The cardiac muscles involved in a heartbeat are the specialized cells in conducting system and the contractile cells. The natural pacemaker is the sinoatrial (SA) node, which is comprised of heart cells that are the most active in generating electrical impulses. This impulse propagates throughtout the heart via the conducting system. The electrical signals can then be tracked down by an electrocardiogram (ECG or EKG).
The conducting system includes the SA node, atrioventricular (AV) node, and conducting cells. The conducting cells form the AV bundle, bundle branches, and Purkinje fibers, which distribute the stimulus from the AV node to the ventricular myocardium. The internodal pathway distributes the stimulus from the SA node to the AV node. The contractile cells, about 99% of the the muscle cells in the heart, shorten and contracts when they receive the electrical signals .
The SA node is affected by both sympathetic and parasympathetic stimulations. The vagas nerve brings parasympathetic stimulus and the sympathetic stimulus from the medulla oblongata. When the heart is overly stimulated, the result is tachycardia, which is the increase of heart rate. The opposite is a situation known as bradycardia, where the heart rate is slower than usual.
Direct Heart Stimulation
The heart will be stimulated with direct contact. This is done by placing a conducting wire on the heart to send an electrical signal. This signal is controlled to stimulate right at the beginning of ventricular contraction, near the peak of ventricular contraction, and done in two quick successions. The signal could also be sent as multiple stimulus, where it administer repeated stimuli at a rate of 20 per second to the heart.
Vagus Nerve Stimulation
This uses the electrode to stimulate the vagus nerve which is then connected to the heart. The vagus nerve stimulation can only be applied with multiple stimuli.
Effect of Chemicals
Certain hormones will be applied to the heart at a specified temperature, such as 23 °C. Epinephrine, pilocarpine, atropine, and digitalis are the chemicals experimented with. A few drops will be applied to the heart and then the heart rate will be observed.
When the direct heart stimulation was applied during the beginning of ventricular contraction, no changes were made. However, when the stimulus was applied during the peak of ventricular, a second ventricular systole occurred right after the first. During the multiple stimulus simulation, the heart rate was changed dramatically. Sometimes it would flat line after a few good heart beats or have a double systole right after the atrial contraction. When the double systole occurred, some had a diastole close to normal range, while in other cases, the ventricular diastole was cut short.
Table 1 – The effects of chemical substance added to the heart on the heart rate.
|Chemical Added |Heart Rate |
|Control |59 |
|Epinephrine |78 |
|Pilocarpine |44 |
|Atropine |69 |
|Digitalis |41 |
The control was at 23 °C without any chemical added. It was the value that all other chemicals were compared with.
During the direct heart stimulation, the stimulus applied right at the beginning of the ventricular contraction, no change was observed because at that moment, the heart muscles were under absolute refractory period. In this period, no matter how many or how strong the stimulus applied is, the cardiac muscle cannot contract again. However, during the peak of the ventricular contraction, it is the relative refractory period. This meant any stimulus applied here would cause another contraction of systole.
Refractory period on the multiple stimulus section was shown during the normal cycle and the double ventricular...