Mastering A&P Chapter 10

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.

1. the activated myosin head binds to actin, forming a cross bridge 2. ADP is released and myosin slides the thin filament toward the center of the sarcomere 3. ATP binds to the myosin head and detaches it from actin 4. ATP is hydrolyzed to ADP and Pi and the energy released re-cocks the myosin head.

The cross bridge cycle is a series of molecular events that occur after excitation of the sarcolemma. What is a cross bridge?

A myosin head bound to actin

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.

Which of the following is/are mechanism(s) to end neural transmission at the neuromuscular junction?

ACh is broken down into acetic acid and choline by the enzyme acetylcholinesterase (AChE). ACh diffuses away from the synaptic cleft.

The conversion of an action potential (AP) generated by a motor neuron to contraction of skeletal muscle fiber is called excitation-contraction coupling. This activity will test your understanding of the sequence of events that occur during excitation-contraction coupling.

AP propagates along sarcolemma AP travels down T tubules to triads Voltage-sensitive proteins open Ca2+ channels Sarcoplasmic reticulum releases Ca2

The characteristic muscle stiffness associated with rigor mortis is due to the inability of myosin filaments to detach from the active site on actin filaments. What molecule is essential for this detachment?

ATP

After a power stroke, the myosin head must detach from actin before another power stroke can occur. What causes cross bridge detachment?

ATP binds to the myosin head.

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.

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 is released by axon terminals of the motor neuron.

Tamara's muscle weakness and fatigue becomes progressively worse over the course of the day. This is a hallmark sign of myasthenia gravis, an autoimmune disorder that affects the ability of a motor neuron to communicate with a muscle fiber. What synaptic events must happen first for excitation to occur?

Acetylcholine is released from the axon terminal and diffuses across the synapse to bind to a receptor in the surface of the motor end plate.

Myasthenia gravis is an autoimmune disorder that results in the production of antibodies that either block or cause the destruction of the ACh receptor. Why do you think that Tamara's symptoms become worse as the day progresses?

Acetylcholinesterase is the enzyme that clears acetylcholine from the synapse. Inhibiting this enzyme will enable acetylcholine to be available for a longer period to bind to receptors.

The site where a motor neuron excites a skeletal muscle fiber is called the neuromuscular junction. This activity will test your understanding of the sequence of events that occur at the neuromuscular junction.

Ca2+ enters the axon terminal synpatic vesicles release ACh ACh binds to ACh receptors Ligand-gated cation channels open Na+ enters and K+ exists membrane potential is less negative

The cross bridge cycle starts when _________.

Ca2+ from the sarcoplasmic reticulum binds to troponin

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 sarcoplasmic reticulum

What specific event triggers the uncovering of the myosin binding site on actin?

Calcium ions bind to troponin and change its shape.

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.

When does cross bridge cycling end?

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

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.

Calcium entry into the axon terminal triggers which of the following events?

Synaptic vesicles fuse to the plasma membrane of the axon terminal and release acetylcholine.

How does the myosin head obtain the energy required for activation?

The energy comes from the hydrolysis of ATP

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.

Identify the incorrect statement about a single motor unit.

The more neurons involved, the more powerful the contraction.

Which of the following statements correctly describes the structure (band or line) of the sarcomere indicated by the arrow? (I Band)

The region of the sarcomere that contains only thin filaments

What structure is the functional unit of contraction in a skeletal muscle fiber?

The sarcomere

Which of the following best describes the term Z line?

Thin filaments are anchored here

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.

The most important factor in decreasing the intracellular concentration of calcium ion after contraction is

active transport of calcium into the sarcoplasmic reticulum

Muscles are attached to bones by tendons or __________.

aponeuroses

A(n) ________ can be described as a broad tendinous sheet.

aponeurosis

Triads in skeletal muscle fibers function in __________.

calcium ion release

Muscle tissue, one of the four basic tissue groups, consists chiefly of cells that are highly specialized for

contraction

Titin is a(n) __________.

elastic protein

Action potentials travel the length of the axons of motor neurons to the axon terminals. These motor neurons __________.

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

Which of the following allows muscles to return to their original shape during relaxation?

gravity, opposing muscle contractions, and elastic forces

A fascicle is a

group of muscle fibers that is encased in the perimysium

What can the nervous system do to increase muscle tension?

increase stimulation frequency recruit larger motor units increase the number of active motor units

A weight-lifter strains to lift a heavy weight and there is no movement of the person's arms holding on to the weight. This type of contraction is called a(n) ________ contraction.

isometric

A single motor neuron together with all the muscle fibers it innervates is called a(n)

motor unit.

Interactions between actin and myosin filaments of the sarcomere are responsible for

muscle contraction

In an isotonic contraction,

muscle tension exceeds the load and the muscle lifts the load.

In which of the following would the motor units have the fewest muscle fibers?

muscles that control the eyes

Thick filaments are made of the protein

myosin

What area of the thick filament binds to actin once actin's binding sites are exposed?

myosin cross-bridge (head)

Which of the following is a recognized function of skeletal muscle?

produces movement, maintain posture maintain body temp, guard body entrances and exits.

The increase in muscle tension that is produced by increasing the number of active motor units is called

recruitment

The repeating unit of a skeletal muscle fiber is the

sarcomere

Since each myofibril is attached at either end of the muscle fiber, when sarcomeres shorten, the muscle fiber

shortens

At each end of the muscle, the collagen fibers of the epimysium, perimysium, and endomysium, come together to form a

tendon

Which of the following statements correctly describes the structure (band or line) of the sarcomere indicated by the arrow? (Z Line)

the boundary between adjacent sarcomeres

Acetylcholine binds to its receptor in the sarcolemma and triggers __________.

the opening of ligand-gated cation channels

Which of the following statements correctly describes the structure (band or line) of the sarcomere indicated by the arrow? (M Line)

the point of connection for adjacent tails of the thick filaments

Which of the following statements correctly describes the structure (band or line) of the sarcomere indicated by the arrow? (H Band)

the region of the resting sarcomere that only contains thick filaments

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

transverse or T tubules

At rest, the tropomyosin molecule is held in place by

troponin molecules

Each thin filament consists of

two actin protein strands coiled helically around each other