Cardiac Muscle in Animals - April 24, 2010 by admin

Although even in the adult animals the fundamental modifications in the basic rate of the heart beat are brought about by the brain, the heart can dispense with these com­mands and set its rhythm independently. Figuratively speaking, our heart works on its own initiative, a peculiarity which we somehow do not appreciate. If the fibres of an embryonic cardiac muscle are grown in a tissue culture on a special nutrient medium, they will contract rhythmically in a vial too, without waiting for any orders. They just cannot live without contracting.

Nonetheless, work cannot be well co-ordinated without a headquarters. If every muscle fibre contracted of its own accord, the common contraction could take place only by pure chance. This is what really happens at the earliest stages of embryonic life. In the rat’s embryo individual sections of the heart contract quite independently until the headquarters is set up and starts to operate. In birds and mammals it is located in a special region of the heart known as sino-auricular node.

The cardiac muscle has no nerves, and commands are conducted over the muscle fibres at the rate of one metre per second. This rate is quite adequate for the auricles to contract normally. The ventricles of the heart, which are larger than the auricles and which require commands to be communi­cated more rapidly, have a system, known as Purkinje fibres, over which excitation spreads five or six times more quickly.

In the heart of every self-respecting animal there is only one headquarters known as the pacemaker. More pace­makers would certainly cause a mess. Strange things, however, are not uncommon. The ascidians and some tunicates have two pacemakers, one at each end of the pulsating vessel. In such animals the blood flow periodically changes its direction.