Skeletal Muscle Flashcards
The spiral arrangement of myosin heads jutting off the tails that make up the thick filament allow the heads to
interact with teh surrounding thick filaments
Purpose of the two lobes of the myosin head
One hydrolyzes ATP
One binds actin
Significance of the M line
The myosin heads can only move in one direction, so the polarity of the myosin thick filament hast o change at teh M line in order to pullt he thin filament toward the middle
Sliding filament theory
ATP-myosin hydrolyzes -> binds actin
Pi is released -> conformational change (cock)
ADP is released -> power stroke
New ATP binds -> release actin
Powerstroke
Relaxation of the head back to its resting position
What has to happen for crossbridge cycling to occur?
Ca++ has to bind troponin and induce tropomyosin to move on the thin filament; reveals myosin binding sites
The SR surrounds each myofibril so that
The released calcium doesn’t have to diffuse far to interact with troponin
Ryanodine receptors
Channels on the SR membrane that release calcium from the SR
Skeletal L-type calcium channel
Voltage-sensitive channel on the T-tubule membrane of skeletal muscle that
opens SR Calcium-release channels in response to depolarization
closes it in response to repolarization.
Ways to remove calcium from the cytoplasm so the muscle cell can relax
- Ca2+ ATPase
- Plasma membrane Ca-ATPase (PMCA) sends Ca2+ out the cell
- Sarcoplasmic/endoplasmic reticulum Ca-ATPase (SERCA) puts Ca2+ back into the SR
- Na-Ca exchanger: uses the power of the Na+conc gradient to pump Ca2+ out as Na enters
Na,K-ATPase is responsible for restoring resting levels of Na+ and K+ by pumping Na out and taking in K; however, during intense activity, you can get muscle fatigue and even temporary paralysis if
extracellular K+ accumulates faster than Na,K-ATPase can send back in
Sequence of events leading to contraction and relaxation
- Muscle action potential propagated and triggers DHP receptor-ryanidine complex to open
- Ca2+ enters the cyotplasm
- Ca2+ binds troponin, taking tropomyosin off actin
- Actin-myosin interaction
- Ca2+ is taken back up and removed from troponin
Why is the muscle membrane of the NMJ/motor end plate highly invaginated?
maximizes the amt of Ach receptors there
1-to-1 relationship
there is an action potential in the muscle for each motor nerve action potential
Isometric vs isotonic contractions
Isometric: Muscle length (preload) is fixed - so no shortening- and tension is measured.
Isotonic: Afterload (load against which the muscle contracts) is fixed and lengthening (eccentric) or shortening (concentric) is measured
Preload vs Afterload
Preload: intiial length of muscle before contraction
Afterload: constant force generated by muscle shortening during isotonic contraction
Lengh-tension relationship
Tension developed during isometirc contractions when the muscle is set to fixed lengths (preload)
Passive tension, Active tension, and Total tension
Passive tension: tension developed by stretching the muscle to different lengths
Total tension: tension developed when the muscle is stimulated to contract at different lengths
Active tension: the active force developed from contractions; proportional to the # of cross bridges; total - passive tension.
The duration of contraction/twitch is ___ than the action potential
Twitch takes longer than the action potential
What contributes to the latency period between the action potential and the start of muscle shortneing (aka start of contraction)?
Need to initiate the process of crossbridge cycling, and elasticity counts, but it’s most affected by the force required to overcome the load.
This is why the latency period is longer in isotonic contraction than it is in isometric contractions.
The velocity of a muscle shortening during a contraction ___ as you increase the load
Velocity decreases - the contraction gets slower as you increase load.
Thus, increasing load lengthens the latency period AND slows contraction.
isometric point
The point of a load-velocity relationship where load = max force the muscle can generate while still contracting isometrically
Beyond this, the muscle lengthens (isotonic)
Describe the force-length relationship
A muscle is tested under isometric contraction - the muscle length is set and max force is measured.
Bell shaped curve because too short has too much overlap/interference betwen teh thin and thick filaments and too long has no overlap for contraction .
What determines passive tension (developed by stretching)
elasticity, due to teh sarcolemma, SR, and connective tissue between myofibrils
Ways to increase muscle tension/strengthen contracitons
- Increased action potential frequency
- until fused tetanus (max contraciton)
- Recruiting additional muscle fibers by activating more motor units (motor neuron + all its fibers)
Human muscle consists of mainly what type of muscle fibers? Which generates the wealest force? Strongest force?
Slow, resistant (Type I) - weakest
Fast-fatigable (Type IIb?) - strongest
To generate increased muscle force, which motor units are recuited force? Then? Then
Slow-resistant > Fast-resistant > Fast-fatigable
weakest to strongest force
Muscle fatigue is caused by
- Build up of lactic acid
- Build up of phosphate from high ATP hydrolysis
- Build up of extracellular K+
- Neuronal fatigue /mental state
note: ATP DEPLETION DOES NOT CONTRIBUTE
