A quantitative study of skeletofusimotor innervation in the cat peroneus tertius muscle.

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Abstract
Physiological tests were used to identify skeletofusimotor or .beta. axons to the cat peroneus tertius muscle to assess the proportion of .beta. axons in the motor supply to this muscle. Static .beta. axons (.beta.S) were identified by observation of a delay between the complete block of extrafusal contraction and the failure of spindle activation upon prolonged stimulation, increase of spindle excitation with stimulation frequencies above that eliciting maximal extrafusal contraction, observation of unfused frequencygram of spindle primary afferent discharge during stimulation of the axon at frequencies above that eliciting complete fusion of extrafusal contraction, and static action exerted on the response of the spindle afferent to ramp stretch. Dynamic .beta. axons (.beta.D) were identified by the persistence of spindle activation after selective block of extrafusal neuromuscular junctions and by their dynamic action on spindle primary endings. The actions of 116 motor axons (conduction velocity 56-104 m/s) on 95 spindle afferents (57 from primary and 38 from secondary endings) were examined in 10 experiments. .beta.-Axons (36%; 31% of the total sample) were identified: 24 .beta.S (conduction velocity 69-104 m/s) and 12 .beta.D (conduction velocity 56-91 m/s). Twenty (35%) primary endings were activated by a .beta.S and 16 (28%) by a .beta.D axons. Nineteen (45%) secondary endings were activated by a .beta.S and 5 (13%) by a .beta.D axons. Convergence of .beta.D and .beta.S axons on the same spindle occurred in 10% of instances. .beta.-innervated spindles were also supplied by .gamma. axons. Most of the .beta.S motor units were of the fast-fatigue resistnt (FR) type, with a few units of the fast-fatiguable (FF) type, and nearly all the .beta.D motor units were of the slow (S) type.