Physiology Exam 2 Chapter 12

In a neuromuscular junction, synaptic vesicles in the motor neuron contain which neurotransmitter?

acetylcholine (ACh)

How is acetylcholine (ACh) removed from the synaptic cleft?

acetylcholinesterase (AChE; an enzyme)

A myosin head binds to which molecule to form a cross bridge?

actin

Each myosin head has a binding site for

actin and ATP.

Which of the following is an effect of the parasympathetic nervous system?

airway constriction

What causes the release of calcium from the terminal cisternae of the sarcoplasmic reticulum within a muscle cell?

arrival of an action potential

In order for high force crossbridges to form in contracting skeletal muscle, calcium must

bind to troponin which moves the tropomyosin.

What causes the myosin head to disconnect from actin?

binding of ATP

In smooth muscle cells, Ca2+ binds to ________ which activates the enzyme ________ to phosphorylate myosin and increase force.

calmodulin, myosin light chain kinase

The binding of acetylcholine to its receptor at the neuromuscular junction causes the opening of a

channel for both Na+ and K+.

As ATP binds to the myosin head at the beginning of a muscle contraction cycle, the myosin head immediately

detaches from actin.

The action potential traveling along the t-tubule is detected by the

dihydropyridine (DHP) receptor (L-type calcium channel).

Where in the cross bridge cycle does ATP hydrolysis occur?

during the cocking of the myosin head

What means of membrane transport is used to release the neurotransmitter into the synaptic cleft?

exocytosis

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 fibers fatigue sooner?

fast-twitch fibers

Which fibers generate more force?

fast-twitch fibers

Asynchronous activation of motor units is important because it functions to prevent

fatigue

The brief period of time between the beginning of the action potential in the muscle and the beginning of contraction is referred to as the

latent period.

Which of the following is a characteristic of slow-twitch oxidative skeletal muscle fibers?

long contraction duration and high capillary density

What, specifically, is a cross bridge?

myosin binding to actin

A motor unit consists of

one neuron and all the skeletal muscle fibers it controls.

The function of transverse tubules is to

rapidly conduct action potentials to the interior of the muscle fiber.

What causes the power stroke?

release of ADP and Pi

The molecular event that occurs immediately after the power stroke is the

release of ADP from the myosin.

The hydrolysis of ATP causes myosin to immediately

rotate into a position (cocked) to bind to actin.

The function of the titin protein is to

stabilize the position of the contractile filaments.

The action potential on the muscle cell leads to contraction due to the release of calcium ions. Where are calcium ions stored in the muscle cell?

terminal cisterns (cisternae) of the sarcoplasmic reticulum

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

the opening of ligand-gated cation channels

Compared to fast-twitch glycolytic fibers (type IIb), slow-oxidative muscle fibers (type I) are characterized by a smaller diameter, less stored glycogen, and ________.

the presence of myoglobin

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

transverse or T tubules

The binding of calcium to which molecule causes the myosin binding sites to be exposed?

troponin

Calcium, released by the sarcoplasmic reticulum, binds to ________ to move the ________, which allows actin to form a high-force crossbridge with myosin.

troponin C, tropomyosin

Smooth muscle cells lack which protein(s)?

troponin only

When an action potential arrives at the axon terminal of a motor neuron, which ion channels open?

voltage-gated calcium channels

In comparison to skeletal muscle, smooth muscle cells function over a ________ range of muscle lengths and exhibit a speed of contraction that is ________ than skeletal muscle.

wider, slower

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 event causes cross bridge detachment?

ATP binding to the myosin head

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.

In order to contract and relax, muscle cells require energy in the form of ________. The immediate backup energy source is ________ that requires the enzyme ________.

ATP, phosphocreatine, creatine kinase

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.

How/when does the myosin head cock back to store energy for the next cycle?

After the myosin head detaches, energy from ATP hydrolysis is used to re-cock the myosin head.

Skeletal muscle moves a bone at a joint, such as flexing the forearm at the elbow. How is the opposite action produced (i.e., extension at the elbow)?

An antagonistic muscle produces the opposite movement.

The binding of the neurotransmitter to receptors on the motor end plate causes which of the following to occur?

Binding of the neurotransmitter causes chemically gated sodium channels to open in the motor end plate (junctional folds of the sarcolemma).

The contraction cycle is triggered by the rise in ________ released from the sarcoplasmic reticulum (SR).

Ca2+

Which ion signals the beginning of skeletal muscle contraction?

Ca2+

How does the process of smooth muscle contraction differ from contraction in skeletal muscle?

Ca2+ comes from the extracellular fluid as well as the SR.

What is the role of calcium in the cross bridge cycle?

Calcium binds to troponin, altering its shape.

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.

How is the energy released by ATP hydrolysis used during the contractile cycle in skeletal muscle?

It causes rotation of the myosin head, thus "cocking" it.

Which of the following is NOT a correct comparison of cardiac myocytes to other muscle cell types?

Like skeletal muscle, contraction of cardiac muscle is under autonomic nervous control.

BMD (2,3-butanedione 2-monoximime) inhibits myosin, such that ATP can bind to myosin but myosin is unable to hydrolyze the bound ATP. What effect would BMD have on the cross bridge cycle?

Myosin heads would remain detached, unable to cock.

________ is the backup energy molecule that can be rapidly converted to ATP in active skeletal muscle.

Phosphocreatine

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.

What role does tropomyosin play in the cross bridge cycle?

The displacement of tropomyosin exposes the active sites of actin, allowing cross bridges to form.

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.

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

The sarcomere

During contraction, what prevents actin myofilaments from sliding backward when a myosin head releases?

There are always some myosin heads attached to the actin myofilament when other myosin heads are detaching.

How are cardiac muscle cells similar to smooth muscle cells?

They are electrically linked to one another.

What happens when calcium binds troponin?

Tropomyosin is pulled away from the actin's myosin-binding site.

How does troponin facilitate cross bridge formation?

Troponin controls the position of tropomyosin on the thin filament, enabling myosin heads to bind to the active sites on actin.

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.