CONDUCTING SYSTEM OF HEART

Cardiac Intrinsic Conduction System

The heart's ability to pump blood is facilitated by a specialized intrinsic conduction system (or nodal system)1, which ensures the rhythmic contraction of the cardiac muscle. The cause of the heartbeat lies within the heart itself, specifically in its pacemaker. The autonomic nerves only modify this activity

The conducting system is responsible for generating spontaneous action potentials and conducting them throughout the heart in an orderly fashion. The specialized cells within this system have automaticity, meaning they can spontaneously generate action potentials

The main components of the cardiac conducting system include:

Sinoatrial (SA) node: This group of specialized muscle cells is the primary, or normal, pacemaker of the heart. It is located in the right atrium near the opening of the superior vena cava. SA node cells spontaneously depolarize and fire action potentials at a regular intrinsic rate. In a resting individual, this rate is typically between 60 and 100 times per minute. The spontaneous time-dependent depolarization during diastole is called the "pacemaker potential" or "prepotential," and it is the cause of automaticity. Pacemaker cells in the SA node typically have a membrane potential of about -55 to -60 mV. The action potentials in these cells are largely due to calcium influx through L-type Ca2+ channels, with minimal contribution from sodium influx. Repolarization is brought about by potassium efflux. The fastest pacemaker (normally the SA node) sets the heart rate and overrides slower pacemakers due to its higher intrinsic frequency.

Atrioventricular (AV) node: Located just above the atrioventricular ring in the interatrial septum, the AV node is the secondary site of origin for electrical signals in the mammalian heart. Its primary role is to conduct the impulse from the atria to the ventricles. Conduction through the AV node is characteristically slow. Like the SA node, the AV node contains cells with pacemaker activity, but its intrinsic rhythmicity is slower, around 40 beats per minute. Thus, it is considered a secondary or subsidiary pacemaker that can assume control of the heart if the SA node fails. SA and AV nodes share many properties, including similar action potentials, pacemaker mechanisms, drug sensitivities, and slow conduction. AV nodal action potentials are classified as slow-response action potentials. Pacemaker potentials in AV nodal cells range from -70 to -50 mV during phase. The intrinsic rhythmicity depends on the interaction of time-dependent and voltage-gated currents (IK, ICa, and If).

Bundle of His and Bundle branches: From the AV node, the impulse is conducted down the Bundle of His and then into the left and right bundle branches, which extend into the ventricles.

Purkinje fibers: These specialized conducting fibers rapidly conduct the impulse through the ventricular walls. Purkinje fibers also possess automaticity properties and can function as latent or potential pacemakers, although their intrinsic frequency is typically slower than both the SA and AV nodes. Pacemaker potentials in Purkinje fibers range from -90 to -65 mV during phase. Action potentials in Purkinje fibers are classified as fast-response.

The electrical excitation normally originates in the SA node and spreads throughout the heart from cell to cell. Action potentials are conducted by myocardial cells in the atria and transmitted to the ventricles via the specialized conducting tissue. Cardiac muscle cells are interconnected by gap junctions, which act as electrical synapses. These gap junctions, located in dense regions of the sarcolemma at the ends of cardiac myocytes called intercalated disks, allow current to flow rapidly from cell to cell. This electrical coupling means the entire mass of interconnected cells (myocardium) acts as a single functioning unit, or functional syncytium, where an action potential originating in one cell can be transmitted to all others.

The propagation of action potentials through this system must be carefully timed to synchronize the contraction of the cardiac myocytes within each chamber, optimizing the heart's function as a pump. The heart's electrical activity (represented by the electrocardiogram) is correlated with the mechanical events (contraction and relaxation) of the cardiac cycle; myocardial contraction occurs in response to depolarization, and relaxation begins during repolarization.

The autonomic nervous system regulates the intrinsic pacemaker activity and conduction speed. Sympathetic activity enhances automaticity and increases heart rate, while parasympathetic activity, particularly via the vagus nerve (which uses acetylcholine), inhibits it and decreases heart rate.These neurotransmitters modulate the membrane currents in pacemaker cells.