Properties of myocardium of animals
Rhythmicity
The cardiac muscles have an inherent ability to generate their own impulses spontaneously at regular intervals known as rhythmicity without the influence of nerves and hormones,. This property can be demonstrated by coronary perfusion of the isolated mammalian heart using Ringer’s solution.
Contractility
Contraction of cardiac muscle differs from other contractile tissues in the following aspects:
All or none principle
If at all the cardiac muscle responds to a stimulation, it contracts with maximum strength (i.e. all contractions of the cardiac muscle are maximum) when stimulated with either minimum or maximum strength of stimulus under the prevailing physical and chemical environment. This is due to the syncytial nature of the cardiac muscle.
Staircase phenomenon or Treppe
When an effective stimulus is applied repeatedly with very short interval (1-2 seconds), the cardiac muscle responds with increasing degree of strength for the first 3 to 5 stimuli, after that the response becomes constant. When the interval between two stimuli of same strength is very short, the physico chemical changes known as beneficial effects, occurring during the first response persist and facilitate greater response for the second stimuli.
Refractory period
This period of unresponsiveness of cardiac muscle when it is responding to a stimulus, it may not show another response for a subsequent stimulation for a shorter period. Following first stimulation, there is a total absence of response for a very brief period, called as absolute refractory period. This is followed by a short period of partial responsiveness referred as relative refractory period, during which the cardiac muscle can be stimulated by a stronger stimuli.
The cardiac muscle is in absolute refractory period (atria 0.15 sec., ventricle 0.25 sec.) during systole and in the relative refractory period (atria 0.03 sec, ventricles 0.05 sec) during early diastole.
The refractory period is longer in cardiac muscle than skeletal muscles, hence cardiac muscle can not be tetanized by a series of rapid stimuli.