Contractility
Contractility is ability of the tissue to shorten in length (contraction) after receiving a stimulus. Various factors affect the contractile properties of the cardiac muscle. Following are the contractile properties:
All-Or-None-Law:
According to all-or-none law, when a stimulus is applied, whatever may be the strength, the whole cardiac muscle gives maximum response or it does not give any response at all. Below the threshold level, i.e. if the strength of stimulus is not adequate, the muscle does not give response.
All-or-none law is demonstrated in the quiescent (quiet) heart of frog. Heart is made quiescent by applying the first Stannius ligature in between the sinus venosus and right auricle. Ventricle is stimulated by placing the electrode at the base of ventricle.
First, one stimulus is with a minimum strength of 1 volt at the base of ventricle and the contraction is recorded, Then, after 20 seconds, the strength of stimulus is increased to 2 volt and the stimulus is applied.
The curve is recorded. The procedure is repeated by increasing the strength every time and applying the stimulus with an interval of 20 seconds. Amplitude of all contractions remains the same irrespective of increasing the strength of stimulus. This shows that cardiac muscle obeys all-or-none law.
Cause For All-Or-None Jaw:
All-or-none law is applicable to whole cardiac muscle, It is because of syncytial arrangement of cardiac muscle. In the case of skeletal muscle, all-or-none law is applicable only to a single muscle fiber.
Staircase Phenomenon:
When the ventricle of a quiescent heart of frog is stimulated at a short interval of 2 seconds, without changing the strength, the force of contraction increases gradually for the first few contractions and then it remains same. Gradual increase in the force of contraction is called staircase phenomenon.
Cause For Staircase Phenomenon:
Staircase phenomenon occurs because of beneficial effect, which facilitates the force of successive contraction. So, there is a gradual increase in force of contraction.
Summation Of Subliminal Stimuli:
When a stimulus with a subliminal strength is applied, the quiescent heart does not show any response. When few stimuli with same subliminal strength are applied in succession, the heart shows response by contraction, due to the summation of stimuli.
Refractory Period:
Refractory period is the period in which the muscle does not show any response to a stimulus, It is of two types:
- Absolute refractory period
- Relative refractory period
1- Absolute Refractory Period:
Absolute refractory period is the period during which the muscle does not show any response at all, whatever may be the strength of the stimulus. It is because, the depolarization occurs during this period. So, a second depolarization is not possible.
2- Relative Refractory Period:
Relative refractory period is the period during which the muscle shows response if the strength of stimulus is increased to maximum. It is the stage at which the muscle is in repolarizing state.
Refractory Period in Skeletal Muscle:
In skeletal muscle, the refractory period is short.’ Absolute refractory period extends during the first half of latent period, measuring about 0.005 sec. Relative refractory period extends during the second half of latent period measuring 0.005 sec. So, the total refractory period is 0.01 sec.
Refractory Period in Cardiac Muscle:
Cardiac muscle has a long refractory period compared to skeletal muscle. Absolute refractory period extends throughout the contraction period of cardiac muscle and its duration is 0.27 sec. Relative refractory period extends during first half of relaxation period. which is about 0.26 sec. So. the total refractory period is 0.53 sec.
Significance of Long Refractory Period In Cardiac Muscle:
Long refractory period in cardiac muscle has three advantages:
- Summation of contractions does not occur.
- Fatigue does not occur.
- Tetanus does not occur.
Demonstration of Refractory Period in Heart:
Refractory period is demonstrated in the heart of a pithed frog. Refractory period can be recorded in beating heart as well as the quiescent heart.
Refractory Period In Beating Heart:
First, normal cardiogram is recorded with the heart of a pithed frog. The impulses for heartbeat arise from the sinus venosus. An electrical (external) stimulus is applied by keeping the electrode at the base of the ventricle. When the stimulus is applied during systole, the heart 0 does not show any response. It is because the abs01ute refractory period extends throughout systole. When a stimulus is applied during diastole, the heart contracts because, diastole is the relative refractory period. This contraction of the heart is called extrasystole or premature contraction. Extrasystole is followed by the stoppage of heart in diastole for a while. This diastole is longer than the diastole after regular contraction. Temporary stoppage of the heart before it starts contracting is called compensatory pause. Duration of extrasystole and compensatory pause is equivalent to the duration of two cardiac cycles.
Cause For Compensatory Pause:
A natural impulse from sinus venosus arrives at the time of contraction period of extrasystole. As this period is absolute refractory period, the natural impulse cannot cause contraction of heart. And the heart has to wait for the arrival of next natural impulse from sinus venosus. Till the arrival of next impulse, the heart stops in diastole.
Refractory Period In Quiescent Heart:
Frog’s heart is made quiescent by applying the first Stannius ligature. Electrode is placed over the base of ventricle. When two stimuli are applied successively in such a way that the second stimulus falls during contraction period, the heart contracts only once. It is because of the first stimulus. There is no response to second stimulus because systole is the absolute refractory period. However, when a second stimulus is applied during diastole, the heart contracts again and second contraction superimposes over the first one. This shows that the relative refractory period extends during diastole.