ANP Homework 3

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

bundle of skeletal muscle fibers enclosed by connective tissue called perimysium

fascicle

Made up of several sarcomeres:

myofibrils

: membrane of muscle cell

sarcolemma

Contractile unit:

sarcomere

the cytoplasm of a skeletal muscle fiber

sarcoplasm

Long, cylindrical cells:

skeletal muscle fibers

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

transverse or T tubules

Blocks myosin-binding sites on actin:

tropomyosin

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

The cross bridge cycle starts when _________.

Ca2+ from the sarcoplasmic reticulum binds to troponin

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

Calcium ions

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

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.

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

Myofilament with a knob-like head:

Myosin

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.

The activated myosin head binds to actin, forming a cross bridge. ADP is released and myosin slides the thin filament toward the center of the sarcomere. ATP binds to the myosin head and detaches it from actin. ATP is hydrolyzed to ADP and Pi and the energy released re-cocks the myosin head. 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

Myofilament stiffened and stabilized by tropomyosin:

actin