Neuromuscular transmission at terminals of sprouted mammalian motor neurones

Motor nerve sprouting was induced in the tensor fasciae latae muscle of mice by partial denervation produced either by cutting (to prevent reinnervation) or crushing (to allow subsequent reinnervation) spinal nerve L4 unilaterally. The quantum content (m) of endplate potentials recorded intracellularly in vitro in the presence of high-Mg2+ and low-Ca2+ ion concentrations was determined up to 400 days later in non-reinnervated, reinnervated and contralateral control muscles. The muscles were then either fixed and stained with silver and cholinesterase for light microscopy, or fixed and examined in the electron microscope. The average value of m in control muscles increased by 4-5-fold as the animals matured in the 4 months following the operations. The average value of m at terminals of sprouted motor neurones in the absence of reinnervation also increased with time after partial denervation but was always less than the value in the corresponding control muscle. In electron micrographs of muscles following L4 section the nerve terminals closely apposed on average only two-thirds of the proportion of junctional folds apposed to terminals in control muscles. When muscles were reinnervated following L4 crush the average value of m at terminals of sprouted and reinnervating motor neurones equalled and sometimes exceeded m in contralateral control muscles. A proportion of muscle fibres had endplate potentials from reinnervating and sprouted axons, and the silver stain showed that these muscle fibres were innervated at the site of the original endplate. At these endplates the fraction of the total quantum content contributed by presumed sprout terminals fell significantly in the 4 months following L4 crush. It is concluded that: (i) in the absence of reinnervation, sprout terminals grow in size but a significant number never occupy all endplate site available to them; and (ii) in the presence of reinnervation axons terminals share some endplates with sprout terminals and grow at the expense of the sprout terminals which are eventually withdrawn from some shared endplates.