Topic 10: Skeletal Muscle Physiology - Muscle Tension

Question 1

What is muscle tension?

Answer 1

• force exerted by a muscle or muscle fiber
  in lecture, more globular heads tagging on to myofilaments means more tension

Question 2

What is muscle tension determined by?

Answer 2

• number of cross bridges formed

Question 3

In a muscle fiber, what is muscle tension affected by? (4)

Answer 3

1. Frequency of Stimulation
2. Fiber length
3. Size of Fiber
4. Fatigue

Question 4

What are the factors of frequency of stimulation? (4)

Answer 4

1. Single Stimulus
2. Second stimulus arrives before complete relaxation from first
3. Rapid sequence of stimuli
   4.High frequency of stimuli

Question 5

What happens when you have a single stimulus? (5)

Answer 5

1. Produces a twitch (a weak contraction and relaxation not normally occurring in skeletal muscles)
2. Causes an AP (lasts 1-2 msec)
3. Latent period (2 msec) where excitation-contraction coupling occurring
4. Contraction period (10-100 msec) increased tension
5. Relaxation that decreases tension

Question 6

What happens during the contraction period of a single stimulus? (4)

Answer 6

• increased tension
  1. Cross bridge attachment and sliding filaments
  2. a lot of calcium released from SR on stimulation, but taken back rapidly by SR Ca2+-ATPase so not all myosin heads attach
  3. Does not reach maximum possible tension

Question 7

What happens in relaxation of a single stimulus? (2)

Answer 7

• calcium pumped into SR
• ATP releases myosin, etc

Question 8

What happens if a 2nd stimulus arrives before complete relaxation from first stimulus? (5)

Answer 8

• muscle AP always completed (refractory period) but uptake of calcium by SR is not yet complete (fiber relaxing)
• second stimulus causes release of more calcium, adding to that already in the cytosol
• more myosin heads then attach
• produces second contraction with increased tension, aka wave summation
• this contraction has no refractory period

Question 9

What happens during rapid sequence of stimuli? (2)

Answer 9

• tension increases further (increased calcium availability to wave summation)
• partial relaxation between contractions produces quivering, called INCOMPLETE TETANUS

Question 10

What happens during high frequency of stimuli? (2)

Answer 10

• no relaxation between contraction (ie. sustained contraction) called COMPLETE TETANUS
• all troponin saturated with calcium and fiber warm (ATP synthesis makes heat) so it works faster

Question 11

Which of the following occurs normally in the body, rapid sequence of stimuli or high frequency of stimuli?

Answer 11

high frequency