A Comparative Study of Membrane Properties of Innervated and Chronically Denervated Fast and Slow Skeletal Muscles of the Rat

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Abstract
The electrophysiological properties of the innervated extensor and soleus muscles are in most aspects similar. In low [Na]o, however, the rate of rise and the amplitude of the action potential were lower in the soleus than in the extensor muscle. The ACh sensitivity in the innervated extensor muscle is limited to the end‐plate region, while in the soleus the sensitivity extends beyond the end‐plate towards the tendon. Denervation brought a marked change in membrane properties. The electrical time constant of the membrane increased in both muscles almost threefold, and the calculated transverse resistance of unit are of the membrane in the extensor from 372 Σ cm2 to 759 Σ cm2, while in the soleus the values were 393 Σ cm2 to 458 Σ cm2. Denervation increased the threshold for action potential generation and reduced significantly the rate of rise and the amplitude of the spike. These changes were more apparent in the soleus. It is suggested that the decrease in excitability may reflect the lengthening of the time constant and possibly a reduction in the number of membrane sites connected with the passive ionic transport. The ACh sensitivity of denervated muscles covered the entire membrane surface and was similar in both muscles.

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