SUMMARY1. High frequency vibration was applied to the tendon of the noncontracting triceps surae muscle while recording the background discharges of single y efferents from the nerves to the gastrocnemius muscles in the decerebrate cat. For isolation of y fibres only small nerve bundles were cut, leaving most of the nerve supply to the triceps intact.2. 22 % out of a total of sixty-three y efferents were tonically inhibited by vibration. The inhibition appeared between 25 and 50 #sm peak-to-peak amplitude of vibration and increased to a plateau for amplitudes of about 100 gm. The dependence of the tonic vibration reflex of a efferents on the amplitude of vibration was found to be similar. Increasing the frequency of vibration from 150 to 300 Hz increased the degree of inhibition.3. 33 % of the fusimotor neurones investigated responded to muscle vibration with an increase in discharge rate. The threshold amplitudes of this reflex ranged from 20 to 50 /m. Some features of the reflex, in particular the parallel post-vibratory facilitation found in a and y efferents, pointed to a polysynaptic pathway organized in an c-y linkage.4. All y efferents inhibited by vibration showed inhibitory responses to antidromic stimulation of the parent ventral root, and most of them were inhibited by ramp stretch of the triceps. The y motoneurones facilitated by vibration, however, were excited by muscle stretch and were less susceptible to antidromic inhibition, some lacking it completely.5. Cutting the nerves to triceps abolished the inhibitory as well as the excitatory responses of y efferents to muscle vibration. Both fusimotor reflexes were preserved after spinal section and subsequent administration of L-DOPA. 6. It is concluded that both of the fusimotor reflex effects of vibration are caused by excitation of primary spindle endings within the triceps.

A B STRACT The effect of hypertonic solutions on the tension of isolated twitch muscle fibers of the frog has been investigated. Increased tonicity up to about 1.7 times normal (1.7 T) caused a very small, graded, maintained tension increase. Above about 1.7 T a large, transient contracture response was superimposed on the small tension. The contracture response was graded with tonicity and reached a maximum at 2.5 T of 108 25 mN mm 2 , a third of the maximum tetanic tension in isotonic solution. Contracture tension developed with a delay which decreased with increased tonicity. The contracture threshold was lower and the delay shorter in small fibers than in large. Contractures were obtained equally well in depolarized as in polarized fibers. They were completely suppressed by 0.1-0.5 mM tetracaine. The possible mechanism responsible for the tension-inducing effect of hypertonic solutions is discussed in terms of the close similarity between the properties of these contractures and those caused by caffeine, and it is suggested that the effect is due to a release of calcium from internal stores.

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Resting tension and short-range elastic properties of isolated twitch muscle fibers of the frog have been studied while bathed by solutions of different tonicities. Resting tension in isotonic solution at 2.3-jsm sarcomere spacing averaged 0.46 mN-mm-2 and was proportional to the fiber crosssection area. Hypertonic solutions, containing 0.1-0.5 mM tetracaine to block contracture tension, caused a small sustained tension increase, which was proportional to the fiber cross-section area and which reached 0.9 mN mm: at two times normal tonicity (2T). Further increases in tonicity caused little increase in tension. Hypotonic solutions decreased tension. Thus, tension at 2.3 m is a continuous, direct function of tonicity. The dependence of tension on tonicity lessened at greater sarcomere lengths. At 3.2 /xm either a very small rise or, in some fibers, a fall in tension resulted from an increase in tonicity. Hypertonic solutions also decreased the tension of extended sarcolemma preparations. In constant-speed stretch experiments the elastic modulus, calculated from the initial part of the stretch response, rose steeply with tonicity over the whole range investigated (1-2.5T). The results show that tension and stiffness of the short-range elastic component do not increase in parallel in hypertonic solutions.

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