A&P Ch9 ~ Muscles and Muscle Tissue ~ MasteringA&P HW

Order of occurrence of excitation-contraction coupling in correct order.

1. Action potential is generated at the sarcolemma. 2. Action potential goes down the T tubule. 3. Voltage-gated tubule protein changes shape. 4. Calcium ion channels open. 5. Calcium exits the cistern into sarcoplasm. 6. Calcium binds to troponin. 7. Actin sites are exposed. 8. Cross bridges form between actin and myosin.

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.

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. (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.)

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. (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)

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. (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. (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)

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 sarcolema. (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)

Which selection best describes the initial event in contraction?

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

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. (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 is name given to the regularly spaced infoldings of the sarcolemma?

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