Ch7 Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling

Question 1

What is the neuromuscular junction?

Answer 1

The junction where a nerve ending connects with a skeletal muscle fiber near its midpoint.

Question 2

How many skeletal muscle fibers can a single nerve fiber stimulate?

Answer 2

From three to several hundred skeletal muscle fibers.

Question 3

What is the role of acetylcholine in skeletal muscle excitation?

Answer 3

Acetylcholine excites the muscle fiber membrane.

Question 4

What is the motor end plate?

Answer 4

The entire structure formed by the junction of a nerve fiber and a muscle fiber, covered by Schwann cells.

Question 5

What is the synaptic cleft?

Answer 5

The space between the terminal and the muscle fiber membrane, approximately 20 to 30 nanometers wide.

Question 6

What is the function of acetylcholinesterase?

Answer 6

It destroys acetylcholine a few milliseconds after its release.

Question 7

What triggers the release of acetylcholine from the vesicles?

Answer 7

The entry of calcium ions into the nerve terminal.

Question 8

What are subneural clefts?

Answer 8

Smaller folds of the muscle membrane that increase the surface area for synaptic transmitter action.

Question 9

What happens when acetylcholine binds to its receptors?

Answer 9

It causes a conformational change that opens ion channels, allowing sodium ions to enter the muscle fiber.

Question 10

What is the end plate potential?

Answer 10

A local positive potential change inside the muscle fiber membrane caused by sodium ions flowing in.

Question 11

True or False: Each muscle fiber has more than one neuromuscular junction.

Answer 11

False.

Question 12

Fill in the blank: The acetylcholine receptor complex in adults is composed of five subunit proteins, with _______ substituting for the gamma protein.

Answer 12

epsilon protein.

Question 13

What is the diameter of the acetylcholine-gated channel?

Answer 13

About 0.65 nanometers.

Question 14

What is the effect of curare on end plate potentials?

Answer 14

It weakens the end plate potential by blocking acetylcholine receptor sites.

Question 15

What is the effect of botulinum toxin on acetylcholine release?

Answer 15

It decreases the quantity of acetylcholine released by nerve terminals.

Question 16

What occurs during the fatigue of the neuromuscular junction?

Answer 16

The number of acetylcholine vesicles diminishes, causing impulses to fail to pass into the muscle fiber.

Question 17

What are some drugs that have acetylcholine-like action?

Answer 17

• Methacholine
• Carbachol
• Nicotine

Question 18

What is the safety factor at the neuromuscular junction?

Answer 18

The ratio of end plate potential to the threshold required to stimulate the muscle fiber.

Question 19

What is the primary energy source for the synthesis of acetylcholine?

Answer 19

Adenosine triphosphate (ATP).

Question 20

What is the role of calcium ions in the neuromuscular junction?

Answer 20

They activate Ca2+-calmodulin-dependent protein kinase to trigger acetylcholine release.

Question 21

What happens to acetylcholine after it excites the muscle fiber?

Answer 21

It is rapidly destroyed by acetylcholinesterase.

Question 22

What initiates an action potential in the muscle fiber?

Answer 22

Sufficient depolarization caused by the end plate potential.

Question 23

How long does acetylcholine typically remain in the synaptic space?

Answer 23

A few milliseconds.

Question 24

How is the release of acetylcholine facilitated?

Answer 24

Through the process of exocytosis after docking at release sites.

Question 25

What is fatigue of the neuromuscular junction?

Answer 25

Fatigue of the neuromuscular junction occurs when the number of acetylcholine vesicles diminishes due to prolonged stimulation, leading to failure of impulses to pass into the muscle fiber.

Question 26

Name three compounds that stimulate muscle fibers by acetylcholine-like action.

Answer 26

* Methacholine * Carbachol * Nicotine

Question 27

What happens to acetylcholine during fatigue of synapses in muscle fibers?

Answer 27

The effects are similar to those of acetylcholine, but the drugs are not destroyed by cholinesterase or are destroyed slowly.

Question 28

What are the stages of acetylcholine formation and release at the neuromuscular junction?

Answer 28

* Formation of small vesicles by the Golgi apparatus * Synthesis of acetylcholine in the cytosol and transport to vesicles * Arrival of action potential opening calcium channels, leading to exocytosis

Question 29

What is the primary function of acetylcholinesterase?

Answer 29

To hydrolyze acetylcholine into acetate ion and choline after it is released into the synaptic space.

Question 30

True or False: Myasthenia gravis is caused by antibodies that attack acetylcholine receptors.

Answer 30

True

Question 31

What are the effects of administering neostigmine in myasthenia gravis?

Answer 31

It allows larger amounts of acetylcholine to accumulate in the synaptic space, temporarily improving muscle function.

Question 32

Fill in the blank: The T tubules are internal extensions of the _______.

Answer 32

[cell membrane]

Question 33

What is the role of T tubules in muscle contraction?

Answer 33

They allow action potentials to penetrate deep into the muscle fiber, triggering calcium release for contraction.

Question 34

What is the resting membrane potential in skeletal muscle fibers?

Answer 34

About −80 to −90 millivolts.

Question 35

How does the duration of action potentials in skeletal muscle compare to those in large myelinated nerves?

Answer 35

The duration is about five times longer in skeletal muscle (1 to 5 milliseconds).

Question 36

What is the velocity of conduction in skeletal muscle fibers?

Answer 36

3 to 5 m/sec, which is about 1/13 the velocity in large myelinated nerve fibers.

Question 37

What triggers the release of calcium ions from the sarcoplasmic reticulum?

Answer 37

The action potential in the T tubule.

Question 38

What is the function of the calcium pump in the sarcoplasmic reticulum after contraction?

Answer 38

To remove calcium ions from the myofibrillar fluid, returning calcium concentration to resting levels.

Question 39

What causes malignant hyperthermia?

Answer 39

Exposure to certain anesthetics in susceptible individuals, often linked to genetic mutations.

Question 40

What is the role of calsequestrin in the sarcoplasmic reticulum?

Answer 40

To bind calcium ions, allowing for storage and release during muscle contraction.

Question 41

What is the role of the action potential in skeletal muscle contraction?

Answer 41

The action potential causes a conformational change in the voltage-sensing dihydropyridine (DHP) receptors, opening the ryanodine (RyR) Ca2+ release channels.

Question 42

What happens during repolarization in excitation-contraction coupling?

Answer 42

The conformational change in the DHP receptor closes the Ca2+ release channels, and Ca2+ is transported back into the sarcoplasmic reticulum by SERCA.

Question 43

What does SERCA stand for?

Answer 43

Sarcoplasmic reticulum Ca2+-ATPase

Question 44

Fill in the blank: The calcium pump responsible for transporting Ca2+ into the sarcoplasmic reticulum is called _______.

Answer 44

SERCA

Question 45

What metabolic consequences arise from malignant hyperthermia?

Answer 45

Increased metabolic rate, generation of large amounts of heat, cellular acidosis, and depletion of energy stores.

Question 46

What are common symptoms of malignant hyperthermia?

Answer 46

Muscle rigidity, high fever, rapid heart rate

Question 47

True or False: Malignant hyperthermia can lead to rhabdomyolysis.

Answer 47

True

Question 48

What treatment is typically administered for malignant hyperthermia?

Answer 48

Rapid cooling and dantrolene

Question 49

What is the function of dantrolene in the treatment of malignant hyperthermia?

Answer 49

It antagonizes ryanodine receptors, inhibiting calcium ion release from the sarcoplasmic reticulum.

Question 50

What causes sustained muscle contractions in malignant hyperthermia?

Answer 50

Unregulated passage of calcium from the sarcoplasmic reticulum into intracellular spaces.

Question 51

Fill in the blank: Excessive muscle contractions due to malignant hyperthermia can lead to a high plasma _______ level.

Answer 51

potassium

Question 52

What is the effect of anesthetics on receptors related to malignant hyperthermia?

Answer 52

They interact with abnormal receptors to trigger malignant hyperthermia.

Question 53

What are the consequences of rapid breakdown of skeletal muscle in malignant hyperthermia?

Answer 53

Rhabdomyolysis and high plasma potassium levels.

Question 54

What are calcium sparks?

Answer 54

Localized releases of calcium ions from the sarcoplasmic reticulum.

Question 55

What is the significance of the sarcolemma in muscle contraction?

Answer 55

It is the membrane that surrounds muscle fibers and is involved in transmitting the action potential.

Question 56

What is the role of actin and myosin filaments in muscle contraction?

Answer 56

They interact to facilitate muscle contraction through the sliding filament theory.

Question 57

What occurs when calcium ions diffuse into the sarcoplasm?

Answer 57

It initiates muscle contraction.

Question 58

What is the primary energy source required for the calcium pump during muscle contraction?

Answer 58

ATP (Adenosine triphosphate)