Fatigue and neuromuscular block in mammalian skeletal muscle

Abstract

Records have been taken of the tetanic tensions of the decerebrate cat's soleus and tibialis anterior muscle, while these were excited maximally by direct electrical stimulation, or through the nerve at frequencies up to 250/sec. and for some 20 sec. The tension-time curves with both methods of stimulation were almost identical for any given frequency. Assessment of the neuromuscular block as fatigue progresses was made from observation of the tension changes which followed a sudden switch from direct to indirect excitation or vice versa. At frequencies above 40/sec., block can be demonstrated before there is any fall in tension. The block which develops is not absolute; fibres to which transmission is failing respond to some, but not to all nerve impulses. This enables block to be measured in terms of the average response frequency of the fibres. The response frequency of the muscle fibres during a tetanus depends only on the total number of stimuli which the nerve has received. The relation is of the form $\text{response frequency}=\frac{\alpha}{\text{(total stimuli)}^{b}}$, where a and b are constants independent of frequency and duration of nerve stimulation. These constants are such that the average response frequency of the muscle fibres has fallen to 25/sec. after the nerve has received about 1000 stimuli at a frequency of 100/sec. Fibres which are rested by block can, when they do respond, develop up to three times the tension-time of unfatigued fibres. The development of neuromuscular block is not responsible for the fall of tension which occurs as the system fatigues.