224. Muscle Mechanics + Physiology

Question 1

What are the molecular components of muscle?

Answer 1

One muscle fiber = 1 myocyte
Motor: thick filament, myosin heads, burns ATP
Troponin-C: calcium binds this to expose actin-myosin binding sites
SR: stores Ca in myocytes

Locomotion: ATP binds to release myosin heads
Powerstroke: release of ADP+Pi

Question 2

What are the steps for myocyte contraction?

Answer 2

1. motor command generates AP
2. AP travels down motor neuron axon
3. AP reaches neuromuscular junction
4. Neuron AP depolarizes muscle fiber
5. Depolarization travels down t-tubule, Ca released from SR
6. Ca allows actin-myosin to bind = muscle twitch (not sustained bc Ca reuptake to SR)

Question 3

What is tetanic frequency?

Answer 3

Maximum frequency above which more force cannot be generated (max Ca/actin-myosin contraction)

Question 4

What 3 things control muscle contraction force?

Answer 4

1. Ca controls number of cross-bridges (most important control)
2. muscle length affects #cross bridges: long sarcomere length = zero force no interaction; too short - no binding sites - zero force - need specific operating range for peak force
3. muscle velocity affects #cross bridges: faster a muscle shortens, the less the force generated (muscle generates more force as it is lengthening - x-bridges yield more energy)

Question 5

What are the energy stores for muscle? How long do they last?

Answer 5

1. ATP <5 s
2. Creatine Phosphate <15s
3. Anaerobic Glycolysis (from muscle glycogen) 10s-5min
4. Aerobic Glycolysis (takes time because needs oxygen)
5. Aerobic Fat (slower due to need to mobilize fat)

More vigorous/intense activity = more aerobic glycolysis than aerobic fat burn

Question 6

What is the definition of motor unit

Answer 6

1 motor neuron in ventral root of SC, its axon, and all the fibers innervated by its branches (no overlap in fibers/other motor neurons)

bigger muscle = more motor units, and more muscle fibers per motor unit

Question 7

What are the three types of motor units?

Answer 7

Slow Twitch: low force, slower contraction time (delayed), never fatigues
Fast-Fatigue Resistant: more force, faster contraction time, takes a while to fatigue
Fast-Fatigable: Highest force, fastest contraction time, very fatigable