Electrical Activity of the Heart Flashcards

These vocabulary flashcards cover the cardiac conduction system, pacemaker potentials, myocardial action potentials, and the mechanisms of heart rate regulation and contraction.

Sinoatrial (SA) node

The primary pacemaker located in the right atrium that controls the normal heart rhythm (sinus rhythm) and typically determines a heart rate of about $70-80\,\text{beats per minute}$.

Atrioventricular (AV) node

A component of the conduction system located between the atria and ventricles that receives impulses and delays their transmission briefly.

AV nodal delay

The brief pause in electrical transmission that allows the ventricles to fill with blood before ventricular contraction begins.

Pacemaker potential

The gradual change in voltage inside pacemaker cells that happens automatically during the resting (diastole) phase, leading to an action potential.

Sinus rhythm

The term used to describe the normal heart rhythm that is controlled by the sinoatrial node.

$Na^+$ channels in pacemaker cells

Channels that open with hyperpolarisation (more negative than the RMP) or when adrenaline/noradrenaline bind to $\beta_1$ receptors, causing the pacemaker potential.

$$-\text{40 mV}$$

The threshold membrane potential in pacemaker cells at which voltage-gated $Ca^{2+}$ channels open to complete depolarisation.

Purkinje fibres

Specialised fibres that distribute electrical impulses throughout the ventricular myocardium to initiate ventricular contraction.

Sympathetic stimulation

Neural regulation that increases the SA node firing rate, increases conduction through the AV node, and increases the force of contraction.

Parasympathetic stimulation

Neural regulation that decreases the SA node firing rate and slows conduction through the AV node, but has no effect on the force of contraction.

$$-\text{85 mV}$$

The resting membrane potential of myocardial (contractile) cells.

Fast $Na^+$ channels

Channels in myocardial cells that open upon stimulation to allow $Na^+$ to rush in, raising the membrane potential to around $\text{+15 mV}$.

Plateau phase

A relatively stable voltage period in the myocardial action potential lasting $200-300\,\text{milliseconds}$ caused by the steady inflow of $Ca^{2+}$.

Tetanus

A state of continuous muscle contraction that is prevented in cardiac muscle by the long plateau phase of the action potential.

Excitation-contraction coupling

The process where electrical activity triggers muscle contraction, starting with the opening of voltage-gated $Ca^{2+}$ channels.

Troponin C

The protein to which $Ca^{2+}$ binds in cardiac muscle cells, allowing actin and myosin to interact for contraction.

$$Ca^{2+}\text{ ATPase pumps}$$

Pumps that move $Ca^{2+}$ back into the sarcoplasmic reticulum during repolarisation to end muscle contraction and allow relaxation.

Gap junctions

Specialised connections through which electrical signals are transmitted directly between heart muscle cells via the passage of ions.