Contractility and Fatigue of Skeletal Muscle
Lab #3: Contractility and Fatigue of Skeletal Muscle (iWorx) Study Questions: 1. During muscle contraction, the binding of an ATP molecule to the head of a myosin molecule results in the dissolution of the bond between actin and myosin. What role does this dissociation between actin and myosin have in muscle functioning? •
ATPbinding decreases actinbinding affinity of myosin, allowing it to leave the rigor state
•
Once myosin is detached from actin, ATPase hydrolyzes ATP into ADP and P i which provides much energy for the myosin head
•
It is now able to bind to actin again, power stroke, bring the Zlines closer together, and contract once troponin removes tryptomyosin
•
Without the dissociation, muscles would be unable to contract and would stay in the rigor state, however there is always ATP in our muscles and therefore this dissociation will always occur, unless you are dead
2. Does the amplitude of the EMG signal and the force of contraction, as measured by the absolute areas, increase because a finite number of fibers are firing more often, or because more fibers are recruited to fire as the intensity of signals in the motor neurons increases, or a combination of these two? •
Strength of a contraction is directly proportional to the intensity of electrical activity in the muscle. As frequency of action potential increases, force of contraction increases. If stimuli occur closer together, summation can occur which is able to create maximal tension (tetanus)
•
The strength of a muscle contraction is related to the number of motor units in the muscle that are activated during the contraction
3. What is a “motor unit”? •
A motor unit is a basic unit of contraction in an intact skeletal muscle
•
Composed of a group of muscle fibers that function together and the somatic motor neuron that controls them
•
One somatic neuron innervates multiple fibers, each muscle fiber is innervated by only a single neuron
•
All muscle fibers in a single motor unit are of the same fiber type
•
AP from a somatic motor neuron will cause contraction of all muscle fibers in the motor unit (allornone)
4. What roles do potassium ions have in the functioning of skeletal muscle? •
When a muscle is worked hard, K+ ions come out of the fibers and make the membranes less excitable
•
K+ ions come out of all electricallyactive cells, including muscle fibers, in the second half of each electrical impulse, this causes the muscle fiber to repolarize
5. What role does phosphocreatine have in the functioning of skeletal muscle? •
Amount of ATP in a muscle fiber only sufficient for about 8 twitches
•
Muscles contain phosphocreatine as a backup energy source
•
Highenergy phosphate group of phosphocreatine is transferred to ADP, creating more ATP to power the muscles
•
Resting muscle stores energy from ATP in the highenergy phosphate bonds of phosphocreatine (ATP + Creatine ADP + Phosphocreatine)
•
Working muscle then uses that stored energy (Phosphocreatine + ADP Creatine + ATP)
6. What causes fatigue in living human skeletal muscle? •
Fatigue – reversible condition in which a muscle is no longer able to generate or sustain expected power output
•
Central Fatigue o Control neurons in CNS o Subjective feelings of ‘tiredness’
•
Peripheral Fatigue o Somatic motor neurons o Within muscle fibre: •
Increased Pi concentration will slow Pi release from myosin, which will slow power stroke
•
Ion imbalances: Increased K+ ions in ttubules will alter excitability of muscle membrane
•
Depletion of glycogen will affect Ca2+ release
ATPbinding decreases actinbinding affinity of myosin, allowing it to leave the rigor state
•
Once myosin is detached from actin, ATPase hydrolyzes ATP into ADP and P i which provides much energy for the myosin head
•
It is now able to bind to actin again, power stroke, bring the Zlines closer together, and contract once troponin removes tryptomyosin
•
Without the dissociation, muscles would be unable to contract and would stay in the rigor state, however there is always ATP in our muscles and therefore this dissociation will always occur, unless you are dead
2. Does the amplitude of the EMG signal and the force of contraction, as measured by the absolute areas, increase because a finite number of fibers are firing more often, or because more fibers are recruited to fire as the intensity of signals in the motor neurons increases, or a combination of these two? •
Strength of a contraction is directly proportional to the intensity of electrical activity in the muscle. As frequency of action potential increases, force of contraction increases. If stimuli occur closer together, summation can occur which is able to create maximal tension (tetanus)
•
The strength of a muscle contraction is related to the number of motor units in the muscle that are activated during the contraction
3. What is a “motor unit”? •
A motor unit is a basic unit of contraction in an intact skeletal muscle
•
Composed of a group of muscle fibers that function together and the somatic motor neuron that controls them
•
One somatic neuron innervates multiple fibers, each muscle fiber is innervated by only a single neuron
•
All muscle fibers in a single motor unit are of the same fiber type
•
AP from a somatic motor neuron will cause contraction of all muscle fibers in the motor unit (allornone)
4. What roles do potassium ions have in the functioning of skeletal muscle? •
When a muscle is worked hard, K+ ions come out of the fibers and make the membranes less excitable
•
K+ ions come out of all electricallyactive cells, including muscle fibers, in the second half of each electrical impulse, this causes the muscle fiber to repolarize
5. What role does phosphocreatine have in the functioning of skeletal muscle? •
Amount of ATP in a muscle fiber only sufficient for about 8 twitches
•
Muscles contain phosphocreatine as a backup energy source
•
Highenergy phosphate group of phosphocreatine is transferred to ADP, creating more ATP to power the muscles
•
Resting muscle stores energy from ATP in the highenergy phosphate bonds of phosphocreatine (ATP + Creatine ADP + Phosphocreatine)
•
Working muscle then uses that stored energy (Phosphocreatine + ADP Creatine + ATP)
6. What causes fatigue in living human skeletal muscle? •
Fatigue – reversible condition in which a muscle is no longer able to generate or sustain expected power output
•
Central Fatigue o Control neurons in CNS o Subjective feelings of ‘tiredness’
•
Peripheral Fatigue o Somatic motor neurons o Within muscle fibre: •
Increased Pi concentration will slow Pi release from myosin, which will slow power stroke
•
Ion imbalances: Increased K+ ions in ttubules will alter excitability of muscle membrane
•
Depletion of glycogen will affect Ca2+ release
