Lecture 12 : Muscle 2 Flashcards
Name 5 ways a muscle can produce more force:
- Increase number of fibres innervated by axon
- Increase frequency of stimulation
- Recruitment of motor units
- Optimise the sarcomere length of the muscle
- Build more sarcomeres - hypertrophy & hyperplasia
What is a motor unit?
- A motor unit is comprised of a motor neuron and all the muscle fibres that it innervates
- Motor neuron cell bodies are in the ventral part of the spinal cord
- Axons project to the target muscle
- The axons branch so that each axon innervates one or many myofibres
- One myofibre is innervated by one axon but axon can innervated multiple myofibres
What is recruitment?
The number of motor units activated at any one time can be varied to change the amount of force produced
How does the number of motor units innervating fibres affect the force and control?
- Motor units innervating many fibres produce more force, but will have less precise control
- Motor units innervating few fibres produce less force, but will have more precise control
Describe the relationship between frequency of stimulation and tension:
The amount of tension a muscle fibre can produce is proportional to the frequency of its stimulation
What is a twitch contraction?
A single action potential produces a short duration of contraction
What is summation of action potentials?
As the frequency of action potentials increase the amount of tension produced also increases
What is a tetanic contraction (tetanus)?
Force produced by a fibre at its maximum
What occurs in a single contraction (twitch) of the entire muscle fibre?
- Excitation (trough)
- Contraction (almost at peak)
- Relaxation (slope downwards)
What is excitation?
Neuronal action potential -> myofibre action potential -> Ca2+ release from the SR into the sarcoplasm -> excitation
What is contraction?
Ca2+ release causes cross bridge cycling -> tension in the sarcomere to be developed -> contraction
What is relaxation?
Ca2+ removed from the sarcoplasm -> cross bridge de-attachment -> relaxation
Describe the process of summation in one 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 quickly
- The level of Ca2+ in the sarcoplasm adds up to cause more cross bridges - a higher level of tension -> summation
What is incomplete tentanus?
A muscle fibre producing maximum tension during rapid cycles of contraction and relaxation
- Still some Ca2+ being removed from sarcoplasm between stimuli
What is complete tetanus?
When relaxation phase is eliminated by higher frequency stimuli
What does the amount of tension a muscle produces, depend on?
The tension produced by each fibre AND the number of activated fibres
What does the activation of different motor units (rotating basis) allow for?
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
Describe the length-tension relationship:
- For a short sarcomere, the overlap between actin and myosin is TOO much, the myosin can NOT effectively move the Z-lines closer together
- For a wide sarcomere, the myosin can NOT effectively bind to actin to create cross bridges
- At the optimal sarcomere length, all myosin heads can interact with actin to create cross bridges and tension
What is hypertrophy?
Larger myofibres - more sarcomeres
What is hyperplasia?
More new myofibres - more sarcomeres
How are motor neurons repaired?
Motor unit remodelling - reinnervation of muscle fibre by sprout of neighbouring neuron
What energy is used during each phase of short-duration, high-intensity exercise?
- 6 seconds: ATP storage - ATP stored in muscles is used
- 10 seconds: Creatine phosphate - ATP is formed from creatine phosphate and ADP through direct phosphorylation
- 30-40 seconds to end of exercise: Anaerobic - Glycogen stored in muscles is broken down to glucose, which is oxidised to generate ATP
What energy is used during prolonged-duration exercise?
Aerobic - ATP is generated by the breakdown of several nutrient energy fuels
Describe the properties of white muscles:
- Type II-B
- Fast-twitch muscles fibres
- High intensity
- Short duration
Describe the properties of pink muscles:
- Type II-A
- Intermediate muscle fibre
- Mid intensity
- Mid duration
Describe the properties of red muscles:
- Type I
- Slow-twitch muscle fibres
- Low intensity
- Long duration
- Table on onenote
