10-5 Tension Production and Contraction Types

10-5 Tension Production and Contraction Types
Tension Production by Muscles Fibers As a whole, a muscle fiber is either contracted or relaxed Depends on: The number of pivoting cross-bridges The fiber’s resting length at the time of stimulation The frequency of stimulation Length–Tension Relationships Number of pivoting cross-bridges depends on: Amount of overlap between thick and thin fibers Optimum overlap produces greatest amount of tension Too much or too little reduces efficiency Normal resting sarcomere length Is 75% to 130% of optimal length

Figure 10-14 The Effect of Sarcomere Length on Active Tension
A muscle fiber is either “on” (producing tension) or “off” (relaxed). Skeletal muscle fibers contract most forcefully when stimulated over a narrow range of lengths. Tension (percent of maximum) Normal range Decreased length Increased sarcomere length Optimal resting length: The normal range of sarcomere lengths in the body is 75 to 130 percent of the optimal length. 2

Tension Production by Muscles Fibers The 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

Tension Production by Muscles Fibers Twitches Latent period 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 and tension falls to resting levels

Figure 10-15b The Development of Tension in a Twitch
Maximum tension development Tension Stimulus Resting phase Latent period Contraction phase Relaxation phase The details of tension over time for a single twitch in the gastrocnemius muscle. Notice the presence of a latent period, which corresponds to the time needed for the conduction of an action potential and the subsequent release of calcium ions by the sarcoplasmic reticulum. 5

Figure 10-15a The Development of Tension in a Twitch
Eye muscle Gastrocnemius Soleus Tension Time (msec) Stimulus A myogram showing differences in tension over time for a twitch in different skeletal muscles. 6

Tension Production by Muscles Fibers Treppe A stair-step increase in twitch tension Repeated stimulations immediately after relaxation phase Stimulus frequency <50/second Causes a series of contractions with increasing tension

Tension Production by Muscles Fibers Wave summation Increasing tension or summation of twitches Repeated stimulations before the end of relaxation phase Stimulus frequency >50/second Causes increasing tension or summation of twitches

Tension Production by Muscles Fibers Incomplete tetanus Twitches reach maximum tension If rapid stimulation continues and muscle is not allowed to relax, twitches reach maximum level of tension

Tension Production and Contraction Types
Complete tetanus If stimulation frequency is high enough, muscle never begins to relax, and is in continuous contraction

Tension Production by Skeletal Muscles Depends on: Internal tension produced by muscle fibers External tension exerted by muscle fibers on elastic extracellular fibers Total number of muscle fibers stimulated Motor units (all muscle fibers controlled by a single motor neuron) in a skeletal muscle: Contain hundreds of muscle fibers That contract at the same time Controlled by a single motor neuron

Motor Units and Tension Production Recruitment (multiple motor unit summation) n a whole muscle or group of muscles, smooth motion and increasing tension are produced by slowly increasing the size or number of motor units stimulated Maximum tension Achieved when all motor units reach tetanus Can be sustained only a very short time

Motor Units and Tension Production Sustained tension Less than maximum tension Allows motor units rest in rotation Muscle tone The normal tension and firmness of a muscle at rest Muscle units actively maintain body position, without motion Increasing muscle tone increases metabolic energy used, even at rest

Motor Units and Tension Production Contraction are classified based on pattern of tension production Isotonic contraction Skeletal muscle changes length Resulting in motion If muscle tension > load (resistance): Muscle shortens (concentric contraction) If muscle tension < load (resistance): Muscle lengthens (eccentric contraction)

Figure 10-18a Concentric, Eccentric, and Isometric Contractions
In an isotonic contraction, tension rises and the skeletal Muscle’s length changes. Tendon Muscle contracts (concentric contraction) 2 kg 2 kg Examples: Lifting an object off a desk, walking, running. Amount of load Muscle relaxes Muscle tension (kg) Peak tension production Contraction begins Resting length Muscle length (percent of resting length) Time 15

Figure 10-18b Concentric, Eccentric, and Isometric Contractions
In an eccentric contraction, the peak tension developed is less than the load, and the muscle elongates due to the contraction of another muscle or the pull of gravity. Support removed when contraction begins (eccentric contraction) Muscle tension (kg) Peak tension production Muscle length (percent of resting length) Support removed, contraction begins 6 kg Resting length 6 kg Examples: Bicep curls Time 16

In an isometric contraction, the muscle as a whole does not change length, and The tension produced never exceeds the load. Isometric Contraction Skeletal muscle develops tension, but is prevented from changing length iso- = same, metric = measure Examples: Carrying a bag of groceries; holding head up.

Load and Speed of Contraction Are inversely related The heavier the load (resistance) on a muscle The longer it takes for shortening to begin And the less the muscle will shorten Small load Distance Intermediate load Large load Time Stimulus

Muscle Relaxation and the Return to Resting Length Elastic Forces The pull of elastic elements (tendons and ligaments) Expands the sarcomeres to resting length Opposing Muscle Contractions Reverse the direction of the original motion Are the work of opposing skeletal muscle pairs Gravity Can take the place of opposing muscle contraction to return a muscle to its resting state

10-6 Energy to Power Contractions
ATP Provides Energy For Muscle Contraction Sustained muscle contraction uses a lot of ATP energy Muscles store enough energy to start contraction Muscle fibers must manufacture more ATP as needed ATP and CP Reserves Adenosine triphosphate (ATP) The active energy molecule Creatine phosphate (CP) The storage molecule for excess ATP energy in resting muscle Energy recharges ADP to ATP Using the enzyme creatine kinase (CK) When CP is used up, other mechanisms generate ATP

ATP Generation Cells produce ATP in two ways Aerobic metabolism of fatty acids in the mitochondria Anaerobic glycolysis in the cytoplasm Aerobic Metabolism Is the primary energy source of resting muscles Breaks down fatty acids Produces 34 ATP molecules per glucose molecule Glycolysis Is the primary energy source for peak muscular activity Produces two ATP molecules per molecule of glucose Breaks down glucose from glycogen stored in skeletal muscles 6 Where inside a skeletal muscle does glycolysis occur? Where does cellular respiration?

Table 10-2 Sources of Energy in a Typical Muscle Fiber
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Energy Use and the Level of Muscular Activity Skeletal muscles at rest metabolize fatty acids and store glycogen During light activity, muscles generate ATP through anaerobic breakdown of carbohydrates, lipids, or amino acids At peak activity, energy is provided by anaerobic reactions that generate lactic acid as a byproduct

Figure 10-20 Muscle Metabolism
Fatty acids Fatty acids Blood vessels Glucose Glycogen Glucose Glycogen Pyruvate Mitochondria Creatine To myofibrils to support muscle contraction Resting muscle: Fatty acids are catabolized; the ATP produced is used to build energy reserves of ATP, CP, and glycogen. Moderate activity: Glucose and fatty acids are catabolized; the ATP produced is used to power contraction. Lactate Glucose Glycogen Pyruvate Creatine Lactate To myofibrils to support muscle contraction Peak activity: Most ATP is produced through glycolysis, with lactate as a by-product. Mitochondrial activity (not shown) now provides only about one-third of the ATP consumed. 24

Muscle Fatigue When muscles can no longer perform a required activity, they are fatigued Results of Muscle Fatigue Depletion of metabolic reserves Damage to sarcolemma and sarcoplasmic reticulum Low pH (lactic acid) Muscle exhaustion and pain The Recovery Period The time required after exertion for muscles to return to normal Oxygen becomes available Mitochondrial activity resumes

Lactic Acid Removal and Recycling The Cori Cycle (The removal and recycling of lactic acid by the liver back to muscle cells). Liver converts lactate to pyruvate Glucose is released to recharge muscle glycogen reserves

During exertion, lactate diffuses out of the muscle fibers into the bloodstream. The liver
absorbs the lactate and converts it to pyruvate. About 30% of the pyruvate is broken down in the Citric Acid Cycle, providing the ATP needed to convert the other pyruvate molecules to glucose. Glucose is then released into circulation, where they are absorbed by skeletal muscle fibers and used to rebuild their glycogen reserves. This is the Cori Cycle.

The Oxygen Debt After exercise or other exertion: The body needs more oxygen than usual to normalize metabolic activities Resulting in heavy breathing Also called excess postexercise oxygen consumption (EPOC) Heat Production and Loss Active muscles produce heat Up to 70% of muscle energy can be lost as heat, raising body temperature

Hormones and Muscle Metabolism Growth hormone Testosterone Thyroid hormones Epinephrine Muscle Performance Force The maximum amount of tension produced Endurance The amount of time an activity can be sustained Force and endurance depend on: The types of muscle fibers Physical conditioning

10-7 Types of Muscles Fibers and Endurance
Three Major Types of Skeletal Muscle Fibers Fast fibers Slow fibers Intermediate fibers

Fast Fibers Contract very quickly Have large diameter, large glycogen reserves, few mitochondria Have strong contractions, fatigue quickly Slow Fibers Are slow to contract, slow to fatigue Have small diameter, more mitochondria Have high oxygen supply Contain myoglobin (red pigment, binds oxygen) Intermediate Fibers Are mid-sized Have low myoglobin Have more capillaries than fast fibers, slower to fatigue

Figure 10-21 Fast versus Slow Fibers
Slow fibers Smaller diameter, darker color due to myoglobin; fatigue resistant LM  170 Fast fibers Larger diameter, paler color; easily fatigued LM  170 LM  783 32

Table 10-3 Properties of Skeletal Muscle Fiber Types
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Muscle Performance and the Distribution of Muscle Fibers White muscles Mostly fast fibers Pale (e.g., chicken breast) Red muscles Mostly slow fibers Dark (e.g., chicken legs) Most human muscles Mixed fibers Pink

Muscle Hypertrophy 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

Physical Conditioning Improves both power and endurance Anaerobic activities (e.g., 50-meter dash, weightlifting) Use fast fibers Fatigue quickly with strenuous activity Improved by: Frequent, brief, intensive workouts Causes hypertrophy

Physical Conditioning Improves both power and endurance Aerobic activities (prolonged activity) Supported by mitochondria Require oxygen and nutrients Improves: Endurance by training fast fibers to be more like intermediate fibers Cardiovascular performance

Importance of Exercise What you don’t use, you lose Muscle tone indicates base activity in motor units of skeletal muscles Muscles become flaccid when inactive for days or weeks Muscle fibers break down proteins, become smaller and weaker With prolonged inactivity, fibrous tissue may replace muscle fibers

10-8 Cardiac Muscle Tissue
Cardiac muscle cells are striated and found only in the heart Striations are similar to that of skeletal muscle because the internal arrangement of myofilaments is similar

Structural Characteristics of Cardiac Muscle Tissue Unlike skeletal muscle, cardiac muscle cells (cardiocytes): Are small Have a single nucleus Have short, wide T tubules Have no triads Have SR with no terminal cisternae Are aerobic (high in myoglobin, mitochondria) Have intercalated discs

Intercalated Discs Are specialized contact points between cardiocytes Join cell membranes of adjacent cardiocytes (gap junctions, desmosomes) Functions of intercalated discs: Maintain structure Enhance molecular and electrical connections Conduct action potentials Coordination of cardiocytes Because intercalated discs link heart cells mechanically, chemically, and electrically, the heart functions like a single, fused mass of cells

Functional Characteristics of Cardiac Muscle Tissue Automaticity (contracts without neural stimulation) Controlled by pacemaker cells Variable contraction tension Controlled by nervous system Extended contraction time Ten times as long as skeletal muscle Prevention of wave summation and tetanic contractions by cell membranes Long refractory period

10-9 Smooth Muscle Tissue Smooth Muscle in Body Systems Forms around other tissues In integumentary system Arrector pili muscles cause “goose bumps” In blood vessels and airways Regulates blood pressure and airflow In reproductive and glandular systems Produces movements In digestive and urinary systems Forms sphincters Produces contractions

10-9 Smooth Muscle Tissue Structural Characteristics of Smooth Muscle Tissue Nonstriated tissue Different internal organization of actin and myosin Different functional characteristics

10-9 Smooth Muscle Tissue Characteristics of Smooth Muscle Cells Long, slender, and spindle shaped Have a single, central nucleus Have no T tubules, myofibrils, or sarcomeres Have no tendons or aponeuroses Have scattered myosin fibers Myosin fibers have more heads per thick filament Have thin filaments attached to dense bodies Dense bodies transmit contractions from cell to cell

10-9 Smooth Muscle Tissue Functional Characteristics of Smooth Muscle Tissue Excitation–contraction coupling Length–tension relationships Control of contractions Smooth muscle tone

10-9 Smooth Muscle Tissue Excitation–Contraction Coupling Free Ca2+ in cytoplasm triggers contraction Ca2+ binds with calmodulin In the sarcoplasm Activates myosin light–chain kinase Enzyme breaks down ATP, initiates contraction Length–Tension Relationships Thick and thin filaments are scattered Resting length not related to tension development Functions over a wide range of lengths (plasticity)

10-9 Smooth Muscle Tissue Control of Contractions Multiunit smooth muscle cells Connected to motor neurons Visceral smooth muscle cells Not connected to motor neurons Rhythmic cycles of activity controlled by pacesetter cells Smooth Muscle Tone Maintains normal levels of activity Modified by neural, hormonal, or chemical factors

Table 10-4 A Comparison of Skeletal, Cardiac, and Smooth Muscle Tissues
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