Motor End-plates in Fast and Slow Muscles of the Chick after Cross-union of their Nerves

Abstract

SLOW and fast dorsal muscles of the chick have different types of motor innervation1–3. In the fast posterior latissimus dorsi, muscle fibres are supplied with single en plaque end-plates which have richly branched troughs and strong acetylcholinesterase activity. On the other hand, the slow anterior latissimus dorsi muscle consists of muscle fibres with multiple end-plates of the en grappe configuration, with weak acetylcholinesterase activity2,3. The two muscles differ in their electrical response to the stimulation of their respective motor nerves4. In the fast muscle, indirect stimulation by single shocks produces synchronous propagated action potentials. This is not the case in the slow anterior latissimus dorsi muscle. Although propagated action potentials are readily evoked in this muscle in vitro5, only local potentials are observed in response to a single nerve volley when electromyographic recording is carried out in vivo4. Propagated action potentials are not evoked except in some fibres by repetitive stimulation or during post-tetanic potentiation4.