**5. Physiological properties of heart muscle**

Heart muscle possesses certain special features. These include:


A summary of all these properties is given below:

i. Rhythmicity: It is the capacity of a cardiac tissue to generate its own impulses regularly. This property is also known as self-excitation or autorhythmicity. All of the tissues of the heart own this feature. However, in the human heart, there is an exclusive excitatory structure that produces rapid electrical impulses. This exclusive structure is termed as the "pacemaker of the heart". The sinoatrial node (SA node) serves the purpose of the pacemaker in the mammalian heart. From this node, the impulses propagate to other portions of the heart through a specific conductive system. Moreover, the AV node, the atria, and the ventricles of the heart are also capable of generating impulses and can perform as pacemakers. In spite of this, SA node is referred to be the pacemaker because of its high rate of impulse generation capacity compared to others.

The rhythmicity of different portions of a healthy human heart is shown in **Table 1**.

ii. Conductivity: The human heart possesses a unique conducting system, which is constructed by specialized cardiac muscle fibers known as internodal fibers. These fibers are responsible for the quick transmission of impulses from the SA node to other parts of the heart. The fundamental elements of the conductive system in the heart include—the AV node, Purkinje fibers, right and left bundle branches, and bundle of His. These conductive tissues are also referred to as the junctional tissues of the human heart. The conductivity of the heart muscle is maintained in the following way:


#### **Table 1.**

*Rhythmicity of different portions of a healthy human heart.*


**Table 2.**

*The velocities of impulses at various portions of the conductive system.*

At first, the AV node receives the impulses from the SA node through the internodal fibers. The AV node then sends these impulses to the ventricles via the bundle of His and its branches. The Purkinje fibers further carry these impulses from the top of the heart down to the base.

The velocities of impulses at various portions of the conductive system of the heart are given in **Table 2**.

iii. Contractility: Contractility of the heart muscle refers to its ability to shorten or contract in length in response to a stimulus. Myofibril is the core contractile unit of the heart muscle, which is made up of actin and myosin. These two units are linked together under the presence of ATP during contraction, which causes the fiber to be shortened. However, at the time of rest, these two units become dissociated as the ATP is resynthesized.

As it pertains to contractility, Starling's Law of the heart or Frank-Starling Law has been codified. This law states that the heart has the ability to modify its stroke volume and force of contraction in response to changes in venous return.


vi. The Staircase Phenomenon: When the heart muscle is excited with sequential maximal stimuli, the initial few contractions display a progressive increase in magnitude, which is referred to as a staircase. Following that, the strength of contraction stabilizes at its regular level.

Refractory Period: The refractory period of the heart is the duration of time during which a typical cardiac impulse cannot re-excite a section of cardiac muscle that has already been stimulated or excited. It is of two types: absolute refractory period and relative refractory period.

Absolute Refractory Period: In the absolute refractory period, the muscle does not respond at all, no matter how strong the stimulus is. The reason is that depolarization is taking place at the time. As a result, there cannot be a second depolarization.

Relative Refractory Period: During this period, the muscle responds if the power of stimulus is maximum. At this point, the muscle is in a repolarizing state.

The comparative refractory periods of different portions of the heart muscle are given in **Table 3**.

vii. Tone: The heart muscle of humans has tone. This tone is nerve-independent and adjustable. This allows the heart to keep reasonably constant tension on its different contents.
