Na Hye Eunice Kim
MCB 32L (THURS-Keling Chen)
5 November 2009
LAB7: Frog Heart Study
Amphibian like frog has four chambers: the sinous venosus, two atria, and a
Unlike human heart, frog heart has only one ventricle, causing some mixing of
oxygenated and deoxygenated blood, but the structure of the ventricle and the presence of
a spiral valve in the arterial cone sufficiently separate the blood.
The heart is made up of
its muscle layer called myocardium.
These myocardial cells are joined to each other by
intercalated discs with desmosomes and gap junctions.
Desmosomes hold cells together
during heart contraction and swelling, and gap junctions allow the passage of molecules
and waves of depolarization to spread over the myocardium by using the ion channels.
The action potential produced by the pacemaker cells spreads to neighboring cells result in
contraction of muscle cells in the atria and in the ventricle.
The mechanism of propagation
of the action potential in the heart is similar to propagation along an axon.
difference is that the action potential spreads from one cell to another through gap
In addition, in cardiac muscle, calcium rises because of calcium coming in from
extracellular space as well as from the sarcoplasmic reticulum within the cell.
Depolization in cardiac muscle is longer than that of skeletal muscle, resulting in a longer
absolute and relative refractory period which stops myocardial tetanus from occurring.
Similar to the mammalian heart, the number of impulses that arises from pacemaker in the
sinus venosus is influenced by sympathetic and parasympathetic nerves, temperature,
chemical agents, and other factors.
For example, Through vagus nerve, the
parasympathetic releases acetylcholine which decrease heart rate.
On the other hand, the
sympathetic innervations release norepinephrine, which increases heart rate.
is more like norepinephrine, increasing the heart rate.
The purpose of this lab is to examine the characteristics of cardiac muscle by using many
Direct heart stimulation, vagus nerve stimulation, and the influence
of neurotransmitters, temperature, and ions on cardiac muscle are studied.
: In the trace of normal heartbeat, there will be two rises: smaller rise for atrial
contraction and larger rise for ventricular contraction.
When the heart is stimulated, extra
ventricular systole will result.
Warm ringers will speed up the heart while cold ringers will
slow down the heart.
Epinephrine will speed up the heart and acetylcholine will slow
down the heart.
An excessive potassium and calcium ions in the extracellular environment
reduces heart rate as well as the strength of contraction.