19. Macroscopic events of muscle contraction

Question 1

Elements:

Answer 1

• Amongst contractile components of the muscle (CC/sarcomere), muscle contains serial elastic components (SEC) and parallel elastic components (PEC), too.
• During stimulation, first the SEC elements will reach equilibrium with load because the contraction of CC (no movement yet, only tension).
  
  • This is followed by constant tension.

Question 2

Twitch:

Answer 2

• To an appropriate stimulus muscle answers with a contraction: a muscle twitch occurs.
• Muscle twitch = a single contraction-relaxation cycle
• An AP is not directly followed by a calcium transient:
  
  • The observable latency is partly derived from the latency of measuring instruments, or it’s a realbiological latency.
• The sum of the two latencies is called virtual latency.

Question 3

Isometric contraction:

Answer 3

• Only tension is changed but not the length of the muscle
• Occurs when muscle tries to lift such heavy load which it is not able to move

Question 4

Isotonic contraction:

Answer 4

• Muscle shortens with constant tension
• Regular physiological behaviour

Question 5

Auxotonic contraction:

Answer 5

• In natural conditions, muscle shortens and in the same time, tension is also increasing in it.
• Example: when a muscle works against a spring.

Question 6

Preload:

Answer 6

• After stimulation contractile machinery will first stretch SEC elements (isometric period), and when tension reaches equilibrium with the load, contraction becomes isotonic, twitch is continued with shortening of the muscle.
• Most of the contraction in association with locomotion are like that.

Question 7

Afterload:

Answer 7

• If we block the free movement of the muscle with a frame, then no more shortening is possible from a certain level, but muscle is still able to increase tension.
• At the beginning it is isotonic, then an isometric period follows.

Question 8

Summation

Answer 8

addition of skeletal muscle contraction forms caused by different reasons

• All-or-none: A single fiber under constant metabolic conditions contracts according to the “all-or-none” law = to an adequate stimulus, response is maximal, to smaller stimulus there is no response.
• Quantal summation: If the increase of tension is caused by the participation of more and more fibers (addition of elementary units). If the demand is higher, a more frequent AP recruits more and more fibers.
• Contraction summation: Repetitive stimuli may cause increasing contraction, for the previous calcium transient may not be completed when a new stimulus elicits additional calcium release. Thus, the amplitude of contraction is increased.
• Staircase effect (treppe): New stimuli applied shortly after the end of a twitch may elicit new contractions with gradually increasing amplitudes: it is caused by IC calcium, which has no time to be removed in between stimuli (warming up).
• Tetanus: If we apply stimuli with increasing frequency we enhance possible summation modes: finally muscle reaches maximal contraction state – a tetanus is formed.