The Heart: Electrical Activity and Contraction

Vocabulary flashcards covering cardiac electrical conduction, specific phases of SAN and ventricular action potentials, and the mechanics of muscle contraction and relaxation.

Intercalated discs

Specialized structures that allow for the spread of electrical activity and the flow of nutrients between cardiac myocytes.

Gap junctions

Clusters of ion channels located at intercalated discs that provide a low resistance path between cardiac myocytes.

Myogenic

The property of cardiac muscle being able to generate and propagate its own electrical activity independent of the nervous system.

Sinoatrial node (SAN)

The dominant pacemaker of the heart where electrical activity originates.

Atrioventricular node (AVN)

The part of the conduction system that transmits impulses from the SAN to the ventricles with a delay.

Ventricular conduction system

A system comprising the bundle of His and Purkinje fibres that provides fast conduction to deliver impulses to the ventricles.

SAN Phase 0

The action potential upstroke characterized by $Ca^{2+}$ influx through L-type $Ca^{2+}$ channels, depolarising $V_m$ from $-30~mV$ to approximately $20~mV$.

SAN Phase 3

Action potential repolarisation occurring through $K^+$ efflux via voltage gated $K^+$ channels (KV channels), hyperpolarising $V_m$ to approximately $-60~mV$.

SAN Phase 4

Pacemaker depolarisation characterized by an unstable resting membrane potential that continuously depolarises from $-60~mV$ to $-30~mV$.

NCX (Sodium-Calcium Exchanger)

A transporter that brings $3 imes Na^+$ ions into the cell for every $1 imes Ca^{2+}$ ion it removes, resulting in a net influx of positive charge.

Funny current ($I_f$)

A depolarising current involving the simultaneous efflux of $K^+$ and influx of $Na^+$ through HCN channels.

Ventricular Phase 0

The action potential upstroke resulting from $Na^+$ influx through voltage gated $Na^+$ channels, depolarising $V_m$ from $-85~mV$ to approximately $+40~mV$.

Ventricular Phase 1

Initial repolarisation unique to ventricular action potentials, involving $K^+$ efflux through transient outward voltage gated $K^+$ channels ($I_{to}$).

Ventricular Phase 2

The plateau phase where $Ca^{2+}$ influx is cancelled out by continued $K^+$ efflux, enabling muscle contraction and blood ejection during systole.

Ventricular Phase 3

Action potential repolarisation involving $K^+$ efflux through voltage gated $K^+$ channels (KV channels) to hyperpolarise $V_m$ back to $-85~mV$.

Ventricular Phase 4

The resting potential phase where $V_m$ is maintained at $-85~mV$ via $K^+$ efflux through $I_{K1}$ channels.

Sarcomere

The main contractile unit of a muscle consisting of actin filaments, myosin filaments, and Z-disks.

Calcium-induced calcium release

The process where initial $Ca^{2+}$ influx through L-type channels triggers the release of more Ca^{2+ from the Sarcoplasmic Reticulum (SR) via RYR.

SERCA

An active transporter used during muscle relaxation to reduce intracellular $Ca^{2+}$ concentration by pumping ions back into the Sarcoplasmic Reticulum (SR).