(2) MASTERING A&P
How is acetylcholine (ACh) removed from the synaptic cleft?A) simple diffusion away from the synaptic cleft and acetylcholinesterase (AChE; an enzyme) B) acetylcholinesterase (AChE; an enzyme) and endocytosis into the muscle fiber C) acetylcholinesterase (AChE; an enzyme) only D) simple diffusion away from the synaptic cleft and endocytosis into the muscle fiber
A)
Which of the choices below correctly describes how an action potential generated at the neuromuscular junction (NMJ) is converted to excitation in the muscle fiber? A) An action potential in the motor neuron causes ACh to be released into the synaptic cleft. Binding of ACh to sarcolemma receptors initiates graded potentials. B) ACh binds at a receptor in the motor neuron, which initiates graded potentials. C) An action potential in the motor neuron directly triggers an action potential at the sarcolemma. D) An action potential in the motor neuron causes ACh to be released into the synaptic cleft. Binding of ACh at the membrane receptor initiates a direct action potential.
A) -An action potential traveling down a motor neuron arrives at the axon terminal and causes exocytosis of the neurotransmitter ACh into the synaptic cleft. ACh diffuses into the synaptic cleft, binds to the receptor proteins on the junctional folds of the muscle sarcolemma (motor end plate), and initiates graded potentials. These graded potentials sum to an action potential, thus initiating excitement of the muscle fiber.
How/when does the myosin head cock back to store energy for the next cycle? A) After the myosin head detaches, energy from ATP hydrolysis is used to re-cock the myosin head. B) when ADP is released from the myosin head C) The power stroke cocks the myosin head. D) The sliding of the actin myofilament during the power stroke re-cocks myosin heads that have previously delivered their power stroke.
A) -In addition to breaking the cross bridge, ATP provides energy to cock the heads back after the previous power stroke.
A triad is composed of a T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum. How are these components connected? A) a series of proteins that control calcium release. B) Potassium leak channels. C) Voltage-gated sodium channels. D) Myosin cross-bridge binding sites
A) -When action potentials propagate along T-tubules, a voltage-sensitive protein changes shape and triggers a different protein to open it's channels, resulting in the release of calcium from the terminal cisternae.
A myosin head binds to which molecule to form a cross bridge? A) actin B) troponin C) tropomyosin
A) -the myosin head binds to actin, the major component of thin filaments.
Where in the cross bridge cycle does ATP hydrolysis occur? A) during the cocking of the myosin head B) during the movement of tropomyosin to expose the active sites on actin C) during the power stroke D) during the removal of calcium from troponin
A) -As ATP is broken down, its energy is used to cock the myosin head in preparation for the next power stroke
The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge? A) A myosin head bound to actin B) Calcium bound to troponin C) ATP bound to a myosin head D) Troponin bound to tropomyosin
A) -As soon as the activated myosin head forms a cross bridge with actin, the power stroke begins.
The force of a muscle contraction is NOT affected by __________. A) the amount of ATP stored in the muscle cells B) the degree of muscle stretch C) the number of muscle fibers stimulated D) the frequency of the stimulation E) the size of the muscle fibers stimulated
A) -The force of a muscle contraction is NOT affected by the amount of ATP stored in the muscle cells. Instead of relying on storage of ATP, muscle cells use ATP regenerating pathways, such as glycolysis, to meet the ATP demands of muscle contraction.
When muscle cells break down glucose to generate ATP under oxygen deficient conditions, they will form ________. A) lactic acid B) glucose C) fatty acids D) glycogen
A) -Under anaerobic conditions, (oxygen deficient conditions) most of the pyruvic acid produced during glycolysis (breakdown of glucose) will be converted to lactic acid rather than enter the mitochondria to participate in aerobic respiration.
A muscle cell runs out of ATP. Even though these are cyclic reactions, what step of the cross bridge cycle given is most directly inhibited or terminated? A) cross bridge detachment B) cocking of myosin head C) cross bridge formation D) the power stroke
A) -When ATP is added to the myosin, myosin detaches from actin
The action potential on the muscle cell leads to contraction due to the release of calcium ions. Where are calcium ions stored in the muscle cell? A) terminal cisterns (cisternae) of the sarcoplasmic reticulum B) T tubule C) sarcolemma D) cytosol
A) -calcium is stored in the terminal cisterns (cisternae) of the sarcoplasmic reticulum until it is released by an action potential.
The neuromuscular junction is a well-studied example of a chemical synapse. Which of the following statements describes a critical event that occurs at the neuromuscular junction? A) Acetylcholine is released by axon terminals of the motor neuron. B) When the action potential reaches the end of the axon terminal, voltage-gated sodium channels open and sodium ions diffuse into the terminal. C) Acetylcholine binds to its receptor in the junctional folds of the sarcolemma. Its receptor is linked to a G protein. D) Acetylcholine is released and moves across the synaptic cleft bound to a transport protein.
A. -Acetylcholine is released into the synaptic cleft via exocytosis.
Acetylcholine binds to its receptor in the sarcolemma and triggers __________. A) the opening of ligand-gated cation channels B) the opening of calcium-release channels C) the opening of voltage-gated calcium channels D) the opening of ligand-gated anion channels
A. -These channels permit sodium ions to diffuse inward and potassium ions to diffuse outward.
What is the role of calcium in the cross bridge cycle? A) Calcium binds to active sites on actin, forming the cross bridge. B) Calcium binds to troponin, altering its shape. C) Calcium binds to myosin, causing the myosin head to release from the actin myofilament. D) Calcium binds to troponin, exposing the active site on troponin.
B) -Calcium binding to troponin causes tropomyosin to move away from the active sites on actin.
Which best describes the initial event in contraction? A) Tropomyosin binds to myosin heads and actin bridges with tropomyosin. B) Myosin heads bind to the newly exposed myosin-binding sites on actin to form cross bridges. C) Calcium binds actin to myosin to begin the cross bridge cycle. D) The myofilaments slide closer together.
B) -Contraction begins when myosin binds to actin and forms cross bridges.
Calcium ions couple excitation of a skeletal muscle fiber to contraction of the fiber. Where are calcium ions stored within the fiber? A) Calcium ions are stored in the mitochondria. B) Calcium ions are stored in the sarcoplasmic reticulum. C) Calcium ions are stored in the transverse tubules. D) Calcium ions are stored in the nuclei.
B) -Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle fibers. The sarcoplasmic reticulum is very elaborate in skeletal muscle fibers, allowing for significant storage of calcium ions.
What is name given to the regularly spaced infoldings of the sarcolemma? A) terminal cisternae B) transverse or T tubules C) sarcoplasmic reticulum D) motor endplates
B) -T tubules penetrate a skeletal muscle fiber and provide a pathway for excitation into the interior.
Which selection correctly describes the role of calcium in coupling? A) Calcium binds to actin, which triggers it to bind to myosin. B) Calcium binds to troponin, which moves tropomyosin and exposes the myosin-binding sites on actin. C) Calcium binds to tropomyosin, which moves tropomyosin and exposes the myosin-binding sites on actin. D) Calcium binds to actin, which triggers troponin to fall off and expose myosin-binding sites.
B) -The thin filament in a sarcomere is composed of actin, troponin, and tropomyosin. Troponin and tropomyosin are attached to one another, both overlaying actin. When a muscle is relaxed, tropomyosin blocks actin's myosin-binding sites. Calcium binds to troponin, initiating a shape change that removes the blocking action of tropomyosin. This exposes the myosin-binding sites on actin to the myosin heads for cross bridging.
What causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum within a muscle cell? A) calcium ion pump B) arrival of an action potential C) troponin D) ATP
B) -an action potential in the T tubule causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum.
The response of a motor unit to a single action potential of its motor neuron is called ________. A)recruitment B) a muscle twitch C) a tetanic contraction D) wave summation
B) -A motor unit's response to a single, brief threshold stimulus (a single action potential) is a muscle twitch
Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers? A) Acetylcholine. B) Calcium ions. C) Sodium ions. D) Action potentials.
B) -Action potentials propagating down the T-tubule cause a voltage-sensitive protein to change shape. This shape change opens calcium release channels in the sarcoplasmic reticulum, allowing calcium ions to flood the sarcoplasm. This flood of calcium ions is directly responsible for the coupling of excitation to contraction in skeletal muscle fibers
During contraction, what prevents actin myofilaments from sliding backward when a myosin head releases? A) Calcium blocks the active sites on actin. B) There are always some myosin heads attached to the actin myofilament when other myosin heads are detaching. C) The cross bridge remains in place, preventing the actin myofilament from sliding. D) The actin myofilament can only move in one direction relative to the myosin filament.
B) -During contraction, about half of the myosin heads are attached, preventing the actin myofilament from sliding backwards when any single myosin head detaches. The situation is analogous to a game of tug-of-war. In tug-of-war, individual hands release after they pull on the rope, but not all hands release at the same time.
After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment? A) ADP and inorganic phosphate are bound to the myosin head. B) ATP binds to the myosin head. C) Calcium ions bind to troponin. D) Acetylcholine binds to receptors in the junctional folds of the sarcolemma.
B) -The binding of ATP to the myosin head weakens the bond between myosin and actin, forcing the myosin head to detach. ATP also provides the energy for the next power stroke.
Which of the following best describes how ACh changes the ion permeability of the sarcolemma? A) ACh entry into the muscle fiber causes Ca2+ to be released by the sarcoplasmic reticulum. B) ACh binds to receptors in the junctional folds. C) ACh entry into the axon terminal causes neurotransmitter release. D) ACh activates acetylcholinesterase.
B) -The junctional folds of the muscle fiber's sarcolemma contain ACh receptors. These receptors are chemically gated ion channels. When ACh binds to these receptors, they open, allowing ions to diffuse across the sarcolemma. This represents an increase in the sarcolemma's ion permeability.
What role does tropomyosin play in the cross bridge cycle? A) Tropomyosin moves the actin filament relative to the myosin filament. B) The displacement of tropomyosin exposes the active sites of actin, allowing cross bridges to form. C) Tropomyosin pushes the myosin head away, causing cross bridge detachment. D) Tropomyosin binds to calcium, causing muscle relaxation.
B) -Tropomyosin covers active sites in relaxed muscle. When tropomyosin is displaced, the active sites are exposed for cross bridge formation.
The binding of the neurotransmitter to receptors on the motor end plate causes which of the following to occur? A) Binding causes voltage-gated sodium channels to open in the motor end plate (junctional folds of the sarcolemma) and sodium enters the cell. B) Binding of the neurotransmitter causes chemically gated sodium channels to open in the motor end plate (junctional folds of the sarcolemma) and sodium enters the cell. C) Binding causes potassium voltage-gated channels to open in the motor end plate (junctional folds of the sarcolemma) and potassium enters the cell. D) Binding causes chemically gated potassium channels to open in the motor end plate (junctional folds of the sarcolemma) and potassium enters the cell.
B) -sodium enters the cell and causes depolarization. A small amount of potassium also leaves the motor end plate (junctional folds of the sarcolemma).
Excitation of the sarcolemma is coupled or linked to the contraction of a skeletal muscle fiber. What specific event initiates the contraction? A) Sodium release from the sarcoplasmic reticulum initiates the contraction. B) Calcium release from the sarcoplasmic reticulum initiates the contraction. C) Voltage-sensitive proteins change shape. D) Action potentials propagate into the interior of the skeletal muscle fiber
B. -Sarcoplasmic reticulum is the specific name given to the smooth endoplasmic reticulum in muscle cells. It is especially abundant and convoluted in skeletal muscle cells. It functions in the storage, release, and reuptake of calcium ions.
Action potentials travel the length of the axons of motor neurons to the axon terminals. These motor neurons __________. A) extend from the spinal cord to the sarcolemma of a skeletal muscle fiber B) extend from the brain or spinal cord to the sarcolemma of a skeletal muscle fiber C) extend from the brain to the sarcolemma of a skeletal muscle fiber D) arise in the epimysium of a skeletal muscle and extend to individual skeletal muscle fibers
B. -The cell bodies of motor neurons to muscles in the head and neck are located in the brain. The cell bodies of motor neurons to the rest of our muscles are located in the spinal cord
Which of the following statements best summarizes excitation-contraction coupling? A) A series of events in which actin binds myosin to enact contraction. B) A series of events in which an action potential triggers the release of chemical messengers. C) A series of events in which an electrical stimulus is conveyed to a muscle fiber to enact contraction. D) A series of events in which calcium gradients are used to convey a signal to the muscle fiber to enact contraction.
C)
What specific event triggers the uncovering of the myosin binding site on actin? A) Sodium ions bind to troponin and change its shape. B) Calcium ions bind to tropomyosin and change its shape. C) Calcium ions bind to troponin and change its shape. D) Calcium release channels open in the sarcoplasmic reticulum, and calcium levels rise in the sarcoplasm.
C) -The shape change caused by the binding of calcium to troponin shifts tropomyosin away from the myosin binding sites on actin.
The sliding filament model of contraction states that ________. A) during contraction the thin myofilaments slide past the thick myofilaments so that the actin and myosin myofilaments no longer overlap B) during contraction the thin myofilaments slide past the thick myofilaments so that calcium ions can be released from the sarcoplasmic reticulum C) during contraction the thin myofilaments slide past the thick myofilaments so that the actin and myosin myofilaments overlap to a greater degree D) during contraction the thin myofilaments slide past T tubules so that the Z discs are overlapping
C) -The sliding filament model of contraction states that during contraction, the thin myofilaments slide past the thick myofilaments so that actin and myosin myofilaments overlap to a greater degree. In a relaxed muscle fiber, the thick and thin myofilaments overlap only at the ends of the A band.
What is the relationship between the number of motor neurons recruited and the number of skeletal muscle fibers innervated? A) A motor neuron typically innervates only one skeletal muscle fiber. B) Motor neurons always innervate thousands of skeletal muscle fibers. C) Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron. D) A skeletal muscle fiber is innervated by multiple motor neurons.
C) -There are many more skeletal muscle fibers than there are motor neurons. The ratio of neurons to fibers varies from approximately one to ten to approximately one to thousands.
What causes the myosin head to disconnect from actin? A) binding of calcium B) hydrolysis of ATP C) binding of ATP D) binding of troponin
C) -the binding of ATP causes the myosin head to disconnect from actin
What means of membrane transport is used to release the neurotransmitter into the synaptic cleft? A) a channel B) a protein carrier C) exocytosis
C) -the synaptic vesicles (where the neurotransmitter is stored) merge with the membrane and release the neurotransmitter by exocytosis.
A person dies and within hours, the skeletal muscles develop a locked contraction known as rigor mortis. Calcium ions leak from the sarcoplasmic reticulum into cytoplasm. From your knowledge of cross-bridge cycling, what best explains this rigor? A) Cross bridge formation cannot occur. ATP, which is only produced during life, must to be bound to the myosin head to permit cross bridging. B) The cross bridge is locked up because ATP directly attaches myosin to actin. C) Cross bridge detachment cannot occur. Detachment requires ATP, which is only produced during life. D) The cross bridge is locked up because the power stroke requires ATP, which is only produced during life.
C) -After ATP attaches to the myosin head, the bond between actin and myosin is weakened and the cross-bridge breaks.
The action potential propagates along the sarcolemma. As the action potential spreads down the T tubules of the triads, voltage-sensitive tubule proteins change shape. How does the shape change of these proteins lead to contraction? A) It allows calcium to exit the cytoplasm and enter the cisterns. B) It facilitates ACh binding to the sarcolemma. C) It allows calcium to exit the sarcoplasmic reticulum and enter the cytosol. D) It permits calcium to exit the cistern and enter the T tubule.
C) -As the action potential propagates, it changes the shape of T tubule proteins. These proteins are linked to calcium channels in the terminal cisterns of the sarcoplasmic reticulum. When these proteins calcium channels open, a massive amount of calcium flows into the cytosol.
Sodium and potassium ions do not diffuse in equal numbers through ligand-gated cation channels. Why? A) The outside surface of the sarcolemma is negatively charged compared to the inside surface. Sodium ions diffuse outward along favorable chemical and electrical gradients. B)The outside surface of the sarcolemma is negatively charged compared to the inside surface. Potassium ions diffuse outward along favorable chemical and electrical gradients. C) The inside surface of the sarcolemma is negatively charged compared to the outside surface. Sodium ions diffuse inward along favorable chemical and electrical gradients. D) The inside surface of the sarcolemma is negatively charged compared to the outside surface. Potassium ions diffuse inward along favorable chemical and electrical gradients.
C. -The resting membrane potential of all cells is negative (inside compared to outside). Therefore, given the direction of the chemical and electrical gradients, more sodium ions diffuse inward than potassium ions diffuse outward.
Calcium entry into the axon terminal triggers which of the following events? A) Cation channels open and sodium ions enter the axon terminal while potassium ions exit the axon terminal. B) Acetylcholine binds to its receptor. C) Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine. D) Acetylcholine is released into the cleft by active transporters in the plasma membrane of the axon terminal.
C. -when synaptic vesicles fuse to the plasma membrane, acetylcholine is released via exocytosis.
What structure is the functional unit of contraction in a skeletal muscle fiber? A) The junctional folds of the sarcolemma B) The cross bridge C) The triad D) The sarcomere
D) -A sarcomere is a regular arrangement of thin and thick myofilaments that extends from one Z disc to the next. A myofibril consists of a series of sarcomeres.
Which event causes cross bridge detachment? A) nervous input ends B) release of calcium from troponin C) release of ADP and inorganic phosphate from the myosin head D) ATP binding to the myosin head
D) -As ATP binds, the myosin head releases from the active site on actin
The cross bridge cycle starts when _________. A) ATP binds to troponin and is hydrolyzed to ADP and Pi B) acetylcholine diffuses away from the synaptic cleft C) Ca2+ is actively transported into the sarcoplasmic reticulum D) Ca2+ from the sarcoplasmic reticulum binds to troponin E) Ca2+ from the sarcoplasmic reticulum binds to tropomyosin
D) -The release of Ca2+ is triggered by the propagation of an action potential along a skeletal muscle fiber. Ca2+ is released from the sarcoplasmic reticulum and into the sarcoplasm of the muscle fiber. When Ca2+ concentration is high in the sarcoplasm, Ca2+ binds to troponin causing change in its shape. This shape change alters the position of tropomyosin and moves it away from myosin binding sites on actin, thus allowing the myosin head to bind actin and form a cross bridge.
When does cross bridge cycling end? A) Cross bridge cycling ends when calcium release channels in the sarcoplasmic reticulum open. B) Cross bridge cycling ends when ATP binds to the myosin head. C) Cross bridge cycling ends when calcium ions are passively transported back into the sarcoplasmic reticulum. D) Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin.
D) -The sarcoplasmic reticulum contains Ca2+-ATPases that actively transport Ca2+ into the SR. Without Ca2+, troponin returns to its resting shape, and tropomyosin glides over and covers the myosin binding sites on actin.
When an action potential arrives at the axon terminal of a motor neuron, which ion channels open? A)voltage-gated sodium channels B) voltage-gated potassium channels C) chemically gated calcium channels D) voltage-gated calcium channels
D) -the action potential opens voltage-gated calcium channels and calcium rushes into the axon terminal, leading to the release of the neurotransmitter.
What causes the power stroke? A) binding of ATP B) hydrolysis of ATP C) calcium D) release of ADP and Pi
D) -the hydrolysis of ATP provides the energy for the power stroke. Energy is transferred from ATP to the myosin head.
The binding of calcium to which molecule causes the myosin binding sites to be exposed? A) myosin B) actin C) tropomyosin D) troponin
D) -when calcium binds to troponin, troponin releases tropomyosin, exposing the myosin binding sites.
How does troponin facilitate cross bridge formation? A) Troponin hydrolyzes ATP, which provides the energy necessary for cross bridges to form. B)Troponin gathers excess calcium that might otherwise block actin's progress. C) Troponin moves away from the active sites on actin, permitting cross bridge formation. D) Troponin controls the position of tropomyosin on the thin filament, enabling myosin heads to bind to the active sites on actin.
D) -For cross bridges to form, tropomyosin must not block the active sites. The position of tropomyosin is controlled by the regulatory protein troponin. This protein-protein interaction couples the binding of calcium (to troponin) to the exposure of active sites.
How does the myosin head obtain the energy required for activation? A) The energy comes from the hydrolysis of GTP. B) The energy comes from oxidative phophorylation. C) The energy comes from the direct phosphorylation of ADP by creatine phosphate. D) The energy comes from the hydrolysis of ATP.
D) -Myosin is a large, complex protein with a binding site for actin. It also contains an ATPase. The energy released during the hydrolysis of ATP activates the myosin head.
Slow oxidative muscle fibers are best suited for ________. A) lifting heavy weights at the gym B) hitting a baseball C) running a 100-yard dash D) running a marathon
D) -Slow oxidative muscle fibers are best suited for endurance activities, such as long distance running, cycling, or rowing.
What, specifically, is a cross bridge? A) calcium binding to troponin B) tropomyosin covering the active sites on actin C) ATP binding to the myosin head D) myosin binding to actin
D) -The attachment of a myosin head from the thick filament to an active site on actin on the thin filament is a cross bridge. As soon as the cross bridge forms, the power stroke occurs, moving the thin filament toward the center of the sarcomere.
Excitation-contraction coupling is a series of events that occur after the events of the neuromuscular junction have transpired. The term excitation refers to which step in the process? A) Excitation refers to the release of calcium ions from the sarcoplasmic reticulum. B) Excitation refers to the propagation of action potentials along the axon of a motor neuron. C) Excitation refers to the shape change that occurs in voltage-sensitive proteins in the sarcolemma. D) Excitation, in this case, refers to the propagation of action potentials along the sarcolemma.
D. -These action potentials set off a series of events that lead to a contraction.
Action potential propagation in a skeletal muscle fiber ceases when acetylcholine is removed from the synaptic cleft. Which of the following mechanisms ensures a rapid and efficient removal of acetylcholine? A) Acetylcholine is transported into the postsynaptic neuron by receptor-mediated endocytosis. B) Acetylcholine is transported back into the axon terminal by a reuptake mechanism. C) Acetylcholine diffuses away from the cleft. D)Acetylcholine is degraded by acetylcholinesterase.
D. -Acetylcholinesterase is an enzyme that degrades acetylcholine. This degradation results in a rapid cessation of the acetylcholine signal and a swift removal from the cleft
T or F: Isometric contraction leads to movement of a load.
False. -An isometric contraction generates tension (force), but not enough to move the load. Overall muscle length does not change during iso(same) -metric(length) contractions. In contrast, isotonic contractions result in movement of the load.