Chapter 12b Muscles 2013 Pearson Education Inc Single twitches Muscle relaxes completely between stimuli N A V I G A T O R Figure 12 16a 1 of 4 One twitch i n o s n e T 0 100 200 300 400 500 Time msec Ca2 signal Contraction relaxation Muscle twitch Figure 12 16b 2 of 4 Summation Stimuli closer together do not allow muscle to relax fully N A V I G A T O R i n o s n e T Summed twitches 0 100 200 300 400 500 Time msec Ca2 signal Contraction relaxation Muscle twitch Summation leading to unfused tetanus Stimuli are far enough apart to allow muscle to relax slightly between stimuli N A V I G A T O R Maximum tension Unfused tetanus Figure 12 16c 3 of 4 i n o s n e T Time msec Ca2 signal Contraction relaxation Muscle twitch Figure 12 16d 4 of 4 Summation leading to complete tetanus Muscle reaches steady tension If muscle fatigues tension decreases rapidly N A V I G A T O R Maximum tension Complete tetanus Single twitch tension Fatigue causes muscle to lose tension despite continuing stimuli i n o s n e T 0 Time msec Ca2 signal Contraction relaxation Muscle twitch Figure 12 17 MOTOR UNITS A motor unit consists of one motor neuron and all the muscle fibers it innervates A muscle may have many motor units of different types One muscle may have many motor units of different fiber types SPINAL CORD Neuron 1 Neuron 2 Neuron 3 Motor nerve Muscle fibers KEY Motor unit 1 Motor unit 2 Motor unit 3 Contraction of Motor Units Interactive Physiology Animation Muscular System Contraction of Motor Units 2013 Pearson Education Inc Contraction Force Recruitment of additional motor units by the nervous system increases contraction force Asynchronous recruitment of motor units helps avoid fatigue Different motor units take turns maintaining tension 2013 Pearson Education Inc Mechanics of Body Movement Isotonic contractions create force and move a load Concentric action is a shortening action Eccentric action is a lengthening action Isometric contractions create force without moving a load Series elastic elements Sarcomeres shorten while elastic elements stretch resulting in little change in overall length 2013 Pearson Education Inc Figure 12 18a 1 of 2 Isotonic Contraction Muscle contracts Muscle relaxes Muscle relaxes Force required to move load 35 30 25 20 15 10 5 l g k d e p o e v e d n o s n e T i 20 kg 20 kg Load moves Time Muscle stimulated Figure 12 18b 2 of 2 Isometric Contraction Muscle contracts Muscle relaxes Force required to move load Muscle relaxes 35 30 25 20 15 10 5 l g k d e p o e v e d n o s n e T i 30 kg Load does not move 30 kg Time Muscle stimulated Figure 12 19 SERIES ELASTIC ELEMENTS IN MUSCLE Muscle has both contractile components sarcomeres shown here as a gear and ratchet and elastic components shown here as a spring Triceps muscle Biceps muscle Elastic components Contractile components Elastic elements allow isometric contractions In an isometric contraction sarcomeres shorten generating force but elastic elements stretch allowing muscle length to remain the same In isotonic contractions sarcomeres shorten more but because elastic elements are already stretched the muscles shorten Elastic element Sarcomeres h t g n e l e l c s u M Muscle at Rest Isometric Contraction Muscle has not shortened Isotonic Contraction The entire muscle shortens Levers and Fulcrums A lever is a rigid bar that pivots around a point called a fulcrum The bones form levers Flexible joints form fulcrums Asynchronous recruitment of motor units helps avoid fatigue Different motor units take turns maintaining tension 2013 Pearson Education Inc Figure 12 20a 1 of 4 The lever system of the forearm is like that of a fishing pole The fulcrum is at one end of the lever and the load is at the other end Force is applied between the fulcrum and the load KEY Fulcrum Applied force Movement of load Load Lever Fulcrum Figure 12 20b 2 of 4 The human forearm acts as a lever The fulcrum is the elbow joint The load is gravity acting on the mass of the forearm and hand Biceps muscle Lever Load Fulcrum KEY Fulcrum Applied force Movement of load Load Lever Figure 12 20c 3 of 4 Force calculations Biceps contraction creates upward force F1 The biceps inserts into the lever 5 cm from the fulcrum Rotational forceup biceps force F1 5 cm from the fulcrum F1 F2 2 kg 5 cm 15 cm The weight of the forearm exerts a downward force of 2 kg at its center of gravity which is 15 cm from the fulcrum Rotational forcedown load F2 15 cm 2 kg 15 cm To hold the arm stationary at 90 degrees the rotational force created by the contracting biceps must exactly oppose the downward rotation created by the forearm s weight Rotational forceup Rotational forcedown Biceps forceup 5 cm 2 kg 15 cm Biceps force 30 kg cm 5 cm Biceps force 6 kg A 7 kg load is added to the hand 25 cm from the elbow FIGURE QUESTION How much additional force must the biceps exert to keep from dropping the weight F1 D1 D2 5 cm 25 cm Figure 12 21 LOAD VELOCITY RELATIONSHIP IN SKELETAL MUSCLE A i g n n e t r o h s f o y t i c o e V l 0 GRAPH QUESTIONS 1 At what point on the line is contraction isometric 2 At what point is the muscle contraction at maximum velocity Load on the muscle B Muscle Disorders Muscle cramp sustained painful contraction Overuse Disuse leads to atrophy Acquired disorders Inherited disorders Duchenne s muscular dystrophy Dystrophin McArdle s disease Myophosphorylase deficiency Glycogen not converted to glucose 6 phosphate 2013 Pearson Education Inc Smooth Muscle Classification Vascular gastrointestinal urinary respiratory By location reproductive ocular By contraction pattern Phasic smooth muscles Tonic smooth muscles By communication with neighboring cells Single unit smooth muscle or visceral smooth muscle Multi unit smooth muscle 2013 Pearson Education Inc Figure 12 22 e c r o f n o i t c a r t n o C e c r o f n o i t c a r t n o C Time Time SMOOTH MUSCLE CONTRACTIONS A phasic smooth muscle that is usually relaxed Example esophagus A phasic smooth muscle that cycles between contraction and relaxation Example intestine A tonic smooth muscle that is usually contracted Example a sphincter that relaxes to allow material to pass A tonic smooth muscle whose contraction is varied as needed Example vascular smooth muscle e c r o f n o i t c a r t n o C e c r o f n o i t c a r t n o C Time Time Figure 12 23a 1 of 2 Single unit smooth muscle cells are connected by gap junctions and the cells contract as a single unit Autonomic neuron varicosity Gap junctions Smooth …