PURKINJE FIBERS: HISTOLOGY AND FUNCTION - ANATOMY AND PHYSIOLOGY - 2025

The Purkinje fibers cardiac represent the last stage of the producing system, automatically and repeatedly, the electrical excitation required for ventricular mechanical activity. It focuses on directing the excitation to the ventricular myocytes so that they produce systole (contraction).

The system to which these fibers belong is made up of the sino-atrial node (SA), where the excitation originates; the internodal bundles that reach the atrioventricular (AV) node; the atrioventricular node, in which electrical conduction is somewhat delayed; the bundle of His, with its right and left branches, and the Purkinje fiber system.

Purkinje fibers in stained heart muscle (Source: I, Nathanael Via Wikimedia Commons)

These fibers were named in honor of John Evangelista Purkinje, a Czech anatomist and physiologist who first described them in 1839. They should not be confused with Purkinje cells, discovered by the same author at the level of the cerebellar cortex and implicated in the movement control.

Histology

Like the rest of the components of the cardiac excitation-conduction system, the cells that make up the Purkinje fiber system are muscle cells or cardiac myocytes that have lost their contractile structure and have specialized in conducting electrical excitation.

Its components join the ends of the branches of the bundle of His and the beginning of a sequence of ventricular myocytes, segments between which conduct electrical excitation originating in the sino-atrial node, forming a diffuse network distributed throughout the endocardium that covers the ventricles..

They have characteristics that differentiate them from the other components of the system: they are longer and thicker fibers (40 μm) even than ventricular contractile fibers and they have the highest conduction speed: 4 m / s; compared to the 1.5 m / s of those that follow, the His bundle fibers.

This high conduction speed is due, apart from its large diameter, to the fact that, at their contact sites, the intercalary discs, there is a high density of gap junctions that allow the easy passage of ionic currents between them. and the rapid transmission of excitement.

Due to this high conduction velocity and the diffuse distribution of Purkinje fibers, excitation reaches almost simultaneously the contractile myocardium of both ventricles, requiring only 0.03 s (30 ms) to complete activation of the entire myocardium ventricular.

Function

- Electrical properties

The cells of the Purkinje system are excitable cells that show, at rest, a potential difference of -90 to -95 mV between both faces of the membrane that separates its interior from the surrounding extracellular fluid, its interior being negative with respect to the exterior.

When excited, these cells respond with a depolarization known as the action potential (AP) and during which the membrane potential rapidly becomes less negative and can become reversed, momentarily reaching a positive value of up to +30 mV (positive inside).

Action potential (Source: en: Memenen Via Wikimedia Commons)

According to the speed with which this depolarization occurs, the different excitable cell types of the heart have been included in one of two categories: fast response fibers or slow response fibers. Purkinje fibers are part of the latter category.

- Action potential on Purkinje fibers

The physiological stimulus for the Purkinje fibers to produce an action potential is a depolarizing ionic current, coming from cellular elements that are earlier in the conduction sequence, and which reaches them through the gap junctions that join them with those elements..

In the action potential of a Purkinje fiber, several phases are distinguished: an abrupt depolarization (phase 0) to +30 mV, a rapid repolarization to 0 mV (phase 1), a sustained depolarization around 0 mV (phase 2 or plateau) and rapid repolarization (phase 3) leading back to resting potential (phase 4).

These events are the result of the activation and / or deactivation of ionic currents that modify the charge balance between the inside and outside of cells. Currents that, in turn, result from changes in the permeability of specific channels for different ions and are designated by the letter I, followed by a subscript that identifies them.

The positive ion input currents or negative ion exit currents are considered negative by convention and produce depolarizations, those of positive ion exit or negative ion exit are positive currents and favor the internal polarization or negativization of the cell.

Phases of the action potential of Purkinje fibers

Phase 0 occurs when the initial depolarization that serves as a stimulus brings the membrane potential to a level (threshold) between -75 and -65 mV, and voltage-dependent sodium (Na +) channels are then opened that allow Na + to enter (Ina current) as in an avalanche, bringing the potential to about +30 mV.

Phase 1 begins at the end of phase 0, when the Na + channels close again and depolarization stops, producing transient currents (Ito1 and Ito2) of K + output and Cl- input, which produce rapid repolarization down to the 0 mV level.

Phase 2 is a long duration "plateau" (300 ms). It results from the opening of slow calcium channels and the production of a Ca ++ input current that maintains, together with a remanent Na + input, the relatively high potential (0 mV) and counteracts the repolarizing K + currents (IKr and IKs) that have started to occur.

In phase 3 the Ca ++ and Na + currents are minimized and the repolarizing K + outflow currents become very pronounced. This increasing K + output drives the membrane potential to the initial resting level of -90 to -95 mV where it remains (phase 4) until the cycle is repeated again.

- Values ​​of some electrical properties of Purkinje fibers

- Idle level: -90 to -95 mV.

- Maximum depolarization level (overshoot): + 30 mV.

- Amplitude of the action potential: 120 mV.

- Duration of the action potential: between 300 and 500 ms.

- Depolarization speed: 500-700 V / s.

- Threshold level for triggering the action potential: between -75 and -65 mV.

- Driving speed: 3-4 m / s.

- Purkinje fibers as secondary pacemakers

Slow-responding myocardial fibers include cells of the sino-atrial and atrio-ventricular nodes, which, during rest (phase 4), undergo a slow depolarization (diastolic prepotential) that brings the membrane potential to its level threshold and an action potential is triggered automatically.

This property is more developed, that is, depolarization occurs more rapidly, in the sino-atrial node, which serves as a cardiac pacemaker and marks a rate of between 60 and 80 beats / min. If it fails, the atrioventricular node can take over the command, but with a lower rate of between 60 and 40 beats / min.

Purkinje fibers, when they are not excited via the normal conduction system, can also undergo the same process of slow depolarization that brings their membrane potential to the threshold level, and end up firing action potentials automatically.

In case the normal excitation of the sino-atrial node and the secondary one of the atrio-ventricular node fail, or the passage of excitation to the ventricles is blocked, some fibers of the Purkinje system begin to discharge on their own and maintain an activation rhythmic ventricular, but at a lower rate (25-40 beats / min).

References

1. Piper HM: Herzerregung, in: Physiologie des Menschen mit Pathophysiologie, 31th ed; RF Schmidt et al (eds). Heidelberg, Springer Medizin Verlag, 2010.
2. Schrader J, Gödeche A, Kelm M: Das Hertz, in: Physiologie, 6th ed; R Klinke et al (eds). Stuttgart, Georg Thieme Verlag, 2010