Chapter 12 muscles
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
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.
What causes the release of calcium ions into the sarcoplasm from the terminal cisterns?
An action potential traveling along the t tubule
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.
What is the role of calcium in the cross bridge cycle?
Calcium binds to troponin, altering its shape.
Which of the following is responsible for preventing a muscle from contracting?
Calcium ions are removed from the sarcoplasm by active transport.
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.
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
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.
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.
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.
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.
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.
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)
In a neuromuscular junction, synaptic vesicles in the motor neuron contain which neurotransmitter?
acetylcholine (ACh)
A myosin head binds to which molecule to form a cross bridge?
actin
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
Tropomyosin
block(s) binding sites on actin.
Which of the following hormones is currently thought to decrease plasma calcium levels in pregnant women and children?
calcitonin
PTH promotes the formation of which hormone?
calcitriol
Which hormone works directly in the intestine to increase plasma calcium levels?
calcitriol
Troponin
change(s) shape upon binding with calcium ions.
T tubule
conduct(s) action potentials throughout the interior of the muscle fiber.
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
What means of membrane transport is used to release the neurotransmitter into the synaptic cleft?
exocytosis
What, specifically, is a cross bridge?
myosin binding to actin
Junctional folds
receive(s) stimulus from the motor neuron.
What causes the power stroke?
release of ADP and Pi
Terminal cistern
release(s) calcium ions into the sarcoplasm.
How is acetylcholine (ACh) removed from the synaptic cleft?
simple diffusion away from the synaptic cleft and acetylcholinesterase (AChE; an enzyme)
Which of the following would NOT be a way that parathyroid hormone (PTH) could alter plasma calcium levels? (Which one of the following is FALSE?)
stimulates increased osteoblast activity
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
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
When an action potential arrives at the axon terminal of a motor neuron, which ion channels open?
voltage-gated calcium channels
