What happens when summation occurs in cardiac muscle?

Summation in cardiac muscle refers to the phenomenon where successive contractions become stronger (increased force of contraction) due to the buildup of calcium ions within the cardiomyocytes. Summation is influenced by several factors, including heart rate, duration of the refractory period, and calcium handling capabilities of the cardiac tissue. Here's an explanation of what happens during summation in cardiac muscle:

1. Initial Contraction:

When an action potential reaches the cardiac muscle cells, it triggers the release of calcium ions from the sarcoplasmic reticulum. Calcium ions bind to the troponin complex, which initiates the sliding of actin and myosin filaments, leading to muscle contraction.

2. Relaxation:

After the initial contraction, calcium ions are actively pumped back into the sarcoplasmic reticulum through Ca2+-ATPase pumps, and the muscle relaxes.

3. Incomplete Relaxation:

At higher heart rates, the relaxation phase may be incomplete due to the reduced duration of the refractory period. This means that subsequent action potentials arrive before the complete relaxation of the muscle.

4. Accumulation of Calcium Ions:

As action potentials occur in rapid succession, calcium ions released from the sarcoplasmic reticulum accumulate within the cytoplasm due to the incomplete relaxation of the muscle. This phenomenon is known as "calcium accumulation" or "calcium staircase."

5. Increased Force of Contraction:

With the accumulation of intracellular calcium, more actin and myosin filaments are activated, leading to an increased number of cross-bridge formations. This results in a more forceful contraction compared to the previous one.

6. Limitation by Refractory Period:

At extremely high heart rates, the refractory period may prevent successful propagation of action potentials, limiting the occurrence of summation. Additionally, reduced time for calcium reuptake can eventually lead to muscle fatigue and dysfunction.

In summary, summation in cardiac muscle occurs when successive contractions become stronger due to the progressive accumulation of calcium ions within the cardiomyocytes. This phenomenon is more pronounced at higher heart rates and can significantly affect the overall contractile performance of the heart. Proper regulation of calcium handling and coordination of heart rate are crucial for the heart to function efficiently.