Physiological Properties Physiological Properties Of Heart MuscleOf Heart Muscle Frog Dissection Frog Dissection

BackgroundBackground

Frog’s myocardiogram gives the medical students a clear understanding of the normal sequence of atrial and ventricular contraction and the effect of temperature, sympathetic stimulation and parasympathetic stimulation on frog’s heart.

The Frog Heart And The Frog Heart And CirculationCirculation

The frog heart has 3 chambers: two atria ( Rt and Lt) and a single ventricle.

The Rt atrium receives deoxygenated blood from the blood vessels (veins) that drain the various organs of the body.

The Lt atrium receives oxygenated blood from the lungs and skin

Both atria empty into the single ventricle. While this might appear to waste the opportunity to keep oxygenated and deoxygenated bloods separate, the ventricle is divided into narrow chambers that reduce the mixing of the two blood

So when the ventricle contracts oxygenated blood from the left atrium is sent, relatively pure, into the carotid arteries taking blood to the head (and brain) Deoxygenated blood from the right atrium is sent, relatively pure, to the pulmocutaneous arteries taking blood to the skin and lungs where fresh oxygen can be picked up. Only the blood passing into the aortic arches has been thoroughly mixed, but even so it contains enough oxygen to supply the needs of the rest of the body

Frog HeartFrog Heart

Frog CirculationFrog Circulation

Physiology Of HeartPhysiology Of HeartThe heart's primary function is simply to

act as a pump that provides pressure to move blood to its ultimate destination.

Cardiac muscle differs from skeletal muscle both morphologically and functionally. It is able to initiate its own rhythmic contractions without requiring a stimulation from outside the heart. This is due to "leaky" cell membranes, in which calcium and sodium ions slowly leak into the cells. This leaking causes a slow depolarization to threshold, thus firing an action potential and initiating contractions of cardiac muscle

The cells that are most leaky to ions and that depolarize fastest , control the rate of contraction of all other cardiac cells; thus, they act as pacemakers for the rest of the heart. In the mammalian heart, the pacemaker is the sinoatrial (SA) node. In the frog heart, the pacemaker is the sinus venosus, an enlarged region between the vena cava and the right atrium, that receives blood from the anterior and posterior caval veins and empties blood into the right atrium.

Equipments Equipments FrogDissection setKymographFrog Ringer solutionsPinsDroppersSuture thread

Composition of frog ringer Composition of frog ringer solutionsolution Ringer’s solution, composed of:– NaCl: 200 ml – KCl: 20 ml– MgCl2: 20 ml – CaCl2: 4 ml – NaOH: 25.8 ml – Glucose: 1.8 g– pH: 7.4– Distilled water: to make 2 L– Total Volume: 2 L. 

Frog Preparation And Frog Preparation And DissectionDissection

Prepare the frog for dissection by PITHING the frog

Fix the frog on its back using the big needlesOpen the thorax of the frog with a central

incision and two flapsHold the pericardium with forceps and carefully

cut away the sac from the heart, using scissorsDo not cut or remove any organs other than the

skin and some of the ribs covering the heartKeep the heart ,moist during the procedure by

putting drops of ringer lactate by syringe

Cont…Cont…Attach a clip to frogs heart and

by thread connect it to your kymograph machine.

Now start your experiment.

A- Heart BeatA- Heart Beat Identify the part of the heart and examine its

beatingMeasure the normal heart rateUsing single stimuli of 100mV and 1-msec duration,

stimulate the ventricle at different times in the cardiac cycle, and note the following

What is the result of stimulating during the systolic phase of the cycle?

What is the result of stimulating during the diastolic phase of the cycle?

Can a second contraction be elicited before the normal rhythmic contraction occurs

B- Temperature effect B- Temperature effect Record the heart contractions at room

temperature. Drop warm (40oC) frog Ringer's solution

on the heart until significant changes are seen in rate and contractility.

Drop cold (5oC)) Ringer's on the heart and record when changes are observed. Then rinse the heart with room-temperature Ringer's to return the beat to normal before continuing the

Temperature is always a controlling factor for chemical reactions.the heart beat is a function of chemical processes. Ca2+ protein interaction brings about the contractile event and is a chemical reaction using ATP as the energy source to drive each contraction. A chemical reaction approximately doubles its rate every time you increase the temperature 10 degrees centigrade, and approximately halves its rate every time you decrease the temperature 10 degrees centigrade.

THE COOLER THE

SLOW

THE WARMER THE FAST

C- Drug effect C- Drug effect (NEUROTANSMITTERS)(NEUROTANSMITTERS)

Using a syringe apply the following drugs to the heart until you see significant changes in rate, contractility, or tone.

Acetylcholine 0.1 mg/ml in Normal Frog Ringer

Atropine 1 mg/ml in Normal Frog RingerEpinephrine 1mg/ml in Normal Frog RingerNote:Best results are seen if the drug is dropped on

the sinus venosus.After the effect is recorded, rinse the heart with

frog Ringer's and allow the heart to return to normal before the next drug is applied.

D- Propagation Heart D- Propagation Heart Block Block Using a heavy thread, tie a loose single-loop ligature

around the heart at the junction of the atria and ventricle.

Note the following type of heart block:• First-degree block. The interval between atrial

and ventricular contraction is prolonged. • Second-degree block. Some impulses fail to

reach the ventricle so that the ratio of atrial to ventricular beats is altered. You may see 2:1, 3:1, 5:1, 8:1 types of heart block.

• Third-degree block. Impulses fail to pass through the AV node and bundle of His and the ventricle may start its own independent rhythm of beating, or you may see only the atria contracting.

After producing a complete heart block (third degree), release the ligature , and see if a normal AV beat is restored.

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