PH - WEEK 6 - Cardiac Muscle Physiology

This set of flashcards covers cardiac muscle function, characteristics, contraction mechanisms, and action potentials.

What is the function of cardiac muscle?

To move blood through the circulatory system by contracting and pumping blood.

What are the characteristics of cardiac muscle?

Spontaneous contraction (pacemaker activity). Cells connected by gap junctions and desmosomes (intercalated discs). Modulated by the sympathetic and parasympathetic nervous systems and hormones.

What are intercalated discs?

Specialized structures connecting cardiac myocytes, containing desmosomes (hold cells together) and gap junctions (facilitate ionic current flow between adjacent cells, aiding in the propagation of action potentials for synchronized contraction).

How does cardiac myocyte contraction differ from skeletal muscle contraction?

Skeletal Myocytes: Contraction initiated by somatic motor neuron innervation, with Ca2+ release from the SR. Cardiac Myocytes: Contraction initiated by depolarization from autorhythmic cells or adjacent myocytes, with Ca2+ influx from ECF triggering further release from the SR (Ca2+-induced Ca2+ release). No neural input is required.

What is the resting membrane potential of a cardiac myocyte?

The resting membrane potential of a cardiac myocyte is -90 mV, with the inside of the cell being negatively charged relative to the outside.

What is the role of ion channels in cardiomyocytes?

Leak Channels: Always open, allowing ions to flow down their concentration gradients. Voltage-gated Channels: Open when the membrane potential changes, allowing ion movement in response to electrical signals. Ligand-gated Channels: Open when a specific chemical binds to them. Na+/K+ Pump: Maintains the resting membrane potential.

How is the resting membrane potential created in a cardiac myocyte?

K+ leak channels, which allow K+ to exit the cell. Negatively charged proteins that can't leave the cell. Na+ leak channels bringing Na+ into the cell, balanced by the Na+/K+ pump.

What is Excitation-Contraction (E-C) Coupling in cardiac myocytes?

Depolarization from adjacent cells via intercalated discs. Action potential propagation along the sarcolemma into T-tubules, with Ca2+ influx through L-type (voltage-gated) channels.

How is calcium involved in cardiac myocyte contraction?

Ca2+ influx through voltage-gated channels induces further release of Ca2+ from the SR via Ryanodine receptors (Ca2+-induced Ca2+ release). Ca2+ binds to troponin, displacing tropomyosin, which exposes myosin binding sites on actin.

What is the mechanism of contraction in cardiac myocytes?

Contraction occurs when ATP allows actin-myosin crossbridges to form, causing the filaments to slide past each other, resulting in contraction.

How is contraction terminated in cardiac myocytes?

Closure of Ca2+ channels. Active transport pumps return Ca2+ to the SR and extrude it out of the cell via the Ca2+-Na+ exchanger. The absence of Ca2+ leads to re-blocking of myosin binding sites and muscle relaxation.

What are action potentials in excitable cells?

Rapid sequences of events where the membrane potential rapidly reverses and then returns to its resting state, propagating electrical signals through the heart.

What are the phases of the cardiac action potential?

Depolarization: Rapid Na+ influx. Plateau phase: Ca2+ influx and K+ efflux; ensures extended contraction. Repolarization: Ca2+ channels close and K+ continues to exit the cell.

What is the refractory period in cardiac muscle?

The time during which a second action potential cannot be triggered, preventing tetanic contractions and allowing for relaxation between contractions.

Why does tetanus not occur in cardiac muscle?

Tetanus does not occur in cardiac muscle because of the extended refractory period, which prevents sustained contractions.