146 Muscles /8 : 3 



potential of skeletal muscle fibers. An additional difficulty is that the 

 muscle fiber moves during contraction. Provision must be made to 

 permit the microelectrode to move with the fiber. When this is done, 

 consistent records can be obtained of the potentials across the sarco- 

 lemma of single muscle fibers. 



The spike potential always precedes contraction. After the crest of 

 the spike has passed, the membrane potential starts to return to normal. 

 At this time, the rate . of heat production increases. A fraction of a 

 millisecond later, there is a slight relaxation, and then the mechanical 

 contraction of the twitch starts. How the spike potential "signals" the 

 muscle fiber to begin the chemical changes necessary for a twitch is 

 completely unknown. Nonetheless, the spike always precedes a twitch 

 and somehow all the myofibrils do contract simultaneously. 



Within all skeletal muscles are sensing organs known as proprioceptors 

 or pacinian corpuscles. These continuously send back "reports" to the 

 central nervous system on the state of contraction of the muscle. Thus, 

 in any muscular motion, a complex process occurs involving multiloop 

 feedback systems. The nervous system signals the muscle to contract. 

 As it does so, the muscle sends many reports indicating its contraction 

 to the central nervous system. These and similar proprioceptor reports 

 from other muscles reach the central nervous system where they are all 

 "analyzed." As a result of this analysis, the original muscle is "in- 

 structed" or controlled to contract faster or slower so as to achieve the 

 desired location. This process has appealed to servomechanism experts 

 who have carried out quite detailed analyses of muscular contraction. 

 Although such analyses can never supply new facts, they have made it 

 possible to understand qualitatively the organizing principles of the 

 muscle-nervous system relationship. 



The problem of muscular fatigue also appears to involve the nervous 

 system. A denervated muscle can be held in tetany by repeated 

 stimulation until it tires. However, if the motor nerve causing a muscle 

 to contract is stimulated, it can be shown that the myoneural junction 

 fatigues before the muscle does. Similarly, if the entire normal animal is 

 stimulated (for example, by poking it with a hot soldering iron), it can 

 be shown that fatigue sets in at the synapses in the central nervous 

 system before the myoneural junction has fatigued. 



C. Heat Production 



Besides studying forces, work, and electrical changes, several bio- 

 physicists have followed a quite different approach, namely the measure- 

 ment of the heat produced by resting and contracting muscles. Muscles 

 produce extra heat when they are working; the extra heat accompanies 



