EFFECT OF STIMULATION OF VAGUS AND WHITE CRESCENTIC LINE IN FROG’S HEART

 

EFFECT OF STIMULATION OF VAGUS AND WHITE CRESCENTIC

LINE IN FROG’S HEART

 

Aim: To demonstrate the effect of stimulation of vagus nerve and the white cresentic line in frogs heart.

 

Requires: All mechanical and electrical appliances

 

Electrical Connections: Connection are made for Foradic shock.

 

Speed of the Drum: Slow speed with pulleys-S4 D1 and gear slow.

 

Procedure: 1) Dissection: The heart of pitched frog is exposed and pericardium is removed on one side the bronchial plexus is identified the Muscles and other tissues below this plexus are cleaned the brachial nerve is then cut this expose a shiny tendon of muscle over this is the pterygoid muscle lying transversely. By everting this muscle gently, the vagus nerve can be seen underlying it. It can be identified by the fact that it crosses the shining tendon transversely and is underneath the pterygoid muscle accompanied by n artery and is crossed by the hypoglossal nerve, which hooks round the pterygoid muscle. The vagus nerve can also be separated from the pterygoid muscle carefully.

 

2) Recording: After the identification of the vagus nerve. It is stimulated by .Faradic shock with minimum strength and the stoppage of the heart is observed after this confirmatory test, the frog is kept on the myograph board and electrode is kept beneath the nerve during stimulation, the heart does not stop if the nerve is cut while doing dissection. In that Case the nerve of side must be identified.

 

                First, normal cardiogram is recorded. Then the vagus nerve is stimulated, the stimulation is continued even during the stoppage of the heart. Inspite of the continuous stimulation. The heart starts beating again which is called the vagal escape. Then the stimulus is stopped and cardiogram is recorded.

 

                Then the white crescentic line is stimulated and the effects are recorded.

 

Observation: When the vagus nerve is stimulated the heart stops immediately in diastole. When the stimulation is continued the heart escapes from the vagal inhibition and starts beating, and when the stimulation is stopped there is increase in force of contraction (Stair - case) which is followed by the normal cardiogram.

 

                The stimulation of white crescentic line also gives the same effect.

Inference: The vagus nerve carries parasympathetic fibres to the heart, and their stimulation liberates acetyicholine which inhibits toe activity of the heart and the causes for vagal escape are,

 

1. Exhaustion of acetylcholine in the nerve and also the destruction of acetyicholine by the enzyme cholinestrase.
2. The liberation of acetyicholine by the simultaneous stimulation of sympathetic fibres which also run along with vagal nerve fibres.
3. Initiation of indo ventricular rhythm.

 

                The above mentioned are causes or vagal escape in pithed frog and in unpithed frog the following also can cause vagal escape.

1. Bain - Bridge reflex and
2. Mareys reflex

 

                The stimulation of the white crescentic line also gives the’ same effect of vagal stimulation as the white crescentic line is the collection of post ganglionic neurones where the pre-ganglionic fibres of vagus end.