L11. Muscle Force and Energy for Muscle Contraction Flashcards
How do muscles produce more force?
- Increase number of fibres innervated by axon (within a motor unit)
- Increase frequency of stimulation (twitch, summation, and tetanic contraction)
- Recruitment of motor units
- Optimise the sarcomere length of the muscle (the length-tension relationship)
- Build more sarcomeres - hypertrophy and hyperplasia
Motor unit?
A motor unit is comprised of a motor neuron and all the muscle fibres that it innervates
- Motor neuron cell bodies (somas) are in the ventral (front/anterior) part of the spinal cord
- Axons project out of the spinal cord to the target muscle
- Close to the muscle, the nerve axons branch so that each axon innervates one or many myofibres (10-100)
Only one myofibre is innervated by one axon
BUT
One axon can innervate multiple myofibres
A whole muscle contains a collection of different motor units
The number of motor units activated at any one time can be varied to change the amount of force produced, this is called recruitment
Motor units innervating many fibres produce more force, but will have less precise control
BUT
Motor units innervating few fibres produce less force, but will have more precise control
Frequency of action potentials?
The amount of tension a muscle fibre can produce is proportional to the frequency of its stimulation
Twitch = a single action potential produces a short duration of contraction
Summation = as the frequency of action potentials increase, the amount of tension produced also increases
Tetanic contraction (tetanus) = force produced by a fibre at its maximum
Twitch of one muscle fibre?
Excitation –> contraction –> relaxation sequence in a muscle fibre
Summation of one muscle fibre?
When a fibre is stimulated before relaxation is completed, the subsequent contraction develops a higher tension
After the first stimulus only some of the Ca2+ is removed from the sarcoplasm, as the next stimulus comes along very quickly
The level of Ca2+ in the sarcoplasm adds up to cause more cross bridges - a higher level of tension
Tetanic contraction of one fibre?
2 types!
- Incomplete tetanus = a muscle fibre producing maximum tension during rapid cycles of contraction and relaxation
- Complete tetanus = when relaxation phase is eliminated by higher frequency stimuli
Recruitment?
Activation of different motor units in turn (rotating basis) allows recovery of some motor units, whereas whole muscle tension is maintained and muscle fatigue is prevented
- Sustained contraction of muscles to maintain posture will be a team effort
- A single twitch will only occur in ocular muscles
Length-tension relationship?
How at different resting muscle/sarcomere lengths different levels of tension develop
At optimal sarcomere length = all myosin heads can interact with actin to create cross bridges and tension
At short sarcomere length = the overlap between actin and myosin is TOO much, the myosin CANNOT effectively move the Z-lines closer together
At wide sarcomere length = the myosin CANNOT effectively bind to actin to create cross bridges
Hypertrophy?
Larger myofibres (more sarcomeres)
Hyperplasia?
More new myofibres (more sarcomeres)
Motor neuron repair?
Motor unit remodelling -
re-innervation of muscle fibre by sprout of neighbouring neuron
Energy types/length?
- ATP storage (6 seconds) –> energy stored in muscles is used first
- Creatinine phosphate (10 seconds) –> ATP is formed from creatine phosphate and ADP (direct phosphorylation)
- Anaerobic (no oxygen required) (30-40 seconds to end of exercise) –> glycogen stored in muscles is broken down to glucose, which is oxidised to generate ATP
ALL FOR SHORT-DURATION, HIGH-INTENSITY EXERCISE - Aerobic (oxygen required) –> ATP is generated by breakdown of several nutrient energy fuels by aerobic pathway
PROLONGED-DURATION EXERCISE
Muscle fibre subtypes?
White muscles - High intensity and short duration (fast-twitch muscle fibre)
Pink muscles - Mid intensity and mid duration (intermediate muscle fibre)
Red muscles - Low intensity and long duration (slow-twitch muscle fibre)
An endurance athlete will have predominantly large muscles with white muscle fibres BECAUSE endurance activities require predominantly aerobic muscle types
A. Both statements are correct and causally related
B. Both statements are correct and NOT causally related
C. Only the first statement is correct and the second statement is incorrect
D. Only the second statement is correct and the first statement is incorrect
E. Both statements are incorrect
D
When a muscle fibre is stimulated before relaxation is completed, the subsequent contraction develops a higher tension BECAUSE at the optimal sarcomere length, the actin myosin overlap is optimatl for cross bridge formation
A. Both statements are correct and causally related
B. Both statements are correct and NOT causally related
C. Only the first statement is correct and the second statement is incorrect
D. Only the second statement is correct and the first statement is incorrect
E. Both statements are incorrect
B
