Which processes are during the latent period of ventricular contraction?

Question

The latent period is the time between the start of depolarisation and the start of contraction. I know that the contraction of ventricles starts after sufficient influx of Ca2+ and threshold potential is met. The influx of Ca2+ is at start of Phase 2, so Phase 0 and 1 are included in the latent period.

Comparing to electrocardiogram is not simple, since there is always at least some time lag between the event and the recording. I think there not much sense to compare cellular model events to ECG recordings here.

Which cellular processes does the latent period include?

Answer 1

During the latent period of ventricular contraction, excitation-contraction coupling takes place.

There may be other processes, but I think most are these from Wikipedia:

1. An action potential is initiated by pacemaker cells in the Sinoatrial node or Atrioventricular node and conducted to all cells in the heart via gap junctions.
2. The action potential travels along the surface membrane into T-tubules (the latter are not seen in all cardiac cell types) and the depolarisation causes Ca2+ to enter the cell via L-type calcium channels (also known as Dihydropyridine receptors) and possibly sodium-calcium exchanger during the early part of the plateau phase. This Ca2+ influx causes a small local increase in intracellular Ca2+ .
3. The increase in Ca2+ is detected by ryanodine receptors in the membrane of the sarcoplasmic reticulum which releases Ca2+ in a positive feedback physiological response. This positive feedback is known as calcium-induced calcium release and gives rise to Calcium sparks.
4. The spatial and temporal summation of ~30,000 Ca2+ sparks gives a cell-wide increase in cytoplasmic calcium concentration.
5. Contractile mechanism similar as in skeletal muscle about cytoplasmic calcium binding to troponin C and so on.

Note that to fully answer this question you should include figures about mechanisms of excitation-contraction coupling, cardiac action potential -figure and permeabilities of different ions.

Answer 2

The phases from the stimulus to the force contain latent period, contraction phase and relaxation phase. This applies to all muscles, not just muscles associated with ventricular contraction.

So the Latent period, about 5-7ms, contains the following:

• skeletal muscle prepares for contraction

• phase between stimulus and beginning of contraction (response)

• the muscle doesn’t change length or tension

• influx of Ca2+

• inhibitory proteins of contraction:

  • binding to Troponin and removing Tropomyosin
  • Troponin: T binds with tropomyosin, I binds with actin, inhibits ATPase, C has high affinity to calcium ions.

• ATPase activity increases

where some parts such as biomechanics is mostly ignored.