Chapter 9
How does the term epimysium relate to the role and position of this connective tissue sheath?
"Epimysium" literally translates to "outside the muscle" and this connective tissue sheath is the outermost muscle sheath, enclosing the entire muscle.
What is the difference between a tendon and a ligament? (Hint: See Chapter 4, p. 117.)
A tendon attaches muscles to bones, whereas a ligament connects bones at joints.
Compare and contrast the contractile mechanisms and the means of activation of skeletal and smooth muscles.
1. Calcium ions enter the cytosol from the ECF via voltage-gated Ca2+ channels, or from the SR 2. Ca2+ binds to and activates calmodulin 3. Activated calmodulin activates the myosin light chain kinase enzymes 4. The activated kinase enzymes catalyze transfer of phosphate to myosin, activating the myosin ATPases 5. Activated myosin forms cross bridges with actin of the thin filaments just as in skeletal muscle. Shortening begins
Describe the steps of a cross bridge cycle.
1. Cross bridge formation - energized myosin head attaches to an actin myofilament, forming a cross bridge 2. The power stroke - ADP and P are released and the myosin head pivots and bends, pulls actin filament toward M line 3. Cross bridge detachment - ATP attaches to myosin and the link between myosin and actin weakens, myosin head detaches 4. Cocking of the myosin head - myosin hydrolyzes ATP to ADP and P, myosin returns to high-energy/cocked position
Describe three ways in which ATP is regenerated during skeletal muscle contraction.
1. Direct phosphorylation of ADP by creatine phosphate - coupled reaction of creatine phosphate (CP) and ADP 2. Anaerobic pathway - glycolysis and lactic acid formation Aerobic pathway - aerobic respiration
Describe factors that influence the force, velocity, and duration of skeletal muscle contraction.
1. Frequency of stimulation - when a muscle is stimulated more frequently, contractions are summed. The higher the frequency of muscle stimulation, the greater the force the muscle exerts. 2. Number of muscle fibers recruited - more motor units recruited = greater the force 3. Size of muscle fibers - the bulkier the muscle and the greater the cross-sectional area, the more tension it can develop 4. Degree of muscle stretch - the tension the muscle can generate varies with length Muscle fiber type, load, and recruitment
Explain how muscle fibers are stimulated to contract by describing events that occur at the neuromuscular junction.
A motor neuron fires an action potential down its axon, the motor neuron's axon terminal releases acetylcholine into the synaptic cleft, ACh binds receptors on the junctional folds of the sarcolemma, ACh binding causes a local depolarization called an end potential. This local depolarization triggers an AP in the adjacent sarcolemma (end plate potential).
What is a motor unit?
A motor unit is an axon of a motor neuron and all the muscle fibers it innervates.
Follow the events of excitation-contraction coupling that lead to cross bridge activity.
AP in sarcolemma travels down T tubulues, sarcoplasmic reticulum releases Ca2+, Ca2+ binds to troponin, which shifts tropomyosin to uncover the myosin-binding sites on actin. Myosin heads bind actin
How do aerobic and resistance exercise differ in their effects on muscle size and function?
Aerobic exercise increases muscle endurance, whereas anaerobic exercise (such as high-intensity resistance exercise) increases muscle size and strength.
Compare and contrast the effects of aerobic and resistance exercise on skeletal muscles.
Aerobic exercise results in the number of capillaries surrounding the muscle fibers to increase, the number of mitochondria within the muscle fibers to increase, and the fibers to synthesize more myoglobin. These changes result in more efficient muscle metabloism and in greater endurance, strength, and resistance to fatigue. Resistance exercises promote hypertrophy of muscles, increases individual size of muscle fibers, more mitochondria, form more myofilaments and myofibrils, store more glycogen, and develop more connective tissue between muscle cells.
Explain how smooth, graded contractions of a skeletal muscle are produced.
An increase in the frequency of stimulation causes temporal summation. The higher the frequency, the greater the strength of contraction of a given motor unit. An increase in the strength of stimulation causes recruitment. The stronger the stimulation, the more motor units are activated, and the stronger the contraction.
Calcium is the trigger for contraction of all muscle types. How does its binding site differ in skeletal and smooth muscle fibers?
Calcium binds to troponin on the thin filaments in skeletal muscle cells. In smooth muscle cells, it binds to a cytoplasmic protein called calmodulin.
Compare the structures of skeletal and smooth muscle fibers.
Both skeletal and smooth muscle fibers are elongated cells, but unlike smooth muscle cells, which are spindle shaped (tapered at both ends), uninucleate, and nonstriated, skeletal muscle cells are very large cylindrical, multinucleate, striated cells.
Chris joined the cross-country team partway through the season, and has just completed another grueling session of trying to keep up with her teammates. After the run, she is breathing heavily, her legs feel weak, and she is sweating profusely. Why is Chris breathing heavily? Which ATP-generating pathway have her working muscles been using that makes her breathless? What metabolic products might account for her muscle weakness?
Chris is breathing heavily because it takes some time for her heart rate and overall metabolism to return to the resting state after exercise. Moreover, she has likely incurred an oxygen debt that requires her to take in extra oxygen, called EPOC, for the restorative processes. Although running can be an aerobic exercise, there is generally some anaerobic respiration that occurs as well—the amount depends on exercise intensity. The weakness Chris feels is likely in part due to muscle fatigue. Muscle fatigue is likely due to a combination of factors in her muscles including increased inorganic phosphate and magnesium, decreased ATP and glycogen, and other ionic imbalances.
Differentiate between direct and indirect muscle origin or insertion
Direct - epimysium of muscle is fused to periosteum of bone or perichondrium of a cartilage Indirect - muscles connective tissue wrappings extend beyond muscle either as ropelike tendon or as a sheetlike aponeurosis Indirect attachments are much more common because of durability and small size
Describe the sliding filament model of muscle contraction.
During contraction, the thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree - thick and thin filaments don't change length during contraction.
Define EPOC and muscle fatigue. List possible causes of muscle fatigue.
EPOC (Excess Postexercise Oxygen Consumption) - the extra amount of oxygen that the body must take in for these restorative processes Possible causes of muscle fatigue: Ionic imbalances - ionic changes that disturb the membrane potential of muscle cells, also reduce size of AP Increased inorganic phosphate - hamper myosin's power strokes Decreased ATP and increased magnesium - decrease Ca2+ release from SR Decreased glycogen - highly correlated with muscle fatigue
Describe the gross structure of a skeletal muscle.
Each muscle has a nerve along with a vein and artery (blood supply), which all together allow neural control and ensure adequate nutrient delivery and waste removal. Connective tissue sheaths are found at various structural levels of each muscle: endomysium surrounds each muscle fiber, perimysium surrounds groups of muscle fibers, and epimysium surrounds whole muscles. Attachments span joints and cause movement to occur from the movable bone (the muscle's insertion) toward the less movable bone (the muscle's origin) Muscle attachments may be direct or indirect
List four important characteristics of muscle tissue that allow it to do its duties.
Excitability / responsiveness - ability of a cell to receive and respond to a stimulus by changing its membrane potential Contractility - ability to shorten forcibly when adequately stimulated Extensibility - ability to extend or stretch Elasticity - ability to recoil and resume its resting length and stretching
Describe the three connective tissue sheaths of muscles.
External to internal: Epimysium - dense irregular connective tissue that surrounds whole muscle Perimysium and fascicles - within each skeletal muscle, the muscle fibers are grouped into fascicles (resemble bundle of sticks), surrounding each fascicles is perimysium (dense irregular connective tissue) Endomysium - wispy sheath of connective tissue that surrounds each individual muscle fiber, consists of fine areolar connective tissue
List two factors that influence contractile force and two that influence velocity of contraction.
Factors that influence muscle contractile force include the frequency of stimulation of muscle fibers, muscle fiber size, the number of muscle fibers stimulated, and the degree of muscle stretch. Factors that influence velocity of contraction include muscle fiber type, load, and the number of motor units contracting.
Jordan called several friends to help him move. Would he prefer to have those with more slow oxidative muscle fibers or those with more fast glycolytic fibers as his helpers? Why?
Fast glycolytic fibers would provide for short periods of intense strength needed to lift and move furniture.
How does the stress-relaxation response suit the role of smooth muscle in hollow organs?
Hollow organs that have smooth muscle cells helping to form their walls often must temporarily store the organ's contents (urine, food residues, etc.), an ability facilitated by the stress-relaxation response.
Jacob is competing in a chin-up competition. What type of muscle contractions are occurring in his biceps muscles immediately after he grabs the bar? As his body begins to move upward toward the bar? When his body begins to approach the mat?
Immediately after Jacob grabs the bar, his biceps muscles are contracting isometrically. As his body moves upward toward the bar, they are contracting isotonically and concentrically. As he lowers his body toward the mat, the biceps are contracting isotonically and eccentrically.
Which region or organelle—cytosol, mitochondrion, or SR—contains the highest concentration of calcium ions in a resting muscle fiber? Which structure provides the ATP needed for muscle activity?
In a resting muscle fiber, the SR would have the highest concentration of calcium ions. The mitochondria provide the ATP needed for muscle activity.
Intracellular calcium performs other important roles in the body in addition to triggering muscle contraction. Give one example. (Hint: See Chapter 3.)
Intracellular calcium acts as a second messenger (see Chapter 3, p. 73). In addition, calcium is involved in exocytosis in many different cells just as it is in the motor neuron axon terminal.
Differentiate between isometric and isotonic contractions.
Isotonic - muscle develops enough tension to lift the load, once resistance is overcome, the muscle shortens and the tension remains constant for the rest of the contraction. Concentric - muscle shortens and does work Eccentric contractions, muscle generates force as it lengthens Isometric - muscle is attached to a weight that exceeds the muscles peak tension-developing capabilities. When stimulated, the tension increases to the muscles peak tension-developing capability, but the muscle does not shorten.
There are no slow glycolytic fibers. Explain why it would not make sense to have these kinds of fibers.
It makes sense that there are no slow glycolytic fibers because glycolysis is a fast but inefficient pathway for generating ATP. Why use such an inefficient pathway if the contraction is just going to be slow anyway (and so use up ATP slowly)?
Define motor unit and muscle twitch and describe the events occurring during the three phases of a muscle twitch.
Motor unit - one motor neuron and all the muscle fibers it innervates, or supplies. Muscle twitch - response to a single stimulation Three phases of muscle twitch: 1. Latent period - first few miliseconds following stimulation when excitation-coupling is occurring, cross bridges begin to cycle 2. Period of contraction - cross bridges are active, myogram tracing rises to a peak 3. Period of relaxation - pumping of Ca2+ back into the SR, muscle tension decreases to zero and tracing returns to baseline
Describe the microscopic structure and functional roles of the myofibrils, sarcoplasmic reticulum, and T tubules, of skeletal muscle fibers
Myofibrils - rodlike fibers, account for 80% of cellular volume, made up of sarcomeres (contain even smaller rodlike structures called myofilaments), Sarcomeres - region of a myofibril between 2 successive z discs, functional unit of skeletal muscle Sarcoplasmic Reticulum - elaborate smooth endoplasmic reticulum, regulates intracellular levels of ionic calcium, stores calcium and releases it on demand, surround each myofribril, terminal cisterns form perpendicular cross channels at A band- I band junctions T Tubules - increase muscle fiber's surface area, allows changes in membrane potential to rapidly penetrate deep into the muscle fiber
List four important functions muscle tissue does for the body.
Produce movement - responsible for all locomotion and manipulation Maintain posture and body position - muscles adjust to counteract never-ending pull of gravity Stabilize joints - even as they pull on bones to cause movement, they strengthen and stabilize the joints of the skeleton Generate heat - muscles generate heat as they contract, which plays a role in maintaining normal body temperature
Compare and contrast the three basic types of muscle tissue.
Skeletal muscle - attach to and cover the skeleton, fibers are the longest muscle cells and have striations, voluntary muscle Cardiac Muscle - occurs only in heart (bulk of walls), cells are striated, not voluntary Smooth Muscle - found in walls of hollow visceral organs (stomach, urinary bladder, and respiratory passages), role is to force fluids or substances through internal body channels, elongated cells, no striations, not voluntary
Describe the three types of skeletal muscle fibers and explain the relative value of each type
Slow oxidative fibers (endurance-type) - contracts slowly (myosin ATPases are slow), depends on oxygen delivery and aerobic pathways, resists fatigue and has high endurance, thin, relatively little power, many mitochondria, rich capillary supply, is red Fast oxidative fibers - intermediate between slow oxidative and fast glycolytic, contract quickly, oxygen dependent and have a rich supply of myoglobin and capillaries Fast glycolytic fibers (short-term, rapid, intense movements) - contracts rapidly due to the activity of fast myosin ATPases, uses little oxygen, depends on plentiful glycogen, tires quickly, large diameter (contract powerfully), few mitochondria, little myoglobin, few capillaries
Compare the gross and microscopic anatomy of smooth muscle cells to that of skeletal muscle cells.
Smooth muscle fibers are small spindle-shaped cells, smooth muscle lacks the coarse connective tissue sheaths found in skeletal muscle, smooth muscle has varicosities instead of neuromuscular junctions, smooth muscle fibers have less elaborate SR and no T tubules, smooth muscle fibers are usually electrically connected by gap junctions
When describing muscle, what does "striated" mean?
Striated means "with stripes."
What is the final trigger for contraction? What is the initial trigger?
The final trigger for contraction is a certain concentration of calcium ions in the cytosol. The initial trigger is depolarization of the sarcolemma.
Consider a phosphorus atom that is part of the membrane of the sarcoplasmic reticulum in the biceps muscle of your arm. Using the levels of structural organization (described in Chapter 1), name in order the structure that corresponds to each level of organization. Begin at the atomic level (the phosphorus atom) and end at the organ system level.
The levels of organization are: (1) atom: phosphorus atom, (2) molecule: phospholipid molecule, (3) organelle: sarcoplasmic reticulum, (4) cell: skeletal muscle fiber, (5) tissue: muscle tissue, (6) organ: the biceps muscle, and (7) organ system: the muscular system.
What prevents the filaments from sliding back to their original position each time a myosin cross bridge detaches from actin?
There are always some myosin cross bridges bound to the actin myofilament during the contraction phase. This prevents backward sliding of the actin filaments.
From the time an action potential reaches the axon terminal of a motor neuron until cross bridge cycling begins, several sets of ion channels are activated. List these channels in the order that they are activated and state what causes each to open.
There are at least four sets of ion channels involved: (1) Voltage-gated Ca2+ channels in the axon terminal of the motor neuron are opened by the action potential running down the axon. (2) Chemically gated channels permeable to both Na+ and K+ are opened by acetylcholine binding to ACh receptors on the junctional folds of the sarcolemma. (3) Voltage-gated Na+ and K+ channels are required for generating and propagating the action potential along the sarcolemma. The end plate potential initiates the action potential by opening the first set of voltage-gated Na+ channels, and then the action potential itself opens more voltage-gated channels as it moves along the sarcolemma. (4) Ca2+ release channels in the terminal cisterns of the SR open to release Ca2+ into the cytosol. These channels open because an AP running along the T tubules causes the voltage-sensitive tubule proteins to change shape, and this shape change causes the Ca2+ release channels of the SR to open.
Distinguish between unitary and multi unit smooth muscle structurally and functionally.
Unitary (walls of all hollow organs except the heart) - cells are: arranged in opposing sheets, inneravated by varicosisities, electrically coupled by gap junctions and so contract as a unit, and respond to various chemical stimuli Multi (large airways to the lungs and in large arteries, arrector pili muscles, internal eye muscles) - consists of muscle fibers structurally independent, richly supplied with nerve endings, responds to neural stimulation with graded contractions
What would happen if a muscle fiber suddenly ran out of ATP when sarcomeres had only partially contracted?
Without ATP, rigor would occur because the myosin heads could not detach.
