Cardiac Action Potential Properties of Cardiac Muscles PDF

# Cardiac Action Potential Properties of Cardiac Muscles *Dr. Padma Rathore, Assistant Professor, Physiology* ## Learning Objectives * At the end of the lecture, the student will be able to define: * The properties of cardiac muscle * Frank Starling Law * The initiation of action pote...

# Cardiac Action Potential Properties of Cardiac Muscles *Dr. Padma Rathore, Assistant Professor, Physiology* ## Learning Objectives * At the end of the lecture, the student will be able to define: * The properties of cardiac muscle * Frank Starling Law * The initiation of action potential * Different phases of action potential ## Cardiovascular System - The heart consists of two separate pumps, right heart pumps the blood through the lungs and left heart pumps the blood to peripheral organs. - The two chambers: atria are the primer pump for the ventricle whereas the ventricles propel the blood in either pulmonary or peripheral circulation. ## Cardiac Muscle - It consists of contractile fibers (99%) which are atrial and ventricular muscles. - The specialized excitatory and conductive fibers necessary for the production of Action potential. ## Heart Function as Syncytium - When one cardiac cell undergoes an action potential, the electrical impulse spreads to all other cells that are joined by gap junctions so they become excited and contract as a single functional syncytium. - Atrial Syncytium and Ventricular Syncytium. ## Cardiac Muscles Act as a Syncytium Connected Through Gap Junctions and Desmosomes | Feature | Description | | ------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Desmosome | Provides greater strength between cells. | | Gap junction | For rapid communication between cells. | | Intercalated disc | Structure that connects cardiac muscle cells. Contains gap junctions and desmosomes. | | Plasma membrane | The outer layer of the cell. | | Gap Junction Channels | Allows electrical currents to flow between cells. | | Electrical Currents | The flow of electrical signals through gap junctions. | | Sarcomere | The basic unit of the muscle. | ## Properties of Cardiac Muscles 1. **Autorhythmicity:** The ability to initiate a heart beat continuously and regularly without external stimulation. 2. **Excitability:** The ability to respond to a stimulus. 3. **Conductivity:** The ability to conduct excitation through the cardiac tissue. 4. **Contractility:** The ability to contract in response to stimulation. ## Autorhythmicity - **Definition:** Ability of the cell to undergo depolarization spontaneously causing the production of electrical impulses at regular intervals (rhythmicity). - **Mechanism:** It is due to the specialized excitatory & conductive system of the heart. - **Functions:** 1. Act as a pacemaker (set the rhythm of electrical excitation) 2. Form the conductive system (network of specialized cardiac muscle fibers that provide a path for each cycle of cardiac excitation to progress through the heart) ### Mechanism of Auto Rhythmicity * **Autorhythmic cells do not have stable resting membrane potential (RMP)** * **Natural Leakiness to Na & Ca → spontaneous and gradual depolarization.** * **Unstable resting membrane potential (= pacemaker potential)** * **Gradual depolarization reaches threshold (-40 mv) → spontaneous AP generation.** ## Self excitation of SA Node * Because of high Sodium concentration in extracellular fluid and negative potential in resting sinus nodal fibers, the positive sodium ions tend to leak to the inside. * This inherent leakiness of sinus nodal fibers to sodium ions causes their self excitation. ## Resting Membrane Potential - In Sinoatrial Node: -55 to -60 mv - In Ventricular muscles: -85 to -90 mv - Threshold for excitation of SA node: -40 mv ## SA Node Action Potential - **RMP of SA node** : -55 to -60 mv in comparison of ventricular muscles' RMP (-85 to -95 mv). - **When memb.potential remains less negative than -60 mv**. The sodium channels closed. - **So only slow calcium-sodium channels become activated and cause AP.** - **Therefore the AP of SA node is slower to develop and also returns slowly to RMP as compared to ventricular muscle** ## SA Node (As Pacemaker) Action Potential - **Slow depolarization:** Pacemaker potential - **Fast Ca ²⁺ channels open.** - **Ca ²⁺ permeability** - **K⁺ permeability accompanied by slow Na ⁺ entry** - **Action potential** - **Threshold** ## Action Potential in Cardiac Muscle | Phase | Description | | ------- | ------------------------------------- | | Phase 0 | Rapid Depolarization | | Phase 1 | Rapid Repolarization | | Phase 2 | Plateau | | Phase 3 | Final Repolarization | | Phase 4 | Resting membrane potential | ## Phases of the Action Potential * Specific sequence of events * Phases 0 through 4 * **Phase 0:** Depolarization * **Phase 1:** Initial repolarization * **Phase 2:** Plateau * **Phase 3:** Rapid repolarization * **Phase 4:** Resting membrane potential ## Rate of Generation of AP at Different Sites of the Heart | Site | Rate (Times/min) | | ---------------------------------------- | ----------------- | | SA node | 100 | | AV node | 40 – 60 | | AV bundle, bundle branches, & Purkinje fibres | 20 – 35 | - **SA node acts as heart pacemaker because it has the fastest rate of generating action potential** ## Excitability and Conductivity - **Definition:** The ability of cardiac muscle to respond to a stimulus of adequate strength & duration by generating an action potential (AP). - **AP initiated by SA node → travels along conductive pathway → excites atrial & ventricular muscle fibers** ## Conductivity - **Conductivity is the ability to propagate an impulse** - **Normally impulses are conducted in one direction** - **The velocity of conduction of action potential in atrial and ventricular muscle fibers**: 0.3 to 0.5 m/sec. This is about 1/250 the velocity in large nerve fibers; 1/10 the velocity in skeletal muscle fibers. - **The velocity of conduction in specialized Purkinje fibers:** 0.02 to 4 m/sec. ## Contractility - **Cardiac muscle contracts in response to the electrical impulse generated by the SA node**. - **Cardiac muscle begins to contract a few millisecond after the action potential begins and continues to contract until a few msec. after the AP ends.** - Values: - 0.2 sec in atrial muscle. - 0.3 sec in ventricular muscle. ## Refractory Period of Cardiac Muscle - **Cardiac muscle is refractory to restimulation during action potential**. - **It is the interval of time during which a normal cardiac impulse cannot reexcite an already excited area of cardiac muscle**. - **The normal refractory period of ventricles:** 0.25 to 0.30 sec. - **The refractory period of atrial muscles:** 0.15 sec ## Frank Starling Law - **The intrinsic ability of the heart to adapt to increasing volumes of inflowing blood is called the Frank Starling Mechanism of heart**. - **It means that the greater the heart muscle is stretched during filling the greater is the force of contraction and the greater the quantity of blood pumped into the aorta**. - **Within physiological limits, the heart pumps all the blood that returns to it by way of the veins.** ## Excitation Contraction Coupling - **It is the mechanism by which the action potential causes the myofibrils of muscle to contract.** - **The contraction is caused by the release of calcium ions from the sarcoplasmic reticulum and T-tubules.** - **Sarcoplasmic reticulum is much less developed in cardiac muscles as compared to skeletal muscles so the source of calcium in cardiac muscles is from extracellular calcium which then enters the t-tubules.**