20. Macroscopic events of muscle contraction

Question 1

What should be mentioned?

20. Macroscopic events of muscle contraction

Answer 1

Macroscopic Examination of Muscle Function

Summation

All-or-none law

Quantal (Motor Unit) Summation

Contraction (Frequency) Summation

Staircase effect

Tetanus

Question 2

Macroscopic Examination of Muscle Function

Answer 2

Muscles consists of contractile components (CC) and elastic elements (SEC or PEC).
Contractile elements are the sarcomers.
The elastic elements can be seen serially (SEC) and parallelly (PEC) to tendons). These elements create a 3 –5 % extra resistance during contraction .

The type of contractions can be:

Isotonic: contraction with constant tension, e.g.: regular physiological activity.

Isometric: only tension is changed (length does not), e.g.: lifting unliftable load.

Mixed: combination of isometric + izotonic contraction:

Auxotonic: working against increasing tension and resistance; (i.e. against

a spring).

Preload: muscle length is adjusted with (pre)load (a stimulus generates isometric contraction until equilibrium is reached with the load), after this isotonic contraction (muscle shortening) will happen, e.g.: locomotion related muscle work.

Afterload: contraction begins with isotonic contraction (lifting a weight), until the contraction is blocked by something thus isometric contraction will happen, e.g.: biting; musculus masseter.

Question 3

Summation

Answer 3

The „summation” is the addition of (sceletal) muscle contraction forms caused by different reasons which will increase the contractile capacity of individual fibers and/or recruit more fibers into the contraction).

Types:

„All-or-none law”: applies for single fiber only (myocyte). An adequate stimulus (an AP) causes maximal contraction (if treshold is reached) . The strength of stimulus can not influence the amplitude of contraction.

AP frequency can increase; prolonged Ca2+ release occurs; more cross-bridge cycles can happen; stronger contraction will happen (even in a single fiber).

Quantal (Motor Unit) Summation: in any given muscle (not in a single fiber) more and more fibers are contracted due to increased AP frequency, even fibers with increased-treshold will contract (muscle contraction will be adapted to physiological need).

Contraction (Frequency) Summation: in case of repetitive stimuli (increased AP frequency) additional Ca2+ release will happen before the end of Ca2+ transient. This results in increased amplitudes of contraction (due to the extra amount of released Ca2+.

Staircase effect (warming up): If new stimuli arrive immediately after the end of the first twitch, following contractions will happen with increasing amplitudes. Reason: increasing efficiency of ion channels and Ca2+ accumulation in the cell.

Tetanus: additive effect of all summation forms. Cause: extremly increased AP frequency which causes muscle spasm.

Question 4

All-or-none law

Answer 4

Under constant (the same) metabolic conditions a single fiber functions according to the “all-or-none” law: the response is maximal to an adequate stimulus, to smaller stimuli (below the treshold) no response can be observed.

Question 5

Quantal (Motor Unit) Summation

Answer 5

Quantal summation: when more and more fibres participate in a contraction, muscle tension will increase. This is the “way” how physiological muscle contraction happens in the

organism: due to higher demand more fibres will be recruited by frequent APs.

Question 6

Contraction (Frequency) Summation

Answer 6

Contraction summation: repetitive stimuli may cause increased contraction because the previous calcium transient may not be completed when a new stimulus elicits additional calcium release. Thus the amplitude of contraction will increase.

Question 7

Staircase effect

Answer 7

Staircase Effect („warming up”): If a new stimulus is applied shortly after the end of a twitch it may elicit new contractions with gradually increasing amplitudes. The cause is the increased IC calcium level, due to the slow Ca2+ removal apparatus in the cell.

Staircase Effect or Treppe effect is not a way muscles exhibit graded contractions. It is a warmup phenomenon. When muscle cells are initially stimulated, they will exhibit gradually increasing responses until they have „warmed up”.

One of the explanations is the inability of the Na+/K+-ATPase to keep up with influx of sodium at increased muscle work.
The L Type Calcium channel has increased activity. The 3Na+/Ca2+ exchanger works to decrease the levels of intracellular calcium.
The Na+/K+- ATPase does not keep up with the rate of Na+ influx.
This leads to a less efficient Na+/Ca2+ exchange since the gradient is decreasing for sodium and the driving force behind calcium transport is actually the concentration gradient of sodium, therefore Ca++ builds up within the cell. This results in an accumulation of calcium in the myocite via the sodium calcium exchanger and leads to a greater contraction.

Question 8

Tetanus

Answer 8

If we apply stimuli with increasing frequency we will enhance the possible summation modes: muscle will reach the maximal contraction state: a tetanus is formed (the initial part is called incomplete tetanus followed by complete tetanus).