Histochemical properties and innervation pattern of fast and slow-tonic fibre types of the anterior latissimus dorsi muscle of the chick.

The anterior latissimus dorsi muscle of the chick is largely composed of slow-tonic fibres but contains a few fast fibres defined by their acid-labile, alkali-stable myofibrillar ATPase activity. These fibres are referred to as alpha fibres. Differing from the slow-tonic (alpha' and beta') fibres which are multiply innervated with en grappe endings, the alpha (fast) fibres are innervated by typical en plaque endings. The innervation of thirty-two alpha fibres were studied closely and it was concluded that about a half of the alpha fibres were focally innervated whereas a half were innervated in two distinct loci by en plaque endings. In only one case, a fast fibre with three widely spaced en plaque endings was observed. The mean intersynaptic length was significantly larger on alpha fibres than on alpha' and beta' fibres. No variation in the histochemical properties of myofibrillar ATPase was observed either along the entire length of singly and dually innervated alpha fibres, or along alpha' and beta' fibres. It is concluded that the three extrafusal fibre types of the anterior latissimus dorsi muscle maintain uniform histochemical characteristics along their length in spite of a possible innervation by several motoneurons.