A&P Chapter 9

Which of the following choices best summarizes excitation-contraction coupling?

A series of events in which an electrical stimulus is conveyed to a muscle fiber to enact contraction Congratulations! You have completed the final activity in this tutorial. The tutorial covered the sequence of events in excitation-contraction coupling that links the excitation of the membrane to the cross bridge cycling of the contracting proteins. Great job!

A triad is composed of a T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum. How are these components connected?

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.

Consider how the action potential that initiates contraction is delivered to the muscle cell. 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?

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. Yes! 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.

Botulism and myasthenia gravis are conditions that cause muscle weakness. Which of these statements is NOT true?

Both conditions are caused by an inability of the body to produce adequate acetylcholine. Although both conditions affect normal activity at the neuromuscular junction, failure to produce acetylcholine is not the mechanism.

The cross bridge cycle starts when _________.

Ca2+ from the sarcoplasmic reticulum binds to troponin 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.

Which selection correctly describes the role of calcium in coupling?

Calcium binds to troponin, which moves tropomyosin and exposes the myosin-binding sites on actin. 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.

Which of the following is most directly responsible for the coupling of excitation to contraction of skeletal muscle fibers?

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. 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.

Click and drag the description of the part of the excitation-contraction coupling to the correct order of occurrence from left to right. Do not overlap any steps.

Coupling begins with an action potential after the arrival of ACh and graded potentials, and it ends before cross bridge cycling.

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?

Cross bridge detachment cannot occur. Detachment requires ATP, which is produced only during life. After ATP attaches to the myosin head, the bond between actin and myosin is weakened and the cross bridge breaks.

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. Yes! These action potentials set off a series of events that lead to a contraction.

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?

It allows calcium to exit the sarcoplasmic reticulum and enter the cytosol. 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.

What structure most directly stimulates a skeletal muscle fiber to contract?

Motor neuron The motor neuron transmits action potentials from the brain or spinal cord to muscle fibers by releasing acetylcholine at the neuromuscular junction.

Which selection best describes the initial event in contraction?

Myosin heads bind to the newly exposed myosin-binding sites on actin to form cross bridges. Contraction begins when myosin binds to actin and forms cross bridges.

Inadequate calcium in the neuromuscular junction would directly affect which of the following processes?

Release of acetylcholine from the synaptic vesicles Calcium ions enter the axon terminal when voltage-gated calcium channels open in response to the arrival of an action potential. The presence of calcium causes synaptic vesicles to release acetylcholine into the synaptic cleft.

If troponin is a component of both cardiac and skeletal muscle, why is an elevated plasma troponin level useful in diagnosing myocardial damage?

The subunits of the troponin in cardiac muscle are unique to heart muscle. Although both types of striated muscle contain troponin, the subunits found in cardiac muscle are different from those in skeletal muscle. The laboratory test used to diagnose cardiac muscle damage is specific for those subunits.

The sarcomere shortens when the myosin heads of the thick filaments, in a cocked position, form cross bridges with the actin molecules in thin filaments. This activity will test your understanding of the steps that occur in one complete cross bridge cycle. Place the steps that occur during a single cross bridge cycle in the correct order from left to right.

The thin filaments are pulled toward the center of the sarcomere by the myosin heads of the thick filament. This shortens the sarcomeres in the muscle fiber and causes the whole skeletal muscle to contract. The cross bridge cycle ends when Ca2+ are actively transported back into the sarcoplasmic reticulum. The cycle repeats as long as the binding sites on actin remain exposed, and both Ca2+ and ATP are available.

What is the relationship between the number of motor neurons recruited and the number of skeletal muscle fibers innervated?

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.

Myasthenia gravis is a disease resulting from an autoimmune attack on the ACh receptors of the motor end plate. Binding of antibodies to the ACh receptors results in generalized muscle weakness that progresses as more ACh receptors are destroyed. Which of the following medications would help alleviate the muscle weakness?

a drug that binds to and inactivates acetylcholinesterase (neostigmine) The progressive destruction of ACh receptors leads to a progressive decline in the strength of end-plate potentials. To counteract this problem, drug therapy focuses on increasing the concentration of acetylcholine in the synaptic cleft. Drugs that bind to acetylcholinesterase and prevent the breakdown of acetylcholine can help restore some muscle strength by allowing acetylcholine to interact with the ACh receptors longer. Neostigmine is one such drug and is used to treat myasthenia gravis.

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?

cross bridge detachment When ATP is added to the myosin, myosin detaches from actin.

What is name given to the regularly spaced infoldings of the sarcolemma?

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