Physiology of skeletal muscle contraction – events at the myofilaments Resting sarcomere Active-site exposure ADP Myosin head ADP + Sarcoplasm P + P Troponin Ca2+ Ca2+ Tropomyosin Actin Active site ADP ADP P + P + Calcium (Ca+2 ) gates in the SR open, allowing Ca+2 to diffuse into the sarcoplasm Calcium will bind to troponin (on the thin myofilament), causing it to change its shape. This then pulls tropomyosin away from the active sites of actin molecules. Figure 7-5 3 of 7 Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Physiology of skeletal muscle contraction – events at the myofilaments Resting sarcomere Active-site exposure Cross-bridge formation ADP Myosin head ADP + P + Sarcoplasm P Troponin ADP + P Ca2+ Ca2+ Ca2+ ADP Tropomyosin Actin Active site Ca2+ + ADP ADP P P + P + Myosin heads are “energized” by the presence of ADP + PO43- at the ATP binding site (energy is released as phosphate bond of ATP breaks) Once the active sites are exposed, the energized myosin heads hook into actin molecules forming cross-bridges Figure 7-5 4 of 7

The ADP & phosphate group are released from the myosin head Physiology of skeletal muscle contraction – events at the myofilaments Resting sarcomere Active-site exposure Cross-bridge formation ADP Myosin head ADP + + Sarcoplasm P P Troponin ADP + P Ca2+ Ca2+ Ca2+ ADP Tropomyosin Actin Active site Ca2+ P + ADP ADP P + P + Pivoting of myosin head Using the stored energy, the attached myosin heads pivot toward the center of the sarcomere The ADP & phosphate group are released from the myosin head ADP + P Ca2+ Ca2+ ADP + P Figure 7-5 5 of 7 Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Physiology of skeletal muscle contraction – events at the myofilaments Resting sarcomere Active-site exposure Cross-bridge formation ADP Myosin head ADP + + Sarcoplasm P P Troponin ADP + P Ca2+ Ca2+ Ca2+ ADP Tropomyosin Actin Active site Ca2+ P + ADP ADP P + P + A new molecule of ATP binds to the myosin head, causing the cross bridge to detach from the actin strand The myosin head will get re-energized as the ATP  ADP+P Cross bridge detachment Pivoting of myosin head ATP ADP + P Ca2+ Ca2+ Ca2+ Ca2+ ATP ADP + P As long as the active sites are still exposed, the myosin head can bind again to the next active site

Physiology of skeletal muscle contraction – events at the myofilaments Resting sarcomere Active-site exposure Cross-bridge formation ADP Myosin head ADP + P + Sarcoplasm P Troponin ADP + P Ca2+ Ca2+ Ca2+ ADP Tropomyosin Actin Active site Ca2+ P + ADP ADP P + P + Myosin reactivation Cross bridge detachment Pivoting of myosin head ADP ATP ADP + P + P Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ ADP P + ATP ADP + P

Muscle fatigue: Prolonged and strong muscle contraction lead to muscle fatigue. This results from: Depletion of glycogen stores in the muscle (inability of the contractile and metabolic processes to continue supplying the same work). Diminished transmission in the NMJ. Interruption of blood supply or flow results in nutrient and O2 deficiency.

Hypertrophy – “stressing” a muscle (i. e Hypertrophy – “stressing” a muscle (i.e. exercise) causes more myofilaments/myofibrils to be produced within muscle fibers; allows for more “cross bridges” resulting in more force (strength) as well as larger size There are always some motor units active, even when at rest. This creates a resting tension known as muscle tone, which helps stabilize bones & joints, & prevents atrophy Play IP Contraction of motor units p. 3-7

Muscle Hypertrophy Muscle Atrophy Muscle growth from heavy training increases diameter of muscle fibers increases number of myofibrils increases mitochondria, glycogen reserves Muscle Atrophy Lack of muscle activity reduces muscle size, tone, and power

Tension Production Tension of a Single Muscle Fiber The all–or–none principal: as a whole, a muscle fiber is either contracted or relaxed Tension of a Single Muscle Fiber Depends on: the number of pivoting cross-bridges the fiber’s resting length at the time of stimulation the frequency of stimulation

Frequency of Stimulation A single neural stimulation produces: a single contraction or twitch which lasts about 7–100 msec Sustained muscular contractions: require many repeated stimuli

3 Phases of Twitch Latent period before contraction: the action potential moves through sarcolemma causing Ca2+ release Contraction phase: calcium ions bind tension builds to peak Relaxation phase: Ca2+ levels fall active sites are covered tension falls to resting levels

Smooth Muscle Fusiform cells One nucleus per cell Nonstriated Involuntary Slow, wave-like contractions

Smooth Muscle Grouped into sheets in walls of hollow organs Longitudinal layer – muscle fibers run parallel to organ’s long axis Circular layer – muscle fibers run around circumference of the organ Both layers participate in peristalsis

Smooth Muscle Contraction: Mechanism

Smooth Muscle Relaxation: Mechanism

Cardiac muscle tissue Makes up myocardium of heart Unconsciously (involuntarily) controlled Microscopically appears striated Cells are short, branching & have a single nucleus Cells connect to each other at intercalated discs