“Fast” and “slow” skeleto-fusimotor innervation in cat tenuissimus spindles; a study with the glycogen-depletion method

The glycogen-depletion method was used to investigate the motor supply to tenuissimus with respect to the presence of fast β axons and to assess the total proportion of both fast and slow β-innervated spindles in this muscle. In a first series of 5 expts., groups of motor axons with conduction velocities higher than 85 m/s were repetitively stimulated so as to produce glycogen depletion in the muscle fibres they innervated. The whole muscle was then quick-frozen, serially cut, stained to demonstrate glycogen and examined for intrafusal glycogen depletion. Zones of glycogen depletion were found in 16 of the 46 examined spindles; they were most frequently located in the longest of the chain intrafusal muscle fibres. Since it is known that there are no purely fusimotor axons to tenuissimus with conduction velocities above 50 m/s, it was concluded that β axons are present among the fastest axons to this muscle. In a second series of 5 expts. as many motor axons as possible with conduction velocities above 60 m/s were stimulated. Zones of glycogen depletion were found in 19 of the 47 examined spindles. They affected chain fibres in about half of the instances and bag₂ fibres in the others. As this latter location is characteristic of slow dynamic β axons, it was concluded that both slow and fast β axons occur regularly in the motor supply to tenuissimus. β-innervation is present in at least 40% of tenuissimus spindles with almost no convergence of fast and slow β axons onto the same spindle.