Cardiac muscle length

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:

The length of cardiac muscle in a resting individual is not at its optimal length for contraction. Greater cardiac muscle length, such as occurs with greater ventricular filling, will result in additional stretching of the cardiac muscle, increasing the muscle length and increasing the force of contraction.

Instructions:

1. Press the Start button and observe the recording. Allow to record for around 100 seconds.
2. Click the Change length button to change the muscle length to 9 mm and observe your recording. What happens to the force of contraction? Why does this happen?
3. Use the cursor to measure the initial force of contraction and the change in force following the increase in muscle length. Put these values into the table underneath the trace.

Full instructions can be found on the previous tab. In short:

1. Initial muscle length when pressing the start button is 8mm.
2. After 100 seconds click the change length button to increase the muscle length to 9mm.
3. Use the cursor to find the values in the graph necessary to fill out the table below.

Complete the below table based on data from the above simulation.
Make sure you collect data from a point after the contractions have stabilised.

Length	Passive tension (between contractions)	Maximum tension	Active tension (maximum − passive)
8 mm
9 mm