40 Questions
What is the enzyme responsible for phosphorylating creatine in the muscle?
Creatine phosphokinase
What is the source of energy that reconstitutes the ATP molecule?
Phosphocreatine
What is the byproduct of glycolysis that accumulates in the muscle?
Lactate
What is the rate of ATP formation by glycolysis compared to oxidative metabolism?
2.5 times faster
What is the purpose of phosphocreatine in muscle contraction?
To regenerate ATP quickly
Where is creatine produced from amino acids?
Liver
What is the characteristic of glycolysis that allows muscle contraction to be sustained without oxygen delivery from the blood?
It is an anaerobic process
What is the purpose of glycolysis in the muscle?
To release energy and produce pyruvic acid and lactic acid
What occurs when the frequency of muscle contractions reaches a critical level?
The successive contractions fuse together
What is the ideal length for a skeletal muscle to operate with the greatest active tension?
When the muscle is close to its resting length
What happens when the frequency of muscle contractions increases?
The strength of contraction rises progressively
What is the term for a contraction of maximal strength where successive contractions fuse together?
Tetanization
What determines the amount of tension generated during a muscle contraction?
The number of cross-bridge interactions
What happens to the muscle contraction strength when the muscle is shortened or stretched?
It diminishes
What is the relationship between the length of a muscle before contraction and the tension it can develop?
Length-tension relationship
What happens when a muscle fiber is stimulated rapidly?
It does not have a chance to relax at all
What percentage of energy used for contraction is attributed to oxidative metabolism?
95%
What is the primary source of energy for cardiac muscle?
Fatty acids
What is the percentage of cytoplasm volume occupied by mitochondria in cardiac fibers?
40%
What is the storage form of fatty acids in cardiac muscle?
Lipid droplets
What is the approximate percentage of energy that comes from glucose and lactate in cardiac muscle?
10-30%
What happens to cardiac cells after 30 seconds of O2 deprivation?
They stop contracting
For what duration can cardiac muscle rely on carbohydrates for energy?
Short periods
What is the percentage of skeletal muscle fibers occupied by mitochondria?
2%
What determines the number of cross-bridge interactions in a muscle?
The degree of overlap between thick and thin filaments
What is the optimal condition for muscle contraction?
When the muscle is at its optimal resting length
What occurs when a muscle is too shortened?
Muscle contraction cannot progress
What is the primary function of the calcium pump in the SR?
To remove calcium ions from the cytosol
What is the purpose of rephosphorylation of ADP in muscle tissue?
To allow contraction to continue
What are the three sources of energy for muscle contraction?
ATP, glycolysis, and oxidative phosphorylation
What is the type of response seen in muscle fibers when exposed to a threshold stimulus?
All-or-none response
What determines the strength of contraction in the body?
The number of motor units recruited and the frequency of stimulation
What is the result of muscle contraction when the sarcomere is in the optimal resting length?
Maximal tension is achieved
What is the term for the time between a stimulus to the motor neuron and the subsequent contraction of the innervated muscle?
Latent period
What is the consequence of limited ATP in muscle tissue?
Rephosphorylation of ADP must occur
What occurs when another action potential comes before the complete relaxation of a muscle twitch?
Summation
What is the term for a single contraction and relaxation cycle produced by an action potential within the muscle fiber itself?
Twitch
What is the type of summation that occurs when the number of motor units recruited increases?
Multiple fiber summation
What is the term for the state of muscle contraction where the muscle contracts at a maximum frequency?
Tetanus
What is the characteristic of muscle contraction that is controlled in the body by changing the number of motor units recruited and/or the frequency of stimulation?
Strength of contraction
Study Notes
Oxidative Metabolism
- Responsible for 95% of energy used for contraction
- Combines O2 with end products of glycolysis (e.g. lactate) and other energy sources (proteins, lipids, carbohydrates) to produce ATP
- Main energy source for cardiac muscle, derived from oxidative metabolism of fatty acids
- Mitochondria make up 40% of cytoplasm volume in cardiac fibers (compared to 2% in skeletal muscle fibers)
- Numerous lipid droplets containing triglycerides (storage form of fatty acids) found in cardiac muscle
- Only 10-30% of energy comes from glucose and lactate; glycogen granules are found in cardiac muscle
- Cardiac cells stop contracting after 30 seconds of O2 deprivation
Phosphocreatine (Creatine Phosphate)
- Creatine produced from amino acids in liver, phosphorylated in muscle by enzyme creatine phosphokinase to produce phosphocreatine
- Carries high-energy phosphate bond similar to ATP bonds
- Used to reconstitute ATP molecule; cleavage of phosphocreatine releases energy used to bond new phosphate ion to ADP to reconstitute ATP
- Small amount of phosphocreatine found in muscle
Glycolysis
- Enzymatic breakdown of carbohydrates (glucose and glycogen) to produce pyruvic acid and lactic acid, releasing energy
- Energy used to reconstitute both ATP and phosphocreatine
- Can occur in absence of oxygen; muscle contraction can be sustained for over a minute without O2 delivery from blood
- Rate of ATP formation by glycolysis is 2.5 times faster than ATP formation by oxidative metabolism
- Many end products accumulate, including lactate
Muscle Physiology
Muscle Strength and Length-Tension Relationship
- Muscle contraction strength controlled by changing number of motor units recruited and/or frequency of stimulation
- All-or-none response: muscle fiber contracts maximally or not at all
- Strength of contraction can be separated into twitch, summation, and tetanus
- Twitch: single contraction and relaxation cycle produced by action potential within muscle fiber
- Summation: occurs in two ways: multiple fiber summation (increased number of motor units recruited) and frequency summation (increased frequency of stimulation)
- Tetanus: contraction of maximal strength resulting from rapid successive contractions
- Latent period: time between stimulus to motor neuron and subsequent contraction of innervated muscle
Length-Tension Relationship
- Skeletal muscles operate with greatest active tension when close to ideal length (often resting length)
- Length-tension relationship: relation between length of muscle before contraction and tension generated during contraction
- Optimal length for maximal force generation varies for each muscle
- Muscle fibers can contract forcefully when stimulated over a relatively narrow range of resting lengths
This quiz covers the process of oxidative metabolism in cardiac muscle, including the sources of energy and the production of ATP.
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