Week 6: Muscle Physiology

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.

EGTA is a substance that binds calcium ions. Imagine an experimental setup with a motor neuron and a muscle fiber. Stimulation of the motor neuron causes contraction of the muscle fiber through activity at the neuromuscular junction and excitation-contraction coupling. Now, inject the muscle fiber with EGTA. Which of the following effects would EGTA have on excitation-contraction coupling after the neuron releases acetylcholine at the neuromuscular junction?

It would prevent myosin from forming cross bridges with actin.

Which of the following processes produces 36 ATP?

Krebs cycle and oxidative phosphorylation

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.

Match the term in the left column to the blanks near their function on the right.

Troponin: change(s) shape upon binding with calcium ions. Terminal cistern: release(s) calcium ions into the sarcoplasm. Junctional folds: receive(s) stimulus from the motor neuron. Tropomyosin: block(s) binding sites on actin. T tubule: conduct(s) action potentials throughout the interior of the muscle fiber.

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.

What event most directly triggers the release of calcium from the terminal cisternae?

action potential propagating down the T tubule

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

arrival of an action potential

What causes the myosin head to disconnect from actin?

binding of ATP

What is the primary mechanism by which ACh is cleared from the synaptic cleft? -endocytosis by synaptic vesicles -broken down by acetylcholinesterase -reuptake into the synaptic terminal -diffusion away from the synaptic cleft

broken down by acetylcholinesterase

The influx of which ion is directly responsible for triggering the release of acetylcholine from a motor neuron?

calcium

What most directly causes synaptic vesicles to release acetylcholine into the synaptic cleft?

calcium entering the axon terminal

The sliding of the myofilaments is directly initiated by the availability of which chemical at the myofibril?

calcium ions

A muscle fiber represents which organizational level of anatomy?

cellular level

Myofilaments represent which organizational level of anatomy?

chemical level

Acetylcholine receptors are best characterized as what type of channel?

chemically gated Na+-K+ channels

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 Submit

Which of the following best describes the events of "contraction" in "excitation-contraction coupling"?

cross bridge formation

What is the type of chemical reaction used to rebuild ADP into ATP?

dehydration synthesis

Conduction of an action potential along the sarcolemma depends upon ___________.

diffusion of sodium ions through voltage-gated channels

Where in the cross bridge cycle does ATP hydrolysis occur?

during the cocking of the myosin head

Which membrane surrounds the others?

epimysium

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

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

the opening of ligand-gated cation channels

An action potential is propagated down the sarcolemma as a result of which of the following?

the opening of voltage-gated sodium channels

Which of the following are composed of myosin?

thick filaments

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

transverse or T tubules

In muscle fibers, which regulatory protein blocks the attachment of myosin heads to actin?

tropomyosin

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

troponin

What causes the power stroke?

release of ADP and Pi

ACh receptors are found mainly in the __________.

sarcolemma

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

simple diffusion away from the synaptic cleft and acetylcholinesterase (AChE; an enzyme)

When the chemically gated ion channels open, which ion is mainly responsible for depolarizing the sarcolemma?

sodium

The calcium that initiates skeletal muscle contraction is released from what structure(s)?

terminal cisternae

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

Drag and drop the terms to correctly complete the sentences involving cross bridge detachment. Answers may be used once or not at all.

1. The bond between the actin and myosin head weakens as a result of ATP. 2. Only one-half of the myosin heads are detached from actin at any one time while calcium is present and cycling continues. 3. ADP is not bound to myosin during the detachment step.

Drag and drop the phrases to their best description involving the power stroke. Answers may be used once or not at all.

1. The molecules released just before power stroke: ADP and Pi 2. Actin behavior during the power stroke: ratchets toward the M line 3. Myosin head energy status during the power stroke: pivots and returns to low energy

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

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.

Which of the following best describes how ACh changes the ion permeability of the sarcolemma?

ACh binds to receptors in the junctional folds.

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) and sodium enters the cell.

The cross bridge cycle starts when _________.

Ca2+ from the sarcoplasmic reticulum binds to troponin

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

Calcium binds to troponin, altering its shape.

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.

Which of the following is responsible for muscle relaxation?

Calcium ions are removed from the sarcoplasm by active transport.

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.

How do calcium ions initiate contraction in skeletal muscle fibers?

Calcium ions bind to troponin, changing troponin's 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.

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.

Drag the measurements of different regions of the sarcomere to their respective locations (bins) to describe what happens during the sliding filament mechanism of sarcomere shortening.

Decreases in length or distance: Length of I band, Length of H zone, Distance between Z discs No change in length or distance: Length of A band, Length of thick filament, Length of thin filament

Which of the following is/are mechanism(s) to end neural transmission at the neuromuscular junction? Select all the correct answers.

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

Which description is most appropriate for a myofibril?

a single long, thin organelle containing many proteins

Synaptic vesicles at the neuromuscular junction contain __________.

acetylcholine

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

acetylcholine (ACh)

Review another set of terms associated with a skeletal muscle fiber/cell and the sarcomere. Drag and drop each term to the best description of that term.

1. Blocks myosin-binding sites on actin: tropomyosin 2. Long, cylindrical cells: skeletal muscle fibers 3. Contractile unit: sarcomere 4. Made up of several sarcomeres: myofibrils

Drag and drop the terms to match the best description. Some terms are not used.

1. Cistern: the swollen chamber of an organelle that lies on either side of a T tubule 2. Voltage-sensitive tubule protein: a protein that changes shape as a direct result of an action potential 3. Myosin: a protein with heads that attach in cross bridging 4. T tubule: a structure that forms a path from the membrane to the cell interior 5. Troponin: the protein that binds calcium

Drag the provided terms to their matching descriptions. Not all terms will be used.

1. Junctional folds : portion of sarcolemma containing ACh receptors 2. ACh : neurotransmitter at neuromuscular junction 3. Sarcolemma : membrane surrounding muscle fiber 4. Ca2+ : ion triggering exocytosis

Use the provided terms to correctly complete each sentence. Terms may be used more than once.

1. K+ diffuses out of the muscle fiber through open chemically gated ion channels. 2. The end plate potential is primarily, and most directly, caused by the movement of Na+. 3. ACh binds to ACh receptors, causing them to open chemically gated ion channels. 4. Na+ diffuses into the muscle fiber through open chemically gated ion channels.

Drag and drop the terms to the best description of terms involving the cross bridge formation. Answers may be used once or not at all.

1. Location of higher concentration of Ca2+ needed for cross bridge formation and cycling: cytoplasm 2. Actin status to begin cross bridge formation: actin-binding sites exposed 3. Molecule(s) bound to the myosin head at the start of the cross bridge cycle: ADP and Pi 4. Energy state of myosin for beginning cross bridge formation: energized/cocked myosin head

Review some terms associated with a skeletal muscle fiber (also called a muscle cell) and the sarcomere. Drag and drop each term to the best description of that term.

1. Myosin: myofilament with a knob-like head 2. Actin: myofilament stiffened and stabilized by tropomyosin 3. Sarcoplasm: the cytoplasm of a skeletal muscle fiber 4. Fascicle: bundle of skeletal muscle fibers enclosed by connective tissue called perimysium 5. Sarcolemma: membrane of muscle cell

Drag and drop the phrases to the appropriate status during the cocking of the myosin head phase. Answers may be used once or not at all.

1. Status of ATP: hydrolyzed to ADP and Pi 2. Status of the myosin head: returns to the energized position 3. Status of actin and myosin: actin and myosin detached

Drag the provided terms to their matching descriptions. Not all terms will be used.

1. Synaptic vesicle : container of many neurotransmitter molecules 2. Synaptic cleft : location of acetylcholinesterase 3. Na+ : ion entering muscle fiber through open chemically gated ion channels 4. ACh receptor : protein embedded in the sarcolemma

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.

Which of the following occurs during cross bridge cycling?

ATP is hydrolyzed in order to put the myosin head in a high-energy position.

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 transported back into the axon terminal by a reuptake mechanism. -Acetylcholine is degraded by acetylcholinesterase. -Acetylcholine is transported into the postsynaptic neuron by receptor-mediated endocytosis. -Acetylcholine diffuses away from the cleft.

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.

Match the term in the left column to the blanks near their definition on the right.

Acetylcholine receptor: a type of chemically-gated ion channel located on the junctional folds of the muscle fiber. Synaptic vesicle : membranous sac located in the axon terminal that contains neurotransmitter. Calcium channel : a type of voltage-gated ion channel located on the axon terminal. Sodium channel : a type of voltage-gated ion channel located on the sarcolemma of the muscle fiber. Acetylcholine : neurotransmitter that stimulates skeletal muscle contraction. Acetylcholinesterase : enzyme located in the synaptic cleft that breaks down acetylcholine. Synaptic cleft : the space between the axon terminal and junctional folds.

Which of the following best describes the role of acetylcholinesterase molecules at the neuromuscular junction?

Acetylcholinesterase breaks down ACh, which allows chemically gated ion channels to close.

What is the fate of acetylcholine (Ach) after it binds to Ach receptors?

Ach is broken down by acetylcholinesterase.

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.

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.

What causes the release of calcium ions into the sarcoplasm from the terminal cisterns?

An action potential traveling along the t tubule

What action is directly responsible for the generation of an action potential at the sarcolemma?

An end plate potential sufficient to reach threshold must be produced.

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.

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.

Which of the following is NOT a role of ATP in muscle contraction? -Exposing myosin binding sites on actin -Energizing the power stroke of the cross bridge -Transporting calcium ions into the sarcoplasmic reticulum -Detaching the cross bridge from actin

Exposing myosin binding sites on actin

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

Motor neuron

What would happen if acetylcholine was not removed from the synaptic cleft?

Multiple action potentials would occur in the muscle fiber.

Which of the following is true concerning the anatomy of a skeletal muscle fiber?

Myofibrils contain thick and thin filaments.

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.

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.

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

The energy comes from the hydrolysis of ATP.

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

actin

The "rest and recovery" period, where the muscle restores depleted reserves, includes all of the following processes EXCEPT __________. -Glycogen is synthesized from glucose molecules. -Oxygen rebinds to myoglobin. -ATP is used to rephosphorylate creatine into creatine phosphate. -Pyruvic acid is converted back to lactic acid.

Pyruvic acid is converted back to lactic acid. Yes, this is NOT a part of the "rest and recovery" period. When oxygen is available, lactic acid is converted back to pyruvic acid (not vice versa) that then enters the Krebs cycle. Lactic acid is the end product of the anaerobic pathway.

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

Release of acetylcholine from the synaptic vesicles

Arrange the sequence of events at the NMJ from first to last. Rank the sequence of events at the NMJ that initiate an action potential in the muscle fiber, from first to last.

Step 1: Action potential arrives at the axon terminal. Step 2: Calcium ions enter the axon terminal. Step 3: Synaptic vesicles fuse to membrane of axon terminal. Step 4: Acetylcholine is released into the synaptic cleft. Step 5: Acetylcholine binds to its receptors on the junctional folds. Step 6: Junctional folds become depolarized. Step 7: Action potential is initiated on 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.

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

The sarcomere

Which of the following best describes the role of muscle fiber triads?

The triad allows an action potential to activate voltage-sensitive proteins in a T tubule, which in turn opens calcium channels in the paired terminal cisternae, allowing calcium to flood 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 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.

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)

Skeletal muscle cells are grouped into bundles called __________.

fascicles

Which of the following processes produces molecules of ATP and has two pyruvic acid molecules as end products?

glycolysis

What, specifically, is a cross bridge?

myosin binding to actin

What chemical does NOT need to be present in the extracellular fluid for a signal to travel from neuron to muscle cell?

potassium

Which of the following best describes the events of "excitation" in "excitation-contraction coupling"?

propagation of the action potential along the sarcolemma

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

voltage-gated calcium channels

Which type of muscle fiber has a large quantity of glycogen and mainly uses glycolysis to synthesize ATP?

white fast twitch fibers