Pre Class Assignment (Chapter 9)

Drag and drop the pictures to display the proper sequence of events in sarcomere contraction. Myosin heads link to the active sites Ca2+ released from the SR Myosin heads pivot, producing tension Myosin heads reset Ca2+ triggers exposure of active sites ATP binds myosin; cross-bridge broken

1) Ca2+ released from the SR 2) Ca2+ triggers exposure of active sites 3) Myosin heads link to the active sites 4) Myosin heads pivot, producing tension 5) ATP binds myosin; cross-bridge broken 6)Myosin heads reset

Which number represents a myofilament arrangement that will have NEITHER an H zone NOR an I band? 5 4 3 6

4

Which number represents an overly stretched muscle that may be INCAPABLE of contracting? 2 3 5 4

5

Which number represents the structure involved in storing calcium ions? 7 11 1 8

7

The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge? Troponin bound to tropomyosin Calcium bound to troponin ATP bound to a myosin head A myosin head bound to actin

A myosin head bound to actin Yes! As soon as the activated myosin head forms a cross bridge with actin, the power stroke begins.

A triad is composed of a T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum. How are these components connected? Myosin cross-bridge binding sites. A series of proteins that control calcium release. Potassium leak channels. Voltage-gated sodium channels.

A series of proteins that control calcium release. Yes! 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.

After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment? Calcium ions bind to troponin. Acetylcholine binds to receptors in the junctional folds of the sarcolemma. ATP binds to the myosin head. ADP and inorganic phosphate are bound to the myosin head.

ATP binds to the myosin head. Yes! 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.

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? Acetylcholine is degraded by acetylcholinesterase. Acetylcholine diffuses away from the cleft. Acetylcholine is transported into the postsynaptic neuron by receptor-mediated endocytosis. Acetylcholine is transported back into the axon terminal by a reuptake mechanism.

Acetylcholine is degraded by acetylcholinesterase. Yes! 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.

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? Acetylcholine binds to its receptor in the junctional folds of the sarcolemma. Its receptor is linked to a G protein. Acetylcholine is released by axon terminals of the motor neuron. When the action potential reaches the end of the axon terminal, voltage-gated sodium channels open and sodium ions diffuse into the terminal. Acetylcholine is released and moves across the synaptic cleft bound to a transport protein.

Acetylcholine is released by axon terminals of the motor neuron. Yes! Acetylcholine is released into the synaptic cleft via exocytosis.

Calcium ions couple excitation of a skeletal muscle fiber to contraction of the fiber. Where are calcium ions stored within the fiber? Calcium ions are stored in the mitochondria. Calcium ions are stored in the transverse tubules. Calcium ions are stored in the nuclei. Calcium ions are stored in the sarcoplasmic reticulum.

Calcium ions are stored in the sarcoplasmic reticulum. Yes! 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 specific event triggers the uncovering of the myosin binding site on actin? Calcium release channels open in the sarcoplasmic reticulum, and calcium levels rise in the sarcoplasm. Calcium ions bind to troponin and change its shape. Sodium ions bind to troponin and change its shape. Calcium ions bind to tropomyosin and change its shape.

Calcium ions bind to troponin and change its shape. Yes! The shape change caused by the binding of calcium to troponin shifts tropomyosin away from the myosin binding sites on actin.

Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers? Sodium ions. Action potentials. Calcium ions. Acetylcholine.

Calcium ions. Yes! 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.

Excitation of the sarcolemma is coupled or linked to the contraction of a skeletal muscle fiber. What specific event initiates the contraction? Calcium release from the sarcoplasmic reticulum initiates the contraction. Action potentials propagate into the interior of the skeletal muscle fiber. Sodium release from the sarcoplasmic reticulum initiates the contraction. Voltage-sensitive proteins change shape.

Calcium release from the sarcoplasmic reticulum initiates the contraction. Yes! 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.

When does cross bridge cycling end? Cross bridge cycling ends when calcium release channels in the sarcoplasmic reticulum open. Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin. Cross bridge cycling ends when ATP binds to the myosin head. Cross bridge cycling ends when calcium ions are passively transported back into the sarcoplasmic reticulum.

Cross bridge cycling ends when sufficient calcium has been actively transported back into the sarcoplasmic reticulum to allow calcium to unbind from troponin. Yes! 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.

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? Excitation, in this case, refers to the propagation of action potentials along the sarcolemma. Excitation refers to the propagation of action potentials along the axon of a motor neuron. Excitation refers to the shape change that occurs in voltage-sensitive proteins in the sarcolemma. Excitation refers to the release of calcium ions from the sarcoplasmic reticulum.

Excitation, in this case, refers to the propagation of action potentials along the sarcolemma. Yes! These action potentials set off a series of events that lead to a contraction.

Calcium entry into the axon terminal triggers which of the following events? Acetylcholine binds to its receptor. Cation channels open and sodium ions enter the axon terminal while potassium ions exit the axon terminal. Acetylcholine is released into the cleft by active transporters in the plasma membrane of the axon terminal. Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine.

Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine. Yes! When synaptic vesicles fuse to the plasma membrane, acetylcholine is released via exocytosis.

How does the myosin head obtain the energy required for activation? The energy comes from the hydrolysis of ATP. The energy comes from the hydrolysis of GTP. The energy comes from oxidative phophorylation. The energy comes from the direct phosphorylation of ADP by creatine phosphate.

The energy comes from the hydrolysis of ATP. Yes! 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.

Sodium and potassium ions do not diffuse in equal numbers through ligand-gated cation channels. Why? The inside surface of the sarcolemma is negatively charged compared to the outside surface. Sodium ions diffuse inward along favorable chemical and electrical gradients. The outside surface of the sarcolemma is negatively charged compared to the inside surface. Sodium ions diffuse outward along favorable chemical and electrical gradients. The outside surface of the sarcolemma is negatively charged compared to the inside surface. Potassium ions diffuse outward along favorable chemical and electrical gradients. The inside surface of the sarcolemma is negatively charged compared to the outside surface. Potassium ions diffuse inward along favorable chemical and electrical gradients.

The inside surface of the sarcolemma is negatively charged compared to the outside surface. Sodium ions diffuse inward along favorable chemical and electrical gradients. Yes! 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.

What structure is the functional unit of contraction in a skeletal muscle fiber? The sarcomere The triad The cross bridge The junctional folds of the sarcolemma

The sarcomere Yes! 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 of the following changes occurs in the sarcomere during muscle contraction? The I bands increase in size. The zone of overlap increases in size. The A band decreases in size. The H band increases in size.

The zone of overlap increases in size.

What is the relationship between the number of motor neurons recruited and the number of skeletal muscle fibers innervated? A motor neuron typically innervates only one skeletal muscle fiber. Motor neurons always innervate thousands of skeletal muscle fibers. Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron. A skeletal muscle fiber is innervated by multiple motor neurons.

Typically, hundreds of skeletal muscle fibers are innervated by a single motor neuron. Yes! 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.

Action potentials travel the length of the axons of motor neurons to the axon terminals. These motor neurons __________. extend from the brain to the sarcolemma of a skeletal muscle fiber arise in the epimysium of a skeletal muscle and extend to individual skeletal muscle fibers extend from the brain or spinal cord to the sarcolemma of a skeletal muscle fiber extend from the spinal cord to the sarcolemma of a skeletal muscle fiber

extend from the brain or spinal cord to the sarcolemma of a skeletal muscle fiber Yes! 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.

Arrange the following terms in order from most superficial (largest) to deepest (smallest): myosin, sarcoplasmic reticulum, sarcolemma, and myofibril sarcolemma, sarcoplasmic reticulum, myofibril, myosin myosin, myofibril, sarcoplasmic reticulum, sarcolemma sarcoplasmic reticulum, sarcolemma, myofibril, myosin sarcolemma, myofibril, myosin, sarcoplasmic reticulum

sarcolemma, sarcoplasmic reticulum, myofibril, myosin

Acetylcholine binds to its receptor in the sarcolemma and triggers __________. the opening of ligand-gated anion channels the opening of voltage-gated calcium channels the opening of ligand-gated cation channels the opening of calcium-release channels

the opening of ligand-gated cation channels Yes! These channels permit sodium ions to diffuse inward and potassium ions to diffuse outward.

What is name given to the regularly spaced infoldings of the sarcolemma? sarcoplasmic reticulum transverse or T tubules terminal cisternae motor endplates

transverse or T tubules Yes! T tubules penetrate a skeletal muscle fiber and provide a pathway for excitation into the interior.

Contraction is initiated by release of calcium ions from the terminal cisternae of the sarcoplasmic reticulum. To what molecule do the calcium ions bind? actin troponin tropomyosin myosin

troponin

The amount of tension produced by a muscle peaks when the _____________ is large, thus allowing the formation of many cross-bridges. zone of overlap I band A band H band

zone of overlap