86. Properties of Cardiac Muscle – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Cardiac muscle possesses specialized physiological properties that enable the heart to pump blood continuously throughout life. These include automaticity, rhythmicity, conductivity, excitability, contractility, distensibility, a long refractory period, functional syncytium, all-or-none response, staircase phenomenon, length–tension relationship, frequency–force relationship, and load–velocity relationship. These properties ensure efficient cardiac performance and prevent tetanic contraction.
• Automaticity is the ability of the heart to generate impulses without external nerve stimulation. The sinoatrial (SA) node is the normal pacemaker because it has the highest intrinsic discharge rate (60–100 beats/min). Other pacemakers include the atrioventricular (AV) node (40–60 beats/min), His–Purkinje system (25–40 beats/min), and ventricular muscle (15–30 beats/min). This arrangement forms the hierarchy of pacemakers.
• Rhythmicity ensures regular generation of impulses, while conductivity permits rapid spread of electrical impulses throughout the heart. Excitability is the ability to respond to an adequate stimulus, contractility is the ability to develop force, and distensibility allows cardiac chambers to stretch during diastolic filling.
• The long refractory period (absolute refractory period about 200 ms, relative refractory period about 50 ms) overlaps with ventricular contraction and prevents tetanic contraction. This allows complete ventricular relaxation and adequate filling during diastole, which is essential for survival.
• Cardiac muscle behaves as a functional syncytium because adjacent cells are connected by gap junctions in intercalated discs, allowing rapid spread of electrical impulses and synchronized contraction.
• An extrasystole is a premature heartbeat caused by an ectopic impulse, usually during the relative refractory period. It is followed by a compensatory pause, which increases ventricular filling and produces a stronger subsequent contraction (postextrasystolic potentiation) due to increased preload and intracellular calcium.
• According to the all-or-none law, cardiac muscle gives a maximal response once threshold is reached. Stronger stimuli do not increase contraction under identical physiological conditions because the myocardium acts as a functional syncytium.
• The staircase (Treppe) phenomenon refers to the gradual increase in the strength of the first few contractions after a period of rest. It occurs mainly because of progressive accumulation of intracellular calcium and improved calcium handling by the sarcoplasmic reticulum.
• The length–tension relationship states that increasing myocardial fiber length within physiological limits increases the force of contraction. This forms the basis of the Frank–Starling mechanism and is an example of heterometric autoregulation. The frequency–force relationship (Bowditch phenomenon) states that moderate increases in heart rate increase contractile force by increasing intracellular calcium. The load–velocity relationship states that the velocity of myocardial shortening decreases as afterload increases.