervation of synaptic sites was lost in the subsequent 4 weeks.4. It is suggested that the axon forming the initial synaptic contact on myotubes induces a property over an adjacent length of myotube which makes its membrane refractory to synapse formation over this length; this characteristic length is longer for axons forming a focal innervation than it is for those forming distributed innervation.