Proceedings of the Physiological Society, 20‐21 December 1982, Birmingham Meeting: Communications

Abstract

Most skeletal muscles are attached through tendons, the elastic properties of which have a significant effect on their activity. Only those parts of a tendon which lie outside the body of a muscle can be subjected to normal mechanical testing, but tendinous fibres extend into many muscles, and their lengths may far exceed the muscle fibres themselves. In this paper we describe a method of measuring the stiffness of the entire tendinous attachment of the cat's soleus muscle.Muscle spindles are used as indicators of muscle fibre length. Spindles are very sensitive to changes in muscle length, but their afferents become silent when the muscle contracts and this silence is attributed to unloading of the spindles as muscle fibres shorten at the expense of their tendinous attachments.We have sinusoidally stretched the muscle-tendon combination while sinusoidally modulating the frequency of the stimulus to the muscle fibres. We have then adjusted movement and stimulus until their effects on the muscle spindle offset each other, so that the afferent activity from the muscle spindle was no longer modulated by the movement. In this null situation, we assume that the muscle fibres were remaining at constant length, and that all the imposed movement was taken up in the tendinous attachments. Measurements of the movement and of the force necessary to sustain it would then give a value for the stiffness of the tendon alone. By adjusting the parameters of muscle stimulation, we determined this stiffness for different mean forces of contraction.Cats were anaesthetized with pentobarbitone sodium and recordings were made from the primary afferent fibres of soleus while the tendon was subjected to sinusoidal movement of small amplitude (up to 02 mm) at a frequency of about 2 Hz. The ventral nerve roots that supplied soleus were split into four or five divisions, and the muscle was activated by stimulating the divisions in sequence to obtain smooth contractions with low stimulus rates (Rack & Westbury, 1969). The rate of stimulation was then modulated sinusoidally about a mean level, and the mean rate, the depth and the phase of modulation were varied in relation to the movement until the modulation of the afferent discharge from the muscle spindles was minimized.Measurements indicated that the tendon behaves as an elastic structure with a stiffness that is related to the force of contraction, varying between 2 N/mm at low forces and 20 N/mm at high forces. Soleus muscles of young rats are composed of two types of motor units -slow ones, and fast fatigue-resistant ones. Thus they have two populations of motoneurones that supply the muscle. Disconnecting motoneurones from the soleus muscle by nerve crush at birth leads to death of a proportion of motoneurones (Romanes, 1946; Zelenai & Hnik, 1963). In this study we investigated which of the two types of motoneurones is more resistant to nerve injury. In addition we assessed the size of the peripheral field of the surviving motoneurones and their size.The sciatic nerve of newborn ra...