Chapter 14, Rice KINE 301
What is a motor unit?
A motor neuron and all of the muscle fibers it innervates
Smooth muscle has more __________________ filaments (proportionally) than skeletal muscle, and because it has no troponin, _________________________ are responsible for controlling contraction and relaxation.
Actin Phosphorylation and dephosphorylation
What is G-actin vs. F-actin? What are crossbridges, and what role do they play in muscle tension?
Actin is a protein that makes up the thin filaments of the muscle fiber. One actin molecule is a globular protein (G-actin). Usually, multiple G-actin molecules polymerize to form long chains or filaments called F-actin. In skeletal muscle, two F-actin polymers twist together like a double strand of beads, creating the thin filaments of the myofibril. In skeletal muscle, two F-actin polymers twist together like a double strand of beads, creating the thin filaments of the myofibril. Myofibril crossbridges span the space between the filaments. Crossbridges form when the myosin heads of thick filaments bind to actin in the thin filaments. Crossbridges have two states: low-force (relaxed muscles) and high-force (contracting muscles).
Name the three most common sites of muscle disorders.
(1) problem with the signal from the nervous system, (2) miscommunication at the NM junction, and (3) defects in the muscle itself.
How many myofibrils might be found in a skeletal muscle? What determines this number?
1000+. The amount of work the muscle needs to do.
What is the magnitude of increase of calcium ion concentration from resting state to maximal contraction?
100x
What % of total body weight comes from skeletal muscle?
40%
Although calcium-induced calcium release is an important mechanism in cardiac muscle, ______ % of all calcium used to cause contraction comes from the SR.
90
Describe the role of the position of insertion in a 3rd class lever system with regard to a person's ability to exert external force and external speed.
Attachment farther from the fulcrum increases force at the expense of speed.
Define sarcomere. Draw a sarcomere. Label the I band, A band, M line, Z disk, actin, myosin.
Basic unit of muscle
How do some individual muscle cells develop into such large cells?
By merging large numbers of embryonic muscle cells in development.
How does muscle relax following contraction?
Ca2+ unbinds from troponin. In the absence of Ca2+, troponin allows tropomyosin to return to the "off" position, covering most of actin's myosin-binding sites. During the brief portion of the relaxation phase when actin and myosin are not bound to each other, the filaments of the sarcomere slide back to their original positions with the aid of titin and elastic connective tissues within the muscle.
What is calcium-induced calcium release? What types of stimuli might initiate the calcium channels at the cell membrane to open? Are these channels opened by action potentials?
Calcium-induced calcium release: Process in which Ca2+ entry into a muscle fiber triggers the release of additional Ca2+ from the sarcoplasmic reticulum No they are not opened by action potentials, they are opened by electrical or chemical graded potentials.
What is calmodulin? What is the role of MLCK and MLCP in contraction of smooth muscle? What is the role of caldesmon?
Calmodulin: A calcium binding protein found in cytosol. MLCK: The enzyme phosphorylates the myosin light chain MLCP: Decreases myosin ATPase activity Caldesmon: Activates MLCK
Describe the troponin complex. Which part of the complex regulates contraction and how does it exhibit this control?
Complex troponin C binds reversibly to Ca2+. The calcium-troponin C complex pulls tropomyosin completely away from actin's myosin-binding sites. This "on" position enables the myosin heads to form strong, high-force crossbridges and carry out their power strokes, moving the actin filament. Contractile cycles repeat as long as the binding sites are uncovered
Name the two contractile proteins of myofibrils. In addition, name two regulatory proteins and two accessory proteins in myofibrils.
Contractile proteins: Myosin and actin Regulatory proteins: Tropomyosin and troponin Accessory proteins: Titin and nebulin
What is crossbridge tilting? What is the last step of crossbridge cycling prior to the extremely brief (normally) rigor state?
Crossbridge tilting is the power stroke that begins after Ca2+ binds to troponin to uncover the rest of the myosin-binding site. The crossbridges transform into strong, high-force bonds as myosin releases P (inorganic phosphate released when ATP → ADP). The release of P allows the myosin head to swivel. The heads swing toward the M line, sliding the attached actin filament along with them. The ower stroke is also called crossbridge tilting because the myosin head and hinge region tilt from a 90 degree angle to a 45 degree angle. The last step prior to the rigor state occurs when myosin releases ADP, the second product of ATP hydrolysis. With ADP gone, the myosin head is again tightly bound to actin in the rigor state. The cycle is ready to begin once more as a new ATP binds to myosin.
How do the muscle fibers of one motor unit become so similar to one another?
During embryological development, each somatic motor neuron secretes a growth factor that directs the differentiation of all muscle fibers in its motor unit so that they develop into the same fiber type.
Name the two major functions of muscles.
Generate motion, generate force.
How is tension of a muscle related to length of the sarcomeres within the muscle fibers?
Each sarcomere contracts with optimum force if it is at optimum length (neither too long nor too short) before the contraction begins. the normal resting length of skeletal muscles usually ensures that sarcomeres are at optimum length when they begin a contraction. At the molecular level, sarcomere length reflects the overlap between the thick and thin filaments. The sliding filament theory predicts that the tension a muscle fiber can generate is directly proportional to the number of crossbridges formed between the thick and thin filaments. If the fibers start a contraction at a very long sarcomere length, the thick and thin filaments barely overlap and form few crossbridges. This means that in the initial part of the contraction, the sliding filaments interact only minimally and therefore cannot generate much force.
What is excitation-contraction coupling, and what which neurotransmitter is responsible for initiating the action potential in muscle?
Excitation-contraction coupling describes the electrical and mechanical events in a muscle fiber. 1) Acetylcholine is released from the somatic motor neuron. 2) Acetylcholine initiates an action potential in the muscle fiber. 3) The muscle action potential triggers calcium release from the sarcoplasmic reticulum. 4) Calcium combines with troponin and initiates contraction. Acetylcholine
Define fatigue. Name a few of the potential causes of fatigue during exercise. What is central fatigue?
Fatigue is a reversible condition in which a muscle is no longer able to generate or sustain the expected power output. Fatigue is influenced by the intensity and duration of the contractile activity, by whether the muscle fiber is using aerobic or anaerobic metabolism, by the composition of the muscle, and by the fitness level of the individual. Central fatigue mechanisms are factors that have been proposed to play a role in fatigue which arise in the central nervous system.
Using figure 14-25, describe the following parts of the cardiac cycle. Tell which valves are open and closed as well as where the blood is flowing: filling phase; isovolumetric relaxation phase; isovolumetric contraction phase; ejection phase.
Filling phase: The blood is flowing from the atria into the ventricles passively as a result of gravity Isovolumetric contraction phase: With both sets of AV and semilunar valves closed, blood in the ventricles has nowhere to go. Nevertheless, the ventricles continue to contract, squeezing on the blood in the same way that you might squeeze a water balloon in your hand. Ejection phase: As the ventricles contract, they generate enough pressure to open the semi- lunar valves and push blood into the arteries. The pressure created by ventricular contraction becomes the driving force for blood flow. High-pressure blood is forced into the arteries, displacing the low-pressure blood that fills them and pushing it farther into the vasculature. During this phase, the AV valves remain closed and the atria continue to fill. Isovolumetric relaxation phase: Once the semilunar valves close, the ventricles again become sealed chambers. The AV valves remain closed because ventricular pressure, although falling, is still higher than atrial pressure.
Describe the state of the crossbridge during a time when skeletal muscle is relaxed.
Free myosin molecule, ATP is split into ADP and P, myosin hasn't bound to actin yet, tropomyosin is in the way.
Describe the structure of the thick filament.
In skeletal muscle, about 250 myosin molecules join to create a thick filament. Each thick filament is arranged so that the myosin heads are clustered at each end of the filament, and the central region of the filament is a bundle of myosin tails.
Describe how contraction occurs during the sliding filament theory
In this model, overlapping actin and myosin filaments of fixed length slide past one another in an energy-requiring process, resulting in muscle contraction. If you examine a myofibril at its resting length, you see that within each sarcomere, the ends of the thick and thin filaments overlap slightly. In the relaxed state, a sarcomere has a large I band (thin filaments only) and an A band whose length is the length of the thick filament. When the muscle contracts, the thick and thin filaments slide past each other. The Z disks of the sarcomere move closer together as the sarcomere shortens. The I band and H zone (regions where actin and myosin do not overlap in resting muscle) almost disappear. Despite shortening of the sarcomere, the length of the A band remains constant. These changes are consistent with the sliding of thin actin filaments along the thick myosin filaments as the actin filaments move toward the M line in the center of the sarcomere. It is from this process that the sliding filament theory of contraction derives its name. The sliding filament theory explains how a muscle can contract and create force without creating movement.
What are intercalated disks in cardiac muscle?
Intercalated disks: Specialized cell junctions in cardiac muscle that contain gap junctions
Multi-unit smooth muscle only occurs in a few places in the human body. Which tissue in humans can switch from multi-unit to single-unit?
Iris and ciliary body of the eye, part of the male reproductive tract, and in the uterus
Define the following: isotonic contraction; concentric contraction; eccentric contraction; isometric contraction; series elastic elements
Isotonic contraction: Any contraction that creates force and moves a load Concentric contraction: The force generated is sufficient to overcome the resistance, and the muscle shortens as it contracts. Eccentric contraction: A lengthening contraction. Isometric contraction: Contractions that create force without moving a load Series elastic elements: Elastic fibers in the muscle that stretch during isometric contraction
What is the purpose of asynchronous recruitment of motor units?
It helps the nervous system to avoid fatigue in sustained (submaximal) contractions.
In muscle physiology literature, the force created by the contracting muscle is called _________.
Muscle Tension
Name the two binding sites on a myosin head. What is a power stroke?
Myosin heads bind to actin molecules. A calcium signal initiates the power stroke, when myosin crossbridges swivel and push the actin filaments toward the center of the sarcomere. At the end of a power stroke, each myosin head releases actin, then swivels back and binds to a new actin molecule, ready to start another contractile cycle. The power stroke repeats many times as a muscle fiber contracts. The myosin heads bind, push, and release actin molecules over and over as the thin filaments move toward the center of the sarcomere.
Why is myosin referred to as a "motor protein"?
Myosin is the motor, actin filaments are the tracks along which myosin moves, and ATP is the fuel that powers movement.
What is the "rigor state" of skeletal muscle attributed to?
No ATP or ADP is bound to myosin
Be familiar with the speed, metabolic and fatigue-related differences between the three broad groups of muscle fibers.
Pg. 392, Table 12.2
Which sources of energy are used during rest, light exercise, and heavy exercise?
Rest/Light: Fatty acids along with glucose Heavy: Primarily glucose
The AV node and the purkinje fibers both have pacemaker cells, but the cells of the ___________________ have the fastest pace and therefore set the heart rate.
SA node
Define the following terms: sarcolemma, sarcoplasm, myofibril, sarcoplasmic reticulum, terminal cisternae, transverse tubules (t-tubules), triad
Sarcolemma: Cell membrane of a muscle fiber. Sarcoplasm: Cytoplasm of a muscle fiber. Myofibril: Highly organized bundles of contractile and elastic proteins that carry out the work of contraction. Sarcoplasmic reticulum: A form of modified endoplasmic reticulum that wraps around each myofibril like a piece of lace Terminal cisternae: Enlarged end regions of longitudinal tubules within the sarcoplasmic reticulum. Transverse tubules (t-tubules): The membranes of t-tubules are a continuation of the muscle fiber membrane, which makes the lumen of t-tubules continuous with the extracellular fluid. T-tubules allow action potentials to move rapidly from the cell surface into the interior of the fiber so that they reach the terminal cisternae nearly simultaneously. Triad: One t-tubule and its two flanking terminal cisternae.
What controls the contraction of skeletal muscle, and how does this differ from cardiac and smooth muscle?
Signal from a somatic motor neuron. Cardiac/smooth muscle activated by autonomic innervation or hormones, with some types contracting spontaneously.
Which types are striated, and what causes this appearance?
Skeletal and cardiac. Alternating light and dark bands.
Name the three types of muscle tissue.
Skeletal, cardiac, smooth.
What are slow wave potentials vs. pacemaker potentials in smooth muscle?
Slow wave potentials: Cells that exhibit cyclic depolarization and repolarization of membrane potential in smooth muscle. Pacemaker potentials: Regular depolarizations of smooth and cardiac muscle that always reach threshold.
Although each muscle fiber has the potential to express proteins in such a fashion as to be completely different from one another, generally muscle fibers can be classified into three broad groups. Name the three groups.
Slow-twitch fibers (ST or type I), fast-twitch oxidative-glycolytic fibers (FOG or type IIA), and fast twitch glycolytic fibers (FG or type IIB)
What is the shape and size of a smooth muscle cell? Where are the nuclei?
Small, spindle-shaped Single, centrally located, nucleus
Why is smooth muscle more difficult to study than striated muscle?
Smooth muscle is harder to describe because the smooth muscles in the body have so much functional variability.
How does smooth muscle differ from striated muscle in terms of speed and resistance to fatigue? How do these traits help with muscle tone?
Smooth muscle is slower than skeletal muscle, but can sustain contraction for longer without fatiguing. Smooth muscle has less mitochondria than striated muscles and relies more on glycolysis for its ATP production.
Name a few advantages of smooth muscle over skeletal muscle with regard to matching its specific functions?
Smooth muscles operate over a range of lengths. The layers of smooth muscle can run in several directions Smooth muscle uses less energy to generate and maintain a given amount of force. They can sustain contraction for extended periods without fatiguing. Contraction in smooth muscle can be initiated by electrical or chemical signals (or both).
Name three specific locations of smooth muscle.
Stomach, urinary bladder, and blood vessels.
Define the following terms: tendons, origin, insertion, flexor, extensor.
Tendons: Bundles of collagen that attach skeletal muscles to bones Origin: End of muscle attached closest to trunk or to more stationary bone. Insertion: End of muscle attached further to trunk or to more mobile bone. Flexor: Muscle that moves the center of 2 bones closer together. Extensor: Muscle that moves the center of 2 bones further apart.
What is an end-plate potential?
The addition of net positive charge to the muscle fiber that depolarizes the membrane
Why is it so important to have phosphocreatine in skeletal muscle? What enzyme catalyzes the reaction with PCR?
The amount of ATP in a muscle fiber at any one time is only sufficient for ~8 twitches. Phosphocreatine serves as a backup energy source. creatine kinase (CK)
How does visceral smooth muscle exhibit its action as a single unit?
The fibers of single-unit smooth muscles are connected to one another by gap junctions. An electrical signal in one cell spreads rapidly throughout the entire sheet of tissue to create a coordinated contraction. Because all fibers contract every time, no reserve units are left to be recruited to increase contraction force. Instead, the amount of Ca2+ that enters the cell determines the force of contraction.
How do we control the level of contraction of an individual muscle fiber? What is unfused and fused tetanus?
The force generated by the contraction of a single muscle fiber can be increased by increasing the rate (frequency) at which muscle action potentials stimulate the muscle fiber. Unfused tetanus: the stimulation rate of the muscle fiber is not at a max value, and consequently the muscle fiber relaxes slightly between the stimuli. Fused tetanus: the stimulation rate is fast enough that the muscle fiber does not have time to relax. Instead it reaches max tension and remains there.
How do we recruit a greater number of motor units to be active during a whole muscle contraction? Which motor units are recruited first?
The force of contraction in a skeletal muscle can be increased by recruiting additional motor units. Recruitment is controlled by the nervous system and proceeds in a standardized sequence. A weak stimulus directed onto a pool of somatic motor neurons in the central nervous system activates only the neurons with the lowest thresholds.
When CPK is found in blood plasma, it is an indication of skeletal muscle or cardiac muscle damage. How can a clinician tell which tissue is damaged?
The tissues contain different isozymes of creatine phosphokinase.
How long does it take for permanent brain damage to occur if oxygen supply is cut off?
a. 5-10 minutes
Which proteins insure the proper alignment of six thin filaments with each thick filament?
Titin and Nebulin
Describe the structure and role of titin and nebulin.
Titin is a huge elastic molecule and the largest known protein composed of more than 25,000 amino acids. A single titin molecule stretches from one Z disk to the neighboring M line. It stabilizes the position of the contractile filaments and its elasticity returns stretched muscles to their resting length. Nebulin is an inelastic giant protein that lies alongside thin filaments and attaches to the Z disk. Nebulin helps align the actin filaments of the sarcomere.
What is the average blood flow through the aorta in 70-kg man per minute?
a. 5L/min
Why do we have so many isoforms of myosin?
Various isoforms of myosin occur in different types of muscle and help determine the muscle's speed of contraction.
What is the most common cause of muscular atrophy?
When a limb is mobilized in a cast.
What are If channels and how do they contribute to depolarization of the autorhythmic cells?
When the cell membrane potential is -60 mV, If channels that are permeable to both K+ and Na+ open. These channels are called If channels because they allow current (I) to flow and because of their unusual properties.
Define: tachycardia; bradycardia; arrhythmia; diastole; systole.
a. A faster-than-normal rate is known as tachycardia, and a slower-than-normal rate is called bradycardia b. An irregular rhythm, or arrhythmia, can result from a benign extra beat or from more serious conditions such as atrial fibrillation, in which the SA node has lost control of the pacemaking. c. Each cardiac cycle has two phases: diastole, the time dur- ing which cardiac muscle relaxes, and systole, the time during which the muscle contracts
What is the slowest portion of the conduction system, and what function does this slow portion provide?
a. A second function of the AV node is to slow down the transmission of action potentials slightly. This delay allows the atria to complete their contraction before ventricular contraction begins. AV node delay is accomplished by slower conduction of signals the leader. In the heart, the cue to follow the leader is the electrical signal sent from the SA node to the other cells.
How does stretch on myocardial cells regulate contraction?
a. Another factor that affects the force of contraction in cardiac muscle is the sarcomere length at the beginning of contraction. In the intact heart, stretch on the individual fibers is a function of how much blood is in the chambers of the heart.
Which two mechanisms are used to remove calcium from the cell when contraction ends?
a. As in skeletal muscle, Ca2+ is transported back into the sarcoplasmic reticulum with the help of a Ca2 + -ATPase. However, in cardiac muscle Ca2+ is also removed from the cell in exchange for Na+ via the Na+-Ca2+ exchanger (NCX)
Why can't we see a region of an ECG which correlates with atrial repolarization?
a. Atrial repolarization is not represented by a special wave but is incorporated into the QRS complex.
What are EDV and ESV? Give approximate values for these parameters at rest.
a. Because maximum filling occurs at the end of ventricular relaxation (diastole), this volume is called the end- diastolic volume (EDV). In a 70-kg man at rest, end-diastolic volume is about 135 mL, but this value varies under different conditions. b. The amount of blood left in the ventricle at the end of contraction is known as the end-systolic volume (ESV). The ESV (point D) is the minimum amount of blood the ventricle contains during one cycle. An average ESV value in a person at rest is 65 mL, meaning that nearly half of the 135 mL that was in the ventricle at the start of the contraction is still there at the end of the contraction.
Why do the ventricles contract from bottom to top? What is the role of the fibrous connective tissue rings which form the origin and insertion of cardiac muscle?
a. Blood enters each ventricle at the top of the chamber but also leaves at the top. is is because during development, the tubular embryonic heart twists back on itself. This twisting puts the arteries (through which blood leaves) close to the top of the ventricles. Functionally, this means that the ventricles must contract from the bottom up so that blood is squeezed out of the top. b. Four fibrous connective tissue rings surround the four heart valves. These rings form both the origin and insertion for the cardiac muscle, an arrangement that pulls the apex and base of the heart together when the ventricles contract. In addition, the fibrous connective tissue acts as an electrical insulator, blocking most transmission of electrical signals between the atria and the ventricles. This arrangement ensures that the electrical signals can be directed through a specialized conduction system to the apex of the heart for the bottom-to-top contraction.
What is the primary determinant of the viscosity of blood?
a. Blood viscosity is determined by the ratio of red blood cells to plasma and by how much protein is in the plasma. Normally, viscosity is constant, and small changes in either length or viscosity have little effect on resistance.
Define cardiac output. How is it calculated? Give approximate values at rest and during exercise.
a. Cardiac output (CO) is the volume of blood pumped by one ventricle in a given period of time. b. Cardiac output (CO) can be calculated by multiplying heart rate (beats per minute) by stroke volume (mL per beat, or per contraction): Cardiac output = heart rate * stroke volume c. The average CO is 5 L/min d. During exercise, cardiac output may increase to 30-35 L/min. Homeostatic changes in cardiac output are accomplished by varying the heart rate, the stroke volume, or both.
Define contractility of the heart.
a. Contractility is the intrinsic ability of a cardiac muscle fiber to contract at any given fiber length and is a function of Ca2 + interaction with the contractile filaments.
Name the three factors which influence resistance to flow of blood. Which factor is the most important? How is this controlled in blood vessels?
a. For fluid flowing through a tube, resistance is in influenced by three components: the radius of the tube (r), the length of the tube (L), and the viscosity (thickness) of the fluid b. Radius has the greatest affect c. Vasoconstriction and vasodilation
Describe the pathway of blood from a large vein to the aorta Name each chamber, vessel, and valve the blood passes through.
a. From the right atrium, blood flows into the right ventricle of the heart. From there it is pumped through the pulmonary arteries to the lungs, where it is oxygenated. From the lungs, blood travels to the left side of the heart through the pulmonary veins. b. Blood from the lungs enters the heart at the left atrium and passes into the left ventricle. Blood pumped out of the left ventricle enters the large artery known as the aorta.
Describe from beginning to end the electrical conduction of the heart?
a. The depolarization begins in the sinoatrial node (SA node), autorhythmic cells in the right atrium that serve as the main pacemaker of the heart. The depolarization wave then spreads rapidly through a specialized conducting system of noncontractile autorhythmic fibers. A branched internodal pathway connects the SA node to the atrioventricular node (AV node), a group of autorhythmic cells near the floor of the right atrium. b. From the AV node, the depolarization moves into the ventricles. Purkinje fibers, specialized conducting cells, transmit electrical signals very rapidly down the atrioventricular bundle (AV bundle, also called the bundle of His ("hiss")) in the ventricular septum. A short way down the septum, the AV bundle fibers divide into left and right bundle branches. The bundle branch fibers continue downward to the apex of the heart, where they divide into smaller Purkinje fibers that spread outward among the contractile cells.
Define pre-load. How are pre-load and venous return affected by skeletal muscle pumps and respiratory pumps.
a. If additional blood flows into the ventricles, the muscle fibers stretch, then contract more forcefully, ejecting more blood. The degree of myocardial stretch before contraction begins is called the preload on the heart because this stretch represents the load placed on cardiac muscles before they contract. b. Skeletal muscle pump is the name given to skeletal muscle contractions that squeeze veins (particularly in the legs), compressing them and pushing blood toward the heart. During exercise that involves the lower extremities, the skeletal muscle pump helps return blood to the heart. During periods of sitting or standing motionless, the skeletal muscle pump does not assist venous return. c. The respiratory pump is created by movement of the thorax during inspiration (breathing in). As the chest expands and the diaphragm moves toward the abdomen, the thoracic cavity enlarges and develops a subatmospheric pressure. This low pressure decreases pressure in the inferior vena cava as it passes through the thorax, which helps draw more blood into the vena cava from veins in the abdomen. The respiratory pump is aided by the higher pressure placed on the outside of abdominal veins when the abdominal contents are compressed during inspiration. The combination of increased pressure in the abdominal veins and decreased pressure in thoracic veins enhances venous return during inspiration.
What is complete heart block?
a. In a condition known as complete heart block, the conduction of electrical signals from the atria to the ventricles through the AV node is disrupted. The SA node fires at its rate of 70 beats per minute, but those signals never reach the ventricles. So the ventricles coordinate with their fastest pacemaker. Because ventricular autorhythmic cells discharge only about 35 times a minute, the rate at which the ventricles contract is much slower than the rate at which the atria contract. If ventricular contraction is too slow to maintain adequate blood flow, it may be necessary for the heart's rhythm to be set artificially by a surgically implanted mechanical pacemaker. These battery-powered devices artificially stimulate the heart at a predetermined rate.
Name two important functions of the intercalated disks?
a. Intercalated disks have two components: desmosomes and gap junctions. Desmosomes are strong connections that tie adjacent cells together, allowing force created in one cell to be transferred to the adjacent cell. b. Gap junctions in the intercalated disks electrically connect cardiac muscle cells to one another. They allow waves of depolarization to spread rapidly from cell to cell, so that all the heart muscle cells contract almost simultaneously. In this respect, cardiac muscle resembles single-unit smooth muscle.
What is cyanosis?
a. Its when the skin turns blue due to a lack of oxygen. Presents in the fingernails and around the mouth mainly.
How much of a heart muscle cell's volume is from mitochondria? What percent of the oxygen in the coronary arteries is extracted at the myocardium? How do we increase oxygen use in myocardial cells?
a. Mitochondria occupy about one-third the cell volume of a cardiac contractile fiber, a reflection of the high energy demand of these cells. By one estimate, cardiac muscle consumes 70-80% of the oxygen delivered to it by the blood, more than twice the amount extracted by other cells in the body. b. During periods of increased activity, the heart uses almost all the oxygen brought to it by the coronary arteries. As a result, the only way to get more oxygen to exercising heart muscle is to increase the blood flow.
How does the SR of cardiac muscle differ from that of skeletal muscle?
a. Myocardial sarcoplasmic reticulum is smaller than that of skeletal muscle, reflecting the fact that cardiac muscle depends in part on extracellular Ca2+ to initiate contraction. In this respect, cardiac muscle resembles smooth muscle.
If the natural spontaneous depolarization rate of the SA node is 90-100 beats per minute, why is resting heart rate in most people about 70 bpm. How is heart rate increased to values greater than 90-100 beats/ minute?
a. Normally, tonic control of heart rate is dominated by the parasympathetic branch. This control can be shown experimentally by blocking all autonomic input to the heart. When all sympathetic and parasympathetic input is blocked, the spontaneous depolarization rate of the SA node is 90-100 times per minute. To achieve a resting heart rate of 70 beats per minute, tonic parasympathetic activity must slow the intrinsic rate down from 90 beats per minute. b. Alternatively, sympathetic input is required to increase heart rate above the intrinsic rate. Norepinephrine (or epinephrine) on b1-receptors speeds up the depolarization rate of the autorhythmic cells and increases heart rate.
What are the names of the valves which separate right ventricle from pulmonary artery and left ventricle from aorta respectively?
a. Pulmonary valve and the aortic valve b. both are semilunar valves
What is stroke volume? How does it change during periods of increased physical activity?
a. Stroke volume is the amount of blood pumped by one ventricle during a contraction. It is measured in milliliters per beat and can be calculated as follows: Volume of blood before contraction - volume of blood a er contraction = stroke volume b. Stroke volume is not constant and can increase to as much as 100 mL during exercise. Stroke volume, like heart rate, is regulated by mechanisms we discuss later in this chapter.
What type of receptors control catecholamine-stimulated calcium entry to myocardial cells? What is the second messenger in this mechanism? What is the role of phospholamban in cardiac muscle contraction?
a. The catecholamines increase Ca2+ storage through the use of a regulatory protein called phospholamban. Phosphorylation of phospholamban enhances Ca2+ -ATPase activity in the sarcoplasmic reticulum. The ATPase concentrates Ca2 + in the sarcoplasmic reticulum, making more Ca2 + available for calcium-induced calcium release. Because more cytosolic Ca2+ means more active crossbridges, and because the force of contraction is proportional to the number of active crossbridges, the net result of catecholamine stimulation is a stronger contraction.
What function does the spiral arrangement of muscles in the walls of the ventricle provide?
a. The ejection of blood from the ventricles is aided by the spiral arrangement of the muscles in the walls. As these muscles contract, they pull the apex and base of the heart closer together, squeezing blood out the openings at the top of the ventricles.
What are coronary arteries, veins and sinus?
a. The first branch represents the coronary arteries, which nourish the heart muscle itself. Blood from these arteries flows into capillaries, then into the coronary veins, which empty directly into the right atrium at the coronary sinus.
Explain the Frank-Starling mechanism. How is it related to the length-tension curve?
a. The graph shows that stroke volume is proportional to EDV. As additional blood enters the heart, the heart contracts more forcefully and ejects more blood. This relationship is known as the Frank-Starling law of the heart. It means that within physiological limits, the heart pumps all the blood that returns to it. The length-tension curve shows this on a graph
What is the pericardium? What role does it play for the heart?
a. The heart is encased in a tough membranous sac, the pericardium. A thin layer of clear pericardial fluid inside the pericardium lubricates the external surface of the heart as it beats within the sac. Inflammation of the pericardium (pericarditis) may reduce this lubrication to the point that the heart rubs against the pericardium, creating a sound known as a friction rub.
In the systemic circulation, where is blood pressure highest, and where is it lowest?
a. The highest pressure in the vessels of the cardiovascular system is found in the aorta and systemic arteries as they receive blood from the left ventricle. The lowest pressure is in the venae cavae, just before they empty into the right atrium.
What are the functions of chordae tendineae and papillary muscles? What is prolapse?
a. The opening between each atrium and its ventricle is guarded by an atrioventricular (AV) valve. The AV valve is formed from thin flaps of tissue joined at the base to a connective tissue ring. The flaps are slightly thickened at the edge and connect on the ventricular side to collagenous tendons, the chordae tendineae. The chordae tendineae prevent the valve from being pushed back into the atrium. b. Most of the chordae fasten to the edges of the valve flaps. The opposite ends of the chordae are tethered to moundlike extensions of ventricular muscle known as the papillary muscles. These muscles provide stability for the chordae, but they cannot actively open and close the AV valves. The valves move passively when flowing blood pushes on them c. Occasionally, the chordae fail, and the valve is pushed back into the atrium during ventricular contraction, an abnormal condition known as prolapse.
Which hormones primarily increase and decrease heart rate?
a. The parasympathetic neurotransmitter, acetylcholine (ACh), slows heart rate b. Sympathetic stimulation of pacemaker cells speeds up heart rate. The catecholamines norepinephrine (from sympathetic neurons) and epinephrine (from the adrenal medulla) increase ion flow through both If and Ca2+ channels.
Where is the apex of the heart?
a. The pointed apex of the heart angles down to the left side of the body, while the broader base lies just behind the breastbone, or sternum.
What is the primary function of the cardiovascular system?
a. The primary function of the cardiovascular system is the trans- port of materials to and from all parts of the body. Substances transported by the cardiovascular system can be divided into (1) nutrients, water, and gases that enter the body from the external environment, (2) materials that move from cell to cell within the body, and (3) wastes that the cells eliminate
What is autorhythmycity? How do cells of the heart accomplish this task?
a. The signal for myocardial contraction comes not from the nervous system but from specialized myocardial cells known as autorhythmic cells. The autorhythmic cells are also called pacemakers because they set the rate of the heartbeat.
List the three components of the ECG, and describe what each component is associated with.
a. Three major waves can be seen on a normal ECG recorded from lead I. The first wave is the P wave, which corresponds to depolarization of the atria. e next trio of waves, the QRS complex, represents the progressive wave of ventricular depolarization. The final wave, the T wave, represents the repolarization of the ventricles.
What is the cause of the heart sounds?
a. Vibrations following closure of the AV valves create the first heart sound, S1, the "lub" of "lub-dup." b. The vibrations created by semilunar valve closure are the second heart sound, S2, the "dup" of "lub-dup."
What is the hepatic portal vein?
a. a blood vessel that carries blood from the gastrointestinal tract, gallbladder, pancreas and spleen to the liver.
How can one calculate the velocity of blood flow in a vessel or group of vessels?
a. the flow rate is identical along the length of the vessel: 12 cm3 per minute (1 cm3 = 1 cubic centimeter (cc) = 1 mL). This flow rate means that in one minute, 12 cm3 of fluid flows past point X in the narrow section, and 12 cm3 of fluid flows past point Y in the wide section.
The t-tubule membrane contains voltage-sensing receptors called _____________________________________ that are mechanically linked to calcium ion release channels in the adjacent sarcoplasmic reticulum. These release channels are called ___________________________ or ____________________________.
dihydropyridine (DHP) receptors Ryanodine receptors or RyR.
Pressure created within the ventricles is called the _________________________.
driving pressure
When fluid is not moving, the pressure is called __________________________.
hydrostatic pressure
Any chemical that affects contractility of the heart is called an ___________________.
inotropic agent
Although smooth muscles can release calcium in response to stretch, the calcium channels exhibit __________________ and calcium channels begin to close in a time-dependent fashion.
myogenic contraction
Tropomyosin partially blocks the ____________________________________. It can be moved by its regulation by __________________________ so as to allow the power stroke to take place.
myosin binding sites of actin troponin
Flow through a tube is directly proportional to the _________________________.
pressure gradient
The _____________________ separates the right atrium and right ventricle, whereas the _______________________ (or mitral valve) separates the left atrium and left ventricle.
tricuspid valve bicuspid valve
