Smooth Muscle and Cardiac Muscle Contraction

Vocabulary flashcards covering key physiological concepts, anatomical structures, action potential phases, and regulatory mechanisms of smooth and cardiac muscle contraction based on MD102 lecture material.

Dense bodies

Structures in smooth muscle cells attached to actin filaments that serve a function similar to Z disks in skeletal muscle.

Unitary smooth muscle

Also known as visceral or syncytial smooth muscle; a muscle type whose cells are electrically coupled via gap junctions, contain pacemaker cells, and contract together as a unit.

Multi-unit smooth muscle

Smooth muscle composed of distinct, separate fibers that operate independently with little to no electrical coupling and are controlled mainly by autonomic nerve innervation.

Phasic contraction

Intermittent or rhythmic smooth muscle contraction where cytosolic $Ca^{2+}$, cross-bridge phosphorylation, and force reach a peak and then return to baseline.

Tonic contraction

Continuous partial or full smooth muscle contraction where cytosolic $Ca^{2+}$, cross-bridge phosphorylation, and force decline after an initial peak but do not return to baseline.

Calmodulin (CaM)

The calcium-binding protein in smooth muscle that binds cytosolic $Ca^{2+}$ to activate myosin light chain kinase (MLCK).

Myosin light chain kinase (MLCK)

An enzyme activated by the $Ca^{2+}$-calmodulin complex that phosphorylates the myosin regulatory light chain to initiate cross-bridge cycling in smooth muscle.

Latch mechanism

A state in smooth muscle where a dephosphorylated myosin head remains attached to actin for a prolonged period, maintaining tension with very low energy expenditure.

Varicosities

Bulbous swellings along autonomic nerve axons that release neurotransmitters across smooth muscle fibers.

Rho-kinase pathway

A signaling pathway activated by GPCR agonists that phosphorylates and inactivates myosin phosphatase, thereby maintaining smooth muscle contraction.

Intercalated discs

Specialized cell junctions in cardiac muscle that contain gap junctions for rapid electrical coupling and desmosomes/fascia adherens for physical adhesion.

Functional syncytium

A tissue property of cardiac muscle where excitation of one cell spreads rapidly through gap junctions, allowing the interconnected cells to contract as a single unit.

Phase 0 (Cardiac Action Potential)

The rapid upstroke phase of cardiac action potentials caused by the opening of voltage-gated fast $Na^+$ channels and inward $Na^+$ current.

Phase 2 (Cardiac Action Potential)

The plateau phase of the cardiac action potential where inward $Ca^{2+}$ current through L-type $Ca^{2+}$ channels equals outward $K^+$ current.

Effective refractory period

The period from the start of an action potential until cardiac cells can generate a second action potential, lasting $0.25\,s$ to $0.30\,s$ in ventricular myocytes.

Calcium-Induced Calcium Release (CICR)

The process in cardiac muscle where extracellular $Ca^{2+}$ entry through L-type $Ca^{2+}$ channels triggers a larger release of $Ca^{2+}$ from the sarcoplasmic reticulum via ryanodine receptors (RyR2).

Frank-Starling mechanism

The intrinsic relationship where increased end-diastolic filling volume stretches cardiac myocytes to an optimal sarcomere length, resulting in increased force of contraction.

Laplace Law

The physical relationship stating that myocardial wall tension is directly proportional to intraventricular pressure and chamber radius ($T = P \times r$).

Cardiac efficiency

The ratio of mechanical work output to total chemical energy consumed by the heart, normally around $20\,\%$ to $25\,\%$.