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SUNMMARY1. The responsiveness of the spindle secondary ending to sinusoidal stretching has been studied using the soleus muscle of the anaesthetized cat. The sensitivity (impulses/sec firing per mm stretching) and the phase of the response were determined by computer averaging. The small linear range was studied at frequencies of 0-5-500 Hz, and also the larger nonlinear range at 1 Hz.2. In the linear range, stimulation of single fusimotor fibres (which were presumed to be static axons) approximately halved the sensitivity of the ending to low frequency stretching (up to 30 Hz), but did not produce any change in the phase of the response. Thus, from the point of view of motor function, fusimotor activity provides control of gain and a biasing signal, but not control of the relative sensitivity of the secondary ending to length and velocity.3. In contrast, such stimulation slightly increased the responsiveness of the secondary ending to high-frequency stretching (100-500 Hz) and slightly advanced the phase of the response above that of the passive ending.4. The results are discussed in relation to the effect of static fusimotor stimulation on the primary ending, and to findings on secondary endings in the decerebrate cat.

SUMMARY1. Irregularities in the development of tension during the tonic vibration reflex of the soleus muscle of the decerebrate cat have been analysed into their frequency components. The reflex was recorded isometrically and elicited by longitudinal vibration, normally at 150 Hz. The amplitude of vibration was set so as to elicit a maximal reflex response, suggesting 1:1 driving of the majority of the Ia afferents at the frequency of vibration.2. The resulting power spectrum regularly showed a well marked tremor peak separated by a trough from any slow irregularities. The predominant frequency of this tremor varied from 4 to 11 Hz in different preparations, with a mean of 7-4 Hz; on average, frequencies within 1-7 Hz on either side contained over half the power of the predominant frequency. Altering the frequency of vibration did not alter the distribution of tremor frequencies.3. The root mean square value of the tension irregularities, over the range [4][5][6][7][8][9][10][11][12][13][14]

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1. Human subjects attempted to maintain a constant force by flexing their elbow against a spring which was attached to a force transducer at one end and the subject's wrist at the other. The tremor at 8-12 Hz which develops in this situation was enhanced in amplitude with negligible change of frequency by applying vibration at 100 Hz to the tendon either of an agonist muscle (biceps brachii) or of the antagonist (triceps brachii). The tremor was assessed by spectral analysis of the force records and measurement of the peaks in the spectra. The compliance of the spring was normally 2.8 N/mm and target forces of 40-120 N were used. 2. The percentage increase in the tremor on applying vibration was relatively independent of target force, although the absolute amounts of tremor increased markedly with increasing target force. The average increase was greater for vibration of triceps than for biceps (70% as opposed to 37%, averaged between subjects and over a range of forces). 3. When the spring was replaced by a rigid connexion there was usually no clear tremor peak either in the presence or absence of vibration. Vibration, however, tended to increase the general noisiness of the force signal. 4. Qualitatively similar effects were seen when the elbow exerted an extending force so that triceps became the agonist and biceps the antagonist. 5. The tremor peak present in the spectrum of the demodulated electromyogram during vigorous tremor increased in size when vibration made the tremor larger. 6. The effect of a rhythmic afferent input was studied by modulating the amplitude of the vibration at 8-9 Hz, to correspond to the tremor frequency, while the subject pulled against a rigid attachment. Both the e.m.g. and the tension spectra contained peaks at the modulation frequency. The raw force records showed that, with reference to the modulation, the effects of biceps and of triceps vibration were approximately 180 degrees out of phase with each other, as would occur if vibration of one were having an excitatory action, and vibration of the other an inhibitory action. Moreover, in each case the effect on force (whether excitatory or inhibitory) lagged about half a cycle on the vibration envelope, as required for such reflexes to help in the generation of tremor. 7. It is suggested that vibration increases the modulation of Ia firing elicited by a given movement tremor and this, by means of the stretch reflex arc, enhanced the tremor. The powerful action of vibration of the antagonist illustrates, it would seem, the functional effectiveness under normal conditions of a spinal inhibitory pathway, most probably the Ia disynaptic route. The findings are also discussed in relation to the increase in stretch reflex gain that occurs in association with increasing strength of voluntary contraction.