[TRANS] Human Physiology: Smooth Muscle and Cardiac Muscle Contraction

Vocabulary flashcards covering structural features, contraction and relaxation cascades, action potential dynamics, and metabolic characteristics of smooth and cardiac muscle tissue.

Dense Body

A smooth muscle structure that anchors actin filaments and intermediate filaments, serving as the functional equivalent of the Z-disk in skeletal muscle.

Intermediate Filament

A cytoskeletal element in smooth muscle that connects adjacent dense bodies and transmits mechanical contractile force throughout the cell.

Multi-Unit Smooth Muscle

A smooth muscle subtype composed of distinct individual fibers that operate independently as separate motor units, possessing little to no electrical coupling, few gap junctions, and dense autonomic innervation (e.g., iris, ciliary muscle of the lens, vas deferens).

Unitary Smooth Muscle

Also called single-unit, syncytial, or visceral smooth muscle; a subtype where cells are connected by numerous gap junctions and behave together as a functional electrically-coupled syncytium (e.g., GI tract, uterus, ureters, bladder).

Phasic Smooth Muscle

Smooth muscle that displays rhythmic or intermittent contraction-relaxation cycles driven by action potentials to promote periodic movement of contents, such as peristalsis in the gastrointestinal tract.

Tonic Smooth Muscle

Smooth muscle that maintains continuous partial or full contraction (tone) driven by graded potentials and regulatory signaling mechanisms (e.g., sphincters, vascular and respiratory smooth muscle).

Calmodulin (CaM)

An intracellular regulatory protein in smooth muscle that reversibly binds $\text{Ca}^{2+}$ to form a complex that activates myosin light chain kinase (MLCK), substituting for the function of troponin.

Myosin Light Chain Kinase (MLCK)

An enzyme activated by the $\text{Ca}^{2+}\text{--calmodulin}$ complex that uses ATP to phosphorylate the regulatory light chain of myosin, increasing myosin ATPase activity and triggering cross-bridge cycling.

Myosin Phosphatase (MLCP)

A cytosolic enzyme that dephosphorylates the regulatory light chains of myosin in smooth muscle, leading to decreased cross-bridge cycling and muscle relaxation.

Latch Mechanism

A state in smooth muscle where dephosphorylated myosin heads remain attached to actin filaments for an extended period, allowing maintenance of high contractile force and muscle tone with very low ATP consumption.

Rho-associated Protein Kinase (ROK)

A kinase that phosphorylates and inactivates myosin phosphatase (and activates CPI-17), thereby preventing dephosphorylation of myosin light chains and increasing the $\text{Ca}^{2+}$ sensitivity of smooth muscle contraction.

Intercalated Discs

Microscopic structures connecting cardiac muscle cells end-to-end, containing desmosomes and fascia adherens for mechanical anchoring alongside gap junctions for rapid electrical ion diffusion.

Calcium-Induced Calcium Release (CICR)

The mechanism in cardiac muscle where extracellular $\text{Ca}^{2+}$ entering through L-type $\text{Ca}^{2+}$ channels (DHPR) during an action potential triggers a larger release of $\text{Ca}^{2+}$ from the sarcoplasmic reticulum through ryanodine receptors (RyR2).

Phase 0 (Cardiac Action Potential)

The rapid depolarization upstroke of the cardiac ventricular action potential caused by the opening of voltage-gated fast $\text{Na}^+$ channels and a corresponding surge in inward $\text{Na}^+$ conductance.

Phase 2 (Cardiac Action Potential)

The plateau phase of the cardiac action potential where inward $\text{Ca}^{2+}$ current via L-type $\text{Ca}^{2+}$ channels is balanced by outward $\text{K}^+$ current, maintaining prolonged membrane depolarization.

Relative Refractory Period (RRP)

The interval during late Phase 3 and early Phase 4 of the cardiac action potential in which a stronger-than-normal stimulus can elicit a second action potential because a fraction of voltage-gated $\text{Na}^+$ channels have recovered from inactivation.

Frank-Starling Mechanism

The cardiac principle stating that increased end-diastolic volume or filling pressure stretches myocytes to a more optimal sarcomere length, increasing myocardial contractility and stroke volume.

Laplace Law (Cardiac Wall Tension)

The physiological relationship expressed as $T \times P \times r$ (where tension is proportional to pressure times radius), demonstrating that ventricular dilation or pressure overload elevates wall tension and myocardial oxygen demand.

Phospholamban

A regulatory protein in cardiac muscle that, when phosphorylated via $\beta$-adrenergic stimulation and Protein Kinase A, releases its inhibition on SERCA2a to increase the rate of $\text{Ca}^{2+}$ reuptake into the sarcoplasmic reticulum and accelerate relaxation.