Module 4: Skeletal Muscle Structure, Function, and Control

Acetylcholine (ACh) is the neurotransmitter used by motor neurons. Which statement is TRUE about ACh at the neuromuscular junction?

ACh is broken down in the synaptic cleft by acetylcholinesterase and transported back into the axon terminals as choline.

During rigor mortis (choose all correct answers):

ATP levels are low cytoplasmic calcium levels are high myosin is bound to actin

Select the TRUE statement.

Animals and some unicellular organisms use actin and myosin to move.

Chicken or turkey breasts are "white meat" and legs are "dark meat". Why do these differ in color? What is the physiological significance of these color differences?

Chickens and turkeys are poor fliers but can, sometimes, fly for a brief period. Their breast muscles are white, glycolytic muscles. Glycolytic fibers have few mitochondria and low concentrations of myoglobin but possess a high concentration of glycolytic enzymes and a large store of glycogen. They are white because they are surrounded by relatively few blood vessels. Leg muscles of chickens or turkeys are dark because they are "red, oxidative fibers", because these birds are good walkers. Red, oxidative fibers contain numerous mitochondria and thus have a high capacity for oxidative phosphorylation. They are referred to as red muscle fibers because the high level of myoglobin inside them and large number of blood-filled capillaries give them a dark red color.

What is the correct sequence of events for cross-bridge activation in smooth muscle? A. The Ca2+-calmodulin complex binds troponin and removes blocking action of tropomyosin. B. Phosphorylation of myosin drives the cross-bridge away from the thick filament backbone, allowing it to bind to actin. C. Ca2+ removal from troponin restores tropomyosin blocking action D. Ca2+ binds to calmodulin. E. Active myosin light-chain kinase phosphorylates myosin light chains. F. Cross-bridges go through repeated cycles of force generation as long as myosin light chains are phosphorylated. G. The Ca2+-calmodulin complex binds myosin light-chain kinase.

D,G,E,B,F

The thin filament is made mostly of the protein myosin while the thick filament is made mostly of the proteins actin, troponin, and tropomyosin. True or false?

False

Which fiber type is most dependent on glycogen as a fuel source during exercise?

Fast glycolytic

Select the statements that apply to high endurance performers in the Animal Kingdom. (Select all that apply.)

Fat is the primary fuel used, A hyena is an example of a high endurance performer, Mitochondrial content is high.

Select the TRUE statement(s) about single unit smooth muscle. (Select all that apply.)

Has most cells connected by gap junctions, Has the property that most of the cells contract together, An example is the smooth muscle of the uterus

How is smooth muscle different than skeletal muscle? (Select all that apply.)

In smooth but not skeletal muscle, some neurons release inhibitory chemicals, Unlike skeletal muscle, smooth muscle cells have a single nucleus and have the capacity to divide throughout the life of an individual.

1. Compare the activation of smooth muscle to skeletal muscle.

In smooth muscle, a rise in cytoplasmic Ca++ causes a binding of Ca++ to calmodulin, which associates with myosin kinase, causing a phosphorylation of myosin. The myosin cross-bridge then binds to actin, and cleaves ATP to contract. Relaxation occurs due to a fall in Ca++, which activates myosin phosphorylase, which removes the phosphate from myosin, so it will not bind to actin. In contrast, in skeletal muscle, Ca++ binds to troponin, which regulates the shape of tropomyosin, triggering contraction.

1. What is the difference between isometric and isotonic contraction?

Isometric contraction is when a muscle fiber develops tension without changing the length of the muscle fiber. Isotonic contraction is when a muscle fiber maintains a constant tension while changing the length of the muscle fiber.

What is true about muscular dystrophy? (Select all that apply.)

Muscular dystrophy is a genetic disease, Muscular dystrophy is caused by the absence or defect of one or more proteins that link the z disks to the sarcolemma.

Choose the TRUE statement(s) about body weight regulation. There is one correct answer.

Protein can be turned into fat, after amino acids are deaminated.

Relaxation of skeletal muscle requires (choose the one correct answer):

Removal of acetylcholine from the receptor on the neuromuscular junction.

Exam 2 Practice Questions

See Below

Choose the true statement about male reproduction. There is one correct answer.

Suppression of LH and FSH will reduce sperm production.

A drug that blocked calcium uptake by the sarcoplasmic reticulum calcium ATPase would have what effect?

The muscle would show a prolonged contraction.

3. How does the length of a muscle fiber effect the tension of a contraction?

The relationship of the overlap of the thick and thin filaments affects the tension of a contraction since the sliding-filament mechanism is what creates the tension. If there is so much overlap that the thick filament can't physically pull the thin filament any farther, the tension will be low. As the length increases there will be more area for the thick filament to pull on the thin filament. As the fiber is continued to be lengthened, there will be a point where the amount of overlap of the thick and thin filament will decrease and this will cause the tension to decrease as well.

3. You see a weightlifter curling heavy weights. This makes you notice the difference in muscle mass between yourself and the weightlifter. How do muscles increase in size in response to their heavy exercise practices?

The weightlifter has most likely undergone muscle hypertrophy. This muscular hypertrophy likely occurred by a combination of enlargement of existing muscle fibers, splitting of existing muscle fibers, and satellite cell proliferation, differentiation, and fusion.

Choose the true statements about skeletal muscle. There may be more than one correct answer.

a. blocking all ion channels on the t-tubules would block contraction b. ATP must remain high for the muscle to contract and relax d. blocking the calcium ATPase would inhibit relaxation

Choose the true statements about inflammation (choose all that are correct):

cytokines are often elevated neutrophils phagocytize bacteria fever often occurs

A drug that removed calmodulin from smooth muscle would likely? (choose the one correct answer):

inhibit contraction

During the absorptive state: (choose the one correct answer)

insulin levels are high

Which regulatory molecule increases anabolism in muscle after exercise training? There is one correct answer.

insulin-like growth factor

Choose the TRUE statement for how a heavier weight can be lifted.

more motor neurons are activated

Cutting a motor neuron is likely to cause _______.

multiple muscle cells will no longer contract

A drug that blocked calcium release from the sarcoplasmic reticulum of smooth muscle would most likely ___________.

reduce the contraction of smooth muscle

Why does skeletal muscle use more energy per unit time than smooth muscle? Choose all correct answers.

the myosin ATPase is faster more calcium is cycled across the sarcoplasmic reticulum skeletal muscle can contract and relax more quickly

Botulism is caused by the bacterium Clostridium botulinum, and the toxin from this bacteria is the active ingredient in botox. Which statement(s) is/are TRUE about botulism? (Select all that apply.)

Botulinum toxin breaks down proteins of the SNARE complex in the axon terminal, stopping acetylcholine release. Botulinum reduces the amount of acetylcholine bound to the receptor.

2. What is the difference between twitch, tetanus and unfused tetanus? Why is the tension generated greater during tetanus than twitch?

A twitch contraction is the mechanical result of a single action potential and the tension will decrease after the twitch. Tetanus is where repeated twitch contraction at a rate faster than the tension can return to zero causes a maintained contraction with high constant tension. Unfused tetanus is like tetanus except that the rate of twitch contractions is slower and this leads to a fluctuation in the tension of the continuous contraction. Higher tensions are generated during tetanus for a number of reasons. First, during tetanus, cytoplasmic calcium levels reach higher levels as more calcium exits the sarcoplasmic reticulum (SR) due to repeated stimulations than can be pumped into the SR by the calcium ATPase. Higher calcium levels mean that more calcium is bound to troponin, allowing more sites on actin to be available to the myosin cross-bridges. Also, Elastic elements such as tendons and titin tend to absorb more of the tension during a twitch than during tetanus.

Choose the true statement about skeletal muscles.

Some unicellular organisms use actin and myosin to move.

2. Compare the control of smooth muscle contractile activity to skeletal muscle.

The inputs that influence smooth muscle contractile activity are extremely varied, including spontaneous electrical activity in the plasma membrane of the muscle cell, various neurotransmitters released by autonomic neurons, hormones, local induced changes in the chemical composition of the extracellular fluid surrounding the cell, and stretch. Some of these inputs can inhibit smooth muscle contraction, while others stimulate contraction. In contrast, with skeletal muscle, there is only excitatory stimuli, when motor neurons release acetylcholine.

How does an acetylcholinesterase inhibitor (as found in some nerve gases and organophosphate insecticides) cause paralysis?

Acetylcholinesterase in an enzyme found in the synapse at the neuromuscular junction, as well as at muscarinic synapses. Acetylcholinesterase breaks down acetylcholine after it has been released and/or is bound to the receptor, stopping the stimulation. If acetylcholinesterase is inhibited, the acetylcholine stays bound to the receptor. The muscle (or postsynaptic neuron) cannot repolarize, so briefly, the muscle is in a prolonged contraction. Over time, the receptor stops responding to the bound acetylcholine, and the muscle is paralyzed. Death occurs because the respiratory muscles are paralyzed, and oxygen cannot be delivered to the lungs.

Give examples of skeletal muscle disorders which target the central nervous system, peripheral nervous system and muscle tissue respectively. Explain the mode of action for each disorder.

An example of a skeletal muscle disorder that occurs in the central nervous system would be poliomyelitis. Poliomyelitis is a viral disease that destroys motor neurons. This leads to paralysis of skeletal muscle and possibly death due to respiratory failure. An example of a skeletal muscle disorder that occurs in the peripheral nervous system would be muscle cramping. Though the specific cause of muscle cramps is uncertain, that are partly caused by electrolyte imbalances in the extracellular fluid surrounding both the muscle and the nerve fibers or axons. The axons of motor neurons are located in the peripheral nervous system. Finally examples of a skeletal muscle disorders that occurs in the muscle tissue include Hypocalcemic tetany that is caused by a decrease in extracellular Ca2+ which leads to membrane depolarization and spontaneous firing of action potentials in increased muscle contractions. Muscular Dystrophy which is caused by the absence of defect of one or more proteins that make up costameres (clusters of structural and regulatory proteins linking the Z disks to the sarcolemma and extracellular matrix) in striated muscle. Finally the autoimmune disease, Myasthenia gravis that causes progressively worsening muscle fatigue and weakness. Antibodies from the immune system target and destroy nicotinic ACh-receptor proteins of the motor end plate making muscle cells less responsive to action potential firings from motor neurons.

Refer to the figure above. Why does the overall isometric tension of a stimulated muscle fiber drop as the muscle fiber length is increased past the optimal length (L0)?

Filament overlap is near-ideal at length 4, stretching the fiber beyond that drops the maximal number of cross-bridges able to bind to the thin filaments, thereby decreasing the overall tension.

As a person varies their exercise type from a single maximal burst (e.g. lifting the maximal possible weight), to burst performance that lasts under a minute (e.g. sprinting a hundred meters) to a long-term, low intensity exercise (e.g. walking or jogging for two hours), how do the fuels, pathways and end products used to produce ATP differ?

For very short-term maximal exercise, creatine phosphate is used as a very fast supply of ATP. The end product for this reaction is creatine and inorganic phosphate. For a longer-term of still near-maximal exercise like sprinting, glucose (derived from glycogen or blood glucose) is used as a fuel for glycolysis, producing mostly lactate and protons. For long-term, low-intensity exercise such as prolonged walking, glucose is burned for the first 30 min or so, and then fat will be burned aerobically, producing CO2 and water.

2. What regulatory molecules mediate exercise-induced changes in muscle?

Insulin-like growth factor-1, Anabolic steroids (androgens) and myostatin.

1. Does the lever system of most muscles increase the force that muscles can produce on the environment? Explain.

No, muscles operate at a mechanical disadvantage, as they insert a short distance from the fulcrum (joint pivot point) compared to the end of the limb, which acts on the environment. So the force on the environment is less than exerted by the muscle. However, the end of the limb can move faster than the muscle can shorten.

Select the TRUE statement(s) about smooth muscle. (Select all that apply.)

Norepinephrine released from the sympathetic nervous system can inhibit peristalsis in gut smooth muscle, Some smooth muscle cells spontaneously depolarize and contract.

What is the metabolic pathway represented by the number "2" and where does it occur?

Oxidative Phosphorylation; Mitochondria

During a Biology lab you are observing the leg of a recently euthanized frog. The leg of the frog moves freely when you push on it. In contrast, the leg of a frog that died ten hours ago is very stiff (rigor mortis). Explain why rigor mortis occurs. Does a corpse remain in rigor mortis forever? If not, why not?

Rigor mortis is caused by the near-zero levels of ATP in the cytosol. This occurs because after death, the heart is not pumping blood with oxygen to the tissues. Lacking oxygen, the muscle cell has a very limited capacity to produce ATP, so ATP levels in the muscle cytoplasm fall dramatically. Without ATP, the Ca++ ATPase cannot pump Ca++ into the sarcoplasmic reticulum. Calcium continues to leak out of the sarcoplasmic reticulum, so levels of Ca++ in the cell increase to a high level, and bind troponin, allowing binding of myosin to actin. However, rigor mortis is not the same as contraction. ATP is required for myosin to release from actin, thus the stiffness muscle during of rigor mortis arises from a constant binding of myosin to actin. The stiffness caused by rigor mortis will cease as the muscle tissues break down and decompose.

1. Why is skeletal muscle referred to as striated muscle?

Skeletal muscle is referred to as striated muscle because the myofibrils of each muscle cell are made up of a repeating pattern of cellular structures known as the sarcomere that, from a microscopic view, forms light and dark bands perpendicular to the long axis.

What are the structural differences between skeletal muscle and smooth muscle?

Smooth muscle cells are much smaller than skeletal muscle fibers. Smooth muscle cells have a single nucleus and have the capacity to divide throughout the life of an individual. Regulatory protein troponin is absent in smooth muscle and a protein called caldesmon associates with the thin filaments of smooth muscle. The thin filaments are anchored either to the plasma membrane or to cytoplasmic structures known as dense bodies in smooth muscle cells. The filaments are organized diagonally to the long axis of the smooth muscle cells and the filaments are not organized in myofibrils nor do they contain sarcomeres. The concentration of myosin in smooth muscle is about one-third that of skeletal muscle and the actin concentration can be twice that of skeletal muscle. Smooth muscle can maintain more tension production when its length is greatly increased than skeletal muscle can.

You stepped on a rusty nail and contracted the tetanus toxin from Clostridium tetani. The toxin blocks the inhibitory nerve terminals in your motor neurons causing all motor neurons to fire at high rates. What will happen to your muscles? (Select all that apply.)

The skeletal muscles will exhibit tetanus (sustained contractions), Ca2+ levels in skeletal muscle cytoplasm will be at a sustained high level, Skeletal muscles will produce greater tension than during a twitch.

2. What are the proteins that make up the thick and thin filament? Which molecule forms the cross-bridge?

The thick filament is composed of myosin filaments, and myosin forms the crossbridges. The thin filament is composed of actin filaments, which are wrapped with tropomyosin filaments, and studded with troponin molecules.

As you lift a heavier weight, what is happening? (Select all that apply.)

You increase the number of motor units that are active in each activated muscle, You increase the frequency of action potentials in your motor neurons.

You are contracting your bicep muscle to create tension applied to an object you are holding. The object you are holding is placing a load on your muscle. You are not able to hold the object up because it is too heavy and you are slowly dropping the object. What will happen? (Select all that apply.)

Your bicep muscle is experiencing an isotonic contraction, Your bicep muscle is undergoing an eccentric contraction, The load is greater than the maximum isometric tension