9 - Nerve and muscle

Summary

Here is a field of physiology that is full of quantification and mathematics, but opportunities for applying just shop-keeper's arithmetic are fewer than might be expected. In Sections 9.1 and 9.2 we look at conduction in nerve and cardiac Purkinje fibres. Section 9.3 takes us a tiny step in the direction of integrated neuromuscular activity, but also has the humbler aim of supplying a context in which to think about time scales and the speeds of neuromuscular events. Section 9.4 relates muscle mass to the activities of chinning the bar and jumping, in a simple, rough-and-ready way that avoids calculating forces in complicated lever systems. The approach leads naturally to a consideration of creatine phosphate usage in muscle contraction (Section 9.5). Finally, Section 9.6 integrates information on sarcomere dimensions, myofilament spacing and calcium concentration in the context of muscle activation. A few related topics are treated earlier in the book: the energetic efficiency of skeletal muscle in Section 3.9, tensions in arteriolar smooth muscle in Section 4.5, and resting membrane potentials and action potentials in Sections 7.6, 7.11 and 7.12.

One may memorize the numerical dimensions of skeletal muscle fibres, yet not perceive that some of these fibres have the lengths and widths of suitably chosen human hair. A class of students beginning physics was asked to estimate the height of the Empire State Building that could be seen from the window (1250 ft, or 381 m, without its television aerial). Estimates ranged from 50 ft (15 m) to 1 mile (1.6 km).