muscle physiology

joint torque is product of _____________ & _____________

muscle force & moment arm

4 steps of force production

1. neuromuscular transmission

2. excitation-contraction coupling

3. cross-bridge cycling

4. Ca2+re-uptake & relaxation

1. neuromuscular transmission

• nerve & muscle tissue are excitable (can conduct APs)

• AP in alpha motor neuron causes release of Ach from presynaptic nerve terminal

• Ach binds to receptors in end plate, causing depolarization of the end plate

• if depolarization reaches threshold, an AP is generated, which travels along sarcolemma into T-tubules

2. excitation contraction coupling

PASSIVE PROCESS

• AP sensed by dihydropyridine (DHP) receptor on T-tubule

• DHP causes conformational change in ryanodine receptor (RYR) on SR, causing release of Ca2+ into sarcoplasm

3. cross-bridge cycling

• Ca2+ binds to troponin, causing a shift in the position of tropomyosin on actin

• tropomyosin shift exposes myosin binding site on actin

• myosin cross-bridge binds & interacts with actin

• at rest, myosin head is cocked, but in the presence of Ca2+, myosin binds to actin & Pi is released

• myosin power stroke causes filaments to slide; ADP is released

• ATP binds to myosin head, allowing it to release actin

• ATP is hydrolyzed as myosin head returns to cocked position

cross-bridge cycling clinical connection - rigor mortis

depletion of ATP → crossbridge can not release from actin binding site → stiffening of the muscle

4. Ca2+ re-uptake & relaxation

• Ca2+ pumped back into SR by SERCA pump

• ATP consumed by pump/active transport

• Ca2+ in sarcoplasm falls, tropomyosin shifts & covers active site on thin filament

time to peak tension

30-100 msec

twitch time is affected by the _____________________

series elastic component

what causes the series elastic component relax?

when Ca2+ is re-sequestered

one sarcomere produces ______ of force

1 pN

one sarcomere spans ______

1 uM

sarcomeres in parallel

sarcomeres in series

how does pennation angle affect the abilities of a muscle?

• affects orientation of sarcomeres with respect to tendon

• decreases the force transmitted from fiber to tendon

why are our strongest muscles pennate?

there are more fibers per amount of surface area

when fibers are longitudinal (fusiform) …

• cross-sectional area indicates number of sarcomeres in parallel (proportional to force capacity

• muscle length indicates number of sarcomeres in series (proportional to excursion & shortening velocity)

when fibers are pennate …

• physiological CSA accounts for pennation angle & is proportional to force capacity

• fiber length indicates number of sarcomeres in series (proportional to excursion & shortening velocity)

soleus fiber length is ________ (short/long), but physiological cross-sectional area is ________ (small/large)

fiber length is short, but physiological cross-sectional area is large; propels entire body forward, but experiences little excursion

sartorious fiber length is ________ (short/long), but physiological cross-sectional area is ________ (small/large)

fiber length is long, but physiological cross-sectional area is small; large excursion during gait, but produces little force

resting muscle is elastic, as it is stretched, ___________ increases

tension

tension results from elasticity of _____________

parallel elastic component

passive insufficiency

when a multi-joint muscle cannot lengthen enough to allow full movement at all the joints it crosses at the same time

example of passive insufficiency

when the hip is flexed, hamstring length can restrict knee extension ROM

passive tension in lengthened muscle can ______________ & ______________

stabilize joints & compensate for muscle weakness

how does muscle length affect force generating capacity?

length determines degree of myofilament overlap, which affects the number of cross-bridges formed, & therefore the amount of force produced

total force = _______________ + _______________

active force + passive force

the joint-angle torque relationship reflects combined effects of changes in _______________ & _______________

changes in moment arm & muscle length

active insufficiency

the inability of a multi-joint muscle to generate maximum force when it is shortened across all the joints it crosses at the same time

lengthening = ___________ (positive/negative) work

shortening = ___________ (positive/negative) work

lengthening = negative work

shortening = positive work

a muscle’s capacity to produce force is a function of what 3 variables?

1. physiological cross-sectional area

2. muscle length

3. velocity of contraction

PCSA is proportional to _________________

the number of sarcomeres in parallel

what does muscle length determine?

degree of myofilament overlap

chart of motor unit types