Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 8 Histology and Physiology of Muscles Skeletal Muscle Fibers

Muscle Relaxation Calcium ions are transported back into the sarcoplasmic reticulum Calcium ions diffuse away from troponin and tropomyosin moves, preventing further cross-bridge formation

Muscle Twitch contraction of a muscle as a result of one or more muscle fibers contracting lag, contraction, and relaxation phases Table 8.2

Strength of Muscle Contraction For a given condition, a muscle fiber or motor unit contracts with a consistent force in response to each action potential For a whole muscle, stimuli of increasing strength result in graded contractions of increased force as more motor units are recruited (multiple motor unit summation) Stimulus of increasing frequency increase force of contraction (multiple-wave summation)

Motor Unit Fig. 8.13

Strength of Muscle Contraction Incomplete tetanus is partial relaxation between contractions Complete tetanus is no relaxation between contractions force of contraction of a whole muscle increases with increased frequency of stimulation because of an increasing concentration of Ca 2+ around the myofibrils Treppe is an increase in force of contraction during first few contractions of a rested muscle

Multiple Motor Unit Summation in a Muscle Fig. 8.14

Multiple-Wave Summation Fig. 8.15

Treppe Fig. 8.15

Multiple Motor Unit Summation Multiple-Wave Summation Treppe

Types of Muscle Contraction Isometric contractions cause a change in muscle tension but no change in muscle length Isotonic contractions cause a change in muscle length but no change in muscle tension Concentric contractions are isotonic contractions that cause muscles to shorten Eccentric contractions are isotonic contractions that enable muscles to shorten Muscle tone is the maintenance of a steady tension for long periods Asynchronous contractions of motor units produce smooth, steady muscle contractions

Muscle Length and Tension Fig Muscle contracts with less than maximum force if its initial length is shorter or longer than optimal

Fatigue The decreased ability to do work Can be caused by –central nervous system (psychologic fatigue) –Depletion of ATP in muscles (muscular fatigue) Physiologic contracture (inability of muscles to contract or relax) and rigor mortis (stiff muscles after death) result from inadequate amounts of ATP

Energy Sources Creatine phosphate –ATP is synthesized when ADP reacts with creatine phosphate to form creatine and ATP –ATP from this source provides energy for a short time Fig. 8.18

Energy Sources Anaerobic respiration –ATP synthesized provides energy for a short time at beginning of exercise and during intense exercise –Produces ATP less efficiently but more rapidly than aerobic respiration –Lactic acid levels increase because of anaerobic respiration Fig. 8.18

Energy Sources Aerobic respiration –Requires oxygen –Produces energy for muscle contractions under resting conditions or during endurance exercise Fig. 8.18

Speed of Contraction 3 main types of skeletal muscle fibers are –Slow-twitch oxidative (SO) fibers –Fast-twitch glycolytic (FG) fibers –Fast-twitch oxidative glycolytic (FOG) fibers SO fibers contract more slowly than FG and FOG fibers because they have slower myosin ATPases than FG and FOG fibers

Fatigue Resistance SO fibers are fatigue-resistant and rely on aerobic respiration –Many mitochondria, a rich blood supply, and myoglobin FG fibers are fatigable –Rely on anaerobic respiration and have a high concentration of glycogen FOG fibers have fatigue resistance intermediate between SO and FG fibers –Rely on aerobic and anaerobic respiration

Functions SO fibers maintain posture and are involved with prolonged exercise –Long-distance runners have a higher percentage of SO fibers FG fibers produce powerful contractions of short duration –Sprinters have a higher percentage of FG fibers FOG fibers support moderate-intensity endurance exercises –Aerobic exercise can result in the conversion of FG fibers to FOG fibers

Tab. 8.3

Muscular Hypertrophy and Atrophy Hypertrophy is an increase in the size of muscles –Due to an increase in the size of muscle fibers resulting from an increase in the number of myofibrils in the muscle fibers Aerobic exercise –Increases the vascularity of muscle –Greater hypertrophy of slow-twitch fibers than fast-twitch fibers Intense anaerobic exercise –Greater hypertrophy of fast-twitch fibers than slow-twitch Atrophy is a decrease in the size of muscle –Due to a decrease in the size of muscle fibers or a loss of muscle fibers

Effects of Aging on Skeletal Muscle By 80 years of age 50% of muscle mass is gone –Due to a loss in muscle fibers –Fast-twitch muscle fibers decrease in number more rapidly than slow-twitch fibers Can be dramatically slowed if people remain physically active!!!!!!!!!!!!!!!!!!!!!!!!!!!Can be dramatically slowed if people remain physically active!!!!!!!!!!!!!!!!!!!!!!!!!!!

SMOOTH & CARDIAC MUSCLE

Types of Smooth Muscle Visceral smooth muscle fibers have many gap junctions and contract as a single unit Multiunit smooth muscle fibers have few gap junctions and function independently –Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages –Forces food and other substances through internal body channels –It is not striated and is involuntary

Regulation of Smooth Muscle Contraction is involuntary –Multiunit smooth muscle contracts when externally stimulated by nerves, hormones, or other substances –Visceral smooth muscle contracts autorhythmically or when stimulated externally Hormones are important in regulating smooth muscle

Structure of Smooth Muscle Cells Spindle-shaped with a single nucleus Have actin and myosin myofilaments –Actin myofilaments are connected to dense bodies and dense areas Not striated No T tubule system and most have less SR than skeletal muscle No troponin

Contraction and Relaxation of Smooth Muscle Calcium ions enter the cell to initiate contraction –Bind to calmodulin –Activate myosin kinase, which transfers a phosphate group from ATP to myosin –When phosphate groups are attached to myosin, cross-bridges form Relaxation results when myosin phosphatase removes a phosphate group from the myosin molecule

Fig. 8.19

Functional Properties of Smooth Muscle Pacemaker cells are autorhythmic smooth muscle cells that control the contraction of other smooth muscle cells Smooth muscle cells contract more slowly than skeletal muscle cells Smooth muscle tone is the ability of smooth muscle to maintain a steady tension for long periods with little expenditure of energy Smooth muscle in the walls of hollow organs maintain a relatively constant pressure on the contents of the organ despite changes in content volume The force of smooth muscle contraction remains nearly constant despite changes in muscle length

Cardiac Muscle Cells Occurs only in the heart Is striated like skeletal muscle but is not voluntary Have a single nucleus Connected by intercalated disks that allowing them to function as a single unit Capable of autorhythmicity Contracts at a fairly steady rate set by the heart’s pacemaker Neural controls allow the heart to respond to changes in bodily needs

Tab. 8.1

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