Extracellular calcium

The cardiac muscle cells of the heart (called the myocardium) are tightly bound together in layers and encircle the blood filled chambers of the heart. When the heart contracts the myocardium encloses on the blood filled chambers and blood is propelled around the body. With greater filling of blood (i.e. increased end diastolic volume; EDV) the force of contraction of the heart is subsequently greater. Experiments in the late 19th century, using frog hearts, demonstrated this ability of the heart, known as the Frank-Starling Law of the heart, named after Otto Frank and Ernest Starling.

The heart requires a number of important resources to carry out its role. Calcium, oxygen and adrenaline are all required for normal functioning and any changes in their levels can affect the force of contraction.

This simulation examines factors that control the Contraction of Cardiac Muscle. The "data" you will collect is "real" data, taken from recordings made of the test conditions you will see in this simulation, done in previous years. This practical uses isolated strips of toad ventricular muscle that are electrically stimulated to contract. The force of cardiac muscle contraction is recorded (with grams of tension of the Y axis and time in seconds on the X axis of the recording).

Watch the video below before commencing with the simulation:

Calcium is essential for the normal functioning of the heart. During the cardiac action potential (i.e. excitation of the heart), calcium enters cardiac muscle cells from the extracellular fluid, and is also released from internal stores in the sarcoplasmic reticulum (i.e. calcium-induced calcium release). In this simulation you will test the effect of changing the extracellular calcium concentrations on the force of contraction (tension) produced.

Please note that if all of the calcium was removed from the extracellular fluids, there would be no contractions.

Instructions:

1. Press the Start button and observe your recording.
2. Using the dropdown box select Calcium free solution and click the Change calcium button and observe your recording. What happens to the force of contraction? Why does this happen?
3. In order to increase the extracellular calcium concentration, using the dropdown box select:
   (1) 0.2mm calcium
   (2) 0.8mm calcium
   (3) 2.0mm calcium
   
4. Observe what happens to the force of contraction. Why does this happen?