Glycogen depletion of bag1 fibers elicited by stimulation of static gamma axons in cat peroneus brevis muscle spindles.

1. The distribution of static fusimotor axons to intrafusal muscle fibres in cat peroneus brevis spindles has been studied with the glycogen depletion method. 2. In each of six experiments three to seven static axons were stimulated. The muscle was subsequently quick‐frozen and cut in serial transverse sections that were stained for glycogen. In each muscle, nearly all the spindles were examined for depletion. 3. Intrafusal muscle fibres displaying zones of complete glycogen depletion were observed in fifty‐two whole spindles and seven half‐spindles. Chain fibres were depleted in forty‐eight (92%) of the whole spindles, bag2 fibres in thirty‐five spindles (67%) and bag1 fibres in nineteen spindles (36%). 4. Seven other experiments were performed to test whether small amplitude sinusoidal stretching (30‐‐100 Hz) of the muscle might produce glycogen depletion in bag1 fibres. Two hundred and seven spindles were examined. In nearly all of them the glycogen content of the intrafusal muscle fibres was normal. Two muscles had limited atrophic portions within which a few depleted spindles were found. 5. These experiments show that the intrafusal distribution of static gamma axons is not restricted to chain and bag2 fibres but that in a significant number of spindles the bag1 fibres are also supplied by static gamma axons.