The skeleto-fusimotor or beta innervation was compared in cat peroneus brevis and peroneus tertius muscles, which differ in their composition of fatigue-resistant motor units; the slow (S) units predominate in brevis and the fast units (FR) in tertius. 2. In four brevis muscles, of thirty-four beta-axons (from a total of 114 axons supplying extrafusal muscle fibres) twenty-nine were dynamic (beta D) and only five static (beta S). In contrast, in three tertius muscles, of twenty-five beta-axons (from a total of 82 axons) twelve were static and thirteen dynamic. 3. In a population of thirty-five brevis and thirty tertius spindles, the proportion of beta D-innervated spindles was greater in the brevis (68.5%) than in the tertius (50%) whereas that of beta S-innervated spindles was greater in the tertius (40%) than in the brevis (17.1%). In a population of thirty-two brevis and twenty-seven tertius spindles in which the presence of bag1 fibres was deduced from the existence of a dynamic innervation, the proportion of spindles innervated by beta D-axons was 80% in the brevis and 62% in the tertius. 4. In both muscles, the number of beta D effects was greater than that of beta S effects. beta S-axons were rarely found to supply more than one spindle whereas beta D-axons supplying more than one spindle (up to four) were common. Spindles were often coinnervated by beta D- and beta S-axons.(ABSTRACT TRUNCATED AT 250 WORDS)
