PRINCIPLES OF PHYSIOLOGY \Q\ 



relatively fixed when a muscle contracts is known as its origin; the end 

 which moves is called the insertion; and the thick part between the two 

 is called the belly of the muscle. Thus, the biceps, which bends or flexes 

 the forearm, has its origin on the scapula and on the upper end of the 

 humerus, and its insertion on the radius in the forearm, fts antagonist, 

 the triceps, which straightens or extends the forearm, has its origin on the 

 scapula and upper part of the humerus and its insertion on the ulna. 

 The contraction of a muscle is stimulated by a nerve impulse reaching 

 it via a motor nerve fiber from the central nervous system. The drug 

 curare, the chief ingredient of the arrow poison used by the South Ameri- 

 can Indians, blocks the junction between nerve and muscle so that 

 impulses cannot pass and the muscle is paralyzed. A curare-paralyzed 

 muscle can still be caused to contract by direct electric stimulation, a 

 demonstration that muscle is independently irritable. 



The Mechanism of Muscular Contraction. The functional unit of 

 vertebrate muscles is called the motor unit. This consists of a single 

 motor neuron and the group of muscle cells innervated by its axon, all 

 of which will contract when an impulse travels down the motor neuron. 

 In man, it is esthnated that there are some 250,000,000 muscle cells but 

 only some 420,000 motor neurons in spinal nerves. Obviously, some 

 motor neurons must innervate more than one muscle fiber. The degree 

 of fine control of a muscle, its delicacy of action, is inversely proportional 

 to the number of muscle fibers in the motor unit. The muscles of the 

 eyeball, for example, have as few as three to six fibers per motor unit, 

 whereas the leg muscles have perhaps 650 fibers per unit. 



If a single motor unit is isolated and stimulated with brief electric 

 shocks of increasing intensity beginning with shocks too weak to cause 

 contraction, there will be no response until a certain intensity is reached, 

 then the response is maximal. This phenomenon is known as the "all 

 or none effect." In contrast, a whole muscle, made of many individual 

 motor units, can respond in a graded fashion depending upon the num- 

 ber of motor units which are contracting at any given time. 



A muscle given a single stimulus, a single electric shock, responds 

 with a single quick twitch. The changes which accompany a single twitch 

 are shown in Figure 5.10. A twitch consists of (1) a very short latent 

 period, the interval between the application of the stimulus and the 

 beginning of the contraction, (2) a contraction period, during which 

 the muscle shortens and does work, and (3) a relaxation period, longest 

 of the three, during which the muscle returns to its original length. The 

 latent period represents the interval between the conduction of the action 

 current and the completion of the changes in the structure of the 

 actomyosin which enable it to contract. The first event after the stimu- 

 lation of a muscle is the initiation and propagation of an electrical 

 response, the muscle action potential, followed by the changes in the 

 structure of actomyosin observed as a change in the total birefringence of 

 the muscle, by its shortening, and by the production of heat. Following 

 a twitch there is a recovery period during which the muscle is restored 

 to its original condition. If a muscle is stimulated repeatedly at intervals 

 short enough so that succeeding contractions occur before the muscle 



