S1B4 - Cardiac Muscle Flashcards
What do you need to know about the SA node?
Sinoatrial Node (SA Node)
- Located near the base of the right atrium
- Primary pacemaker (automaticity) for cardiac contraction under normal conditions
- Controls heart rate
- Modulated by both the sympathetic and parasympathetic nervous system
- Increasing sympathetic activity increases heart rate
- Increasing parasympathetic activity decreases heart rate
What do you need to know about the AV node?
Atrioventricular Node (AV Node)
- Located at the junction of the right atrial septum with the interventricular septum
- Can act as a pacemaker in pathological conditions (e.g., SA nodal failure)
What do you need to know about the Bundle of His and Purkinje Cell System?
Bundle of His and Purkinje Cell System
- Bundle of His splits into two branches, one to each ventricle
- Purkinje fibers rapidly conduct electrical depolarization cell-to-cell through low resistance junctions throughout both ventricles
- Purkinje cells are also potential pacemakers under certain conditions
What do you need to know about atrial myocytes?
Atrial Myocytes
- Smaller than ventricular myocytes
- Contract before ventricular myocytes to permit final pumping of blood from atria into ventricles
What do you need to know about ventricular myocytes?
Ventricular Myocytes
- The myofibrils run longitudinally and contain well-organized sarcomeres which gives the muscle a “striated” appearance.
- Each muscle fiber contains a single central nucleus (in contrast to skeletal muscle which has many peripheral nuclei).
- Ventricular muscle fibers are interconnected in a branching array and joined by specialized junctions known as intercalated disks. The disks contain gap juntions (connexons) that are low resistance paths between cells, and desmosomes (macula adherens) which are structural attachments.
- Invaginations of the sarcolemma form T-tubules similar to those in skeletal muscle. However, the T-tubules in cardiac muscle are larger and are located near the Z lines of the sarcomere (T-tubules in skeletal muscle are located near the A band). T-tubules permit the rapid penetration of the depolarization into the muscle fiber, which facilitates coordinated contraction and entry of extracelluar Ca2+.
- Ventricular myocytes possess a lot of mitochondria (about 23% by volume, compare to less than 10% in skeletal muscle) which is consistent with the high metabolic rate.
- The sarcoplasmic reticulum is well-developed and acts as storage for calcium ions.
What are some key features of cardiac muscle action potentials?
- In cardiac muscle, even with T-tubules, the action potential is much longer (e.g., 100-200 ms) such that the duration of the action potential and the duration of the contraction are similar.
- In cardiac muscle, the height and duration of the action potential triggers both the magnitude and the duration of the contraction.
- The length of the refractory period (i.e., the time during which another depolarization can be elicited) and the contraction are similar, thus tetanic contractions cannot be obtained in cardiac muscle. (Is this a good thing? If so, why?)
What modulates the contractility of cardiac myocytes?
Contractility is modulated by the concentration of intracellular free Ca2+.
What are two important proteins associated with cardicac myocyte sarcoplasmic reticulum?
Two important proteins are associated with the SR.
- Phospholamban is a protein in the SR membrane and is the major regulator of the SR calcium pumps sequestering calcium
- Calsequestrin is a protein within the terminal cisternae of the SR that binds Ca2+ and further increases the capacity of the SR to concentrate and store Ca2+.
What are the catecholamin (i.e., sympathetic stimulation) effects of excitation and relaxation of cardiac muscle?
Catecholamine (i.e., sympathetic stimulation) effects on excitation and relaxation:
- Ca2+ movement into the cell is increased by phosphorylation of calcium channels by a cyclic AMP dependent protein kinase. The additional calcium increases developed force (contractility).
- The phosphorylation of phospholamban enhances Ca2+ uptake by the sarcoplasmic reticulum. This causes an increased rate of relaxation. The increased calcium sequestration causes increased Ca2+ to be released during a subsequent contraction thereby increasing force (contractility).
- The rate of relaxation is further enhanced by phosphorylating Troponin I which inhibits the Ca2+ binding to Troponin C. This shortens the duration of the contraction and leads to increased heart rate.
What do you need to know about preload in the heart?
Preload is the load on (or length of) the muscle at rest before the onset of a contraction. In the intact heart, preload is the volume of blood in the ventricle just before contraction (i.e., the end-diastolic volume, EDV). EDV is inferred by the pressure within the ventricle at the end of diastolic filling (i.e., the end-diastolic pressure, EDP) and is related and ventricular stiffness.
What do you need to know about afterload in the heart?
Afterload is the load that the muscle is contracting against after the onset of a contraction. In the heart, afterload represents the resistance to the ejection of blood (e.g., the diastolic blood pressure).
