ALBIO5: MECHANISM OF HEART EXCITATION AND CONTRACTION

The eating of the heart is myogenic meaning that it is initiated from within heart muscles, itself and not as a result of nervous imputes, teaching them.

(Myogenic property is used in heart transport sungery) whereas it is initiated by nerves as in insects it is solid be neuragenic.

The initial stimulus for a heart beat originates in a group of hitologically different muscle cells known as the Sino Artial Node

• (SAN). This is located in the wall of right atrium near where the vena covae enter it.
• The SAN determines the basic rate of heartbeat and is therefore known as the PACE MAKER. In humans, the basic rate is 70 beats/minute.
• A wave of excitation spreads out from the SAN across to both atria causing them to contract more or less at the same time.
• When the wave reaches junction between atria and ventricles, it excites another specialized group of cardiac muscle fibres called the Atrio-ventricular- Node (AVN).
• Find then along the Purkinje fibres (tissue) that continuous with the AVN (NB Purkinje tissue is recently known as purkyne tissue).
• The Purkinje fibre are collectively known as Bundle of his. It moves along the septum bond and is finally spread through the walls of the ventricles.

Summary of path taken by electrical excitation during the cardiac cycle.

SAN                              ATRIA      AVN          Purkinje              Ventricles

(PACEMAKER)                                                tissue

(Bundle of It is)

HEART SHOWING THE SPREAD OF ELECTRICAL EXCITATION THAT ACCOMPANIES CONTRACTION.

INNERVAITON OF THE HEART

• The pacemaker (SPN) receives two nerves; a branch of the sympathetic nervous system and a branch of the vagus nerve.
• These do not initiate the beating of the heart but can modify the activity of the pacemaker by speeding up or slowing down the rate at which the heart beats.
• If the sympathetic nerve is stimulated, the heart speeds up, if the vagus nerve is stimulated, the heart slows down.
• The antagonism between the two nerves makes the animals’ transport system adaptive to its nerves.

TYPES OF CIRCULATION SYSTEMS

• All circulating systems share the following basic characteristics

1. Posses a circulatory fluid, e.g. blood, lymph.

1. Possession of a pumping device; (Heart) which creates a pressure difference which forces blood to flow.
2. Blood vessels. The blood must be at least partly constrained in tubes in order to carry out towards the tissues and then back to the heart.

• Are needed to ensure that the blood flow in the correct direction.

The different types of circulatory system may fall under the following:-

1. OPEN AND CLOSED CIRCULATORY SYSTEMS
2. CLOSED CIRCULATORY SYSTEMS.

Closed circulations are where the blood flows within vessels or sinuses and does not come into contact with the tissue directly e.g. Blood flow in annelids e.g. Earthworm.

– Mammals                             – Fish

– Amphibians

The Earth worm has 5 pseudo heart located in segments 7-11

OPEN CIRCULATION SYSTEMS

Is one in which blood does not move within vessels but through the body cavity called harmocoel and moves freely over the tissues e.g. arthropods e.g. insects.

SINGLE AND DOUBLE CIRCULATION

If the heart of a mammal is compared to that of the fish and frogs, the fish has the simplest.

• In fish, deoxygenated blood flows from the body to the heart then gills and vein and to various parts of the body and returns to the heart; which has only one atrium and one ventricle (i.e. two chambers).
• As blood flows only once through the heart for every complete circulation of the body. It is called single circulation.
• The disadvantage of this system is that the resistance created by the blood capillaries in the gills followed by those of the body causes its pressure to fall at these points respectively, resulting in the blood being sluggish especially at venous ends.

Solutions to the problem.

1. In fishes, it has been overcome by replacing veins with large sinuses that offer minimum resistance
2. In mammals, the problem has been overcome by development of a double circulation. Here the blood passes twice through the heart. It is free pumped to the lungs (respiratory surface) it returns to the heart and is then repumped to the body tissues.

• This helps to sustain a high blood pressure and thus allows more rapid circulation.
• In mammals and birds, the complete separation into 2 halves allows deoxygenated blood 2 to be kept separate. This improves rate of those endothermic animals.

FISH’S HEART SHOWING SINGLE CIRCULATION

Difference between the two should follow the above.

Differences ahead, read on

No need to leave this space.

CIRCULAITON IN THE FROG

• The frog provides an intermediate condition. It has double circulation. It has a three chambered heart with two atria and one ventricle. It has canus arteriosus into which blood is pumped before entering the vessels leading to the lungs of the body.

Various foldings into the wall of the ventricle aided by spiral value in the conus anterious may play an important role in separating oxygenated blood from mixing deoxygenated blood.

1. The pressure can also be overcome by possessing two separate hearts one for pumping blood to the body and the other for pumping blood to the lungs (respiratory organs) as in the octopus.

THE AMPHIBIAN WITH DOUBLE CIRCULATION

DOUBLE CIRCULATION OF BLOOD TO ALL BODY PARTS IN MAMMAL.

THE OPEN CIRCULATION IN ARTHROPODS e.g. in insects.

Why an open system but not a closed system exists in insects.

The reason lies in the way arthropods develop. In most animals the embryological cavity called coelom that eventuall becomes the main body cavity expands at the expense of the cavity calle blastocoel which eventually forms blood vessels. In arthropods, the revense takes place.

The coelomic cavities remain small and the blastocoel becomes the main cavity. Insect; arthropods rows faster developing open circulating system.

STRUCTURE OF AN INSECTS’ OPEN CIRCULATION SYSTEM.