Formation and Regression of Synaptic Contacts in the Adult Muscle

Abstract

A large amount of information has been accumulating over the last 30 years indicating that the adult motoneuron is capable of producing extensive peripheral sprouting under a variety of conditions. Sprouts can reoccupy abandoned synaptic sites, form new ones, or might remain or be withdrawn without forming synaptic contacts. Such abnormal outgrowth of branches, which was first described after partial denervation of a muscle (van Harreveld 1945; Weiss and Edds 1945; Edds 1950; Hoffman 1950), includes collateral sprouts which originate from the nodes of Ranvier, preterminal sprouts and sprouts from the endplate itself (‘ultraterminal’ sprouts). Consequently, when searching for sprouts most investigators have been looking for nerve branches which appear “abnormal” in the sense that they leave the vicinity of the parent endplate and do not regularly occur under normal conditions (see Sect. 2.1). This is demonstrated in Fig. 1, which shows a typical endplate from a normal mouse muscle (Fig. 1A) and an endplate after local application of tetanus toxin (Fig. 1B); in the latter unusual branches are present (arrows) which leave the endplate area, often to reach neighboring muscle fibers (cf. Duchen and Tonge 1973).