In the pacemaking cells of the

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...

(e.g., the

sinoatrial node The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximat ...

), the pacemaker potential (also called the pacemaker current) is the slow, positive increase in voltage across the

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

's membrane (the

membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...

) that occurs between the end of one

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

and the beginning of the next action potential. This increase in membrane potential is what causes the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

, which typically maintains a resting membrane potential around -65 mV, to reach the

threshold potential In electrophysiology, the threshold potential is the critical level to which a membrane potential must be depolarized to initiate an action potential. In neuroscience, threshold potentials are necessary to regulate and propagate signaling in both ...

and consequently fire the next action potential; thus, the pacemaker potential is what drives the self-generated rhythmic firing (

automaticity Automaticity is the ability to do things without occupying the mind with the low-level details required, allowing it to become an automatic response pattern or habit. It is usually the result of learning, repetition, and practice. Examples of tas ...

) of pacemaker cells, and the rate of change (i.e., the slope) of the pacemaker potential is what determines the timing of the next action potential and thus the intrinsic firing rate of the cell. In a healthy sinoatrial node (SAN, a complex tissue within the right atrium containing pacemaker cells that normally determine the intrinsic firing rate for the entire heart), the pacemaker potential is the main determinant of the heart rate. Because the pacemaker potential represents the non-contracting time between heart beats (

diastole Diastole ( ) is the relaxed phase of the cardiac cycle when the chambers of the heart are re-filling with blood. The contrasting phase is systole when the heart chambers are contracting. Atrial diastole is the relaxing of the atria, and ventric ...

), it is also called the

diastolic depolarization In mammals, cardiac electrical activity originates from specialized myocytes of the sinoatrial node (SAN) which generate spontaneous and rhythmic action potentials (AP). The unique functional aspect of this type of myocyte is the absence of a stable ...

. The amount of net inward current required to move the cell membrane potential during the pacemaker phase is extremely small, in the order of few pAs, but this net flux arises from time to time changing contribution of several currents that flow with different voltage and time dependence. Evidence in support of the active presence of K⁺, Ca²⁺, Na⁺ channels and Na⁺/K⁺ exchanger during the pacemaker phase have been variously reported in the literature, but several indications point to the “funny”(I_f) current as one of the most important.(see

funny current The pacemaker current (or I''f'', or IK''f'', also referred to as the funny current) is an electric current in the heart that flows through the HCN channel or pacemaker channel. Such channels are important parts of the electrical conduction system ...

). There is now substantial evidence that also sarcoplasmic reticulum (SR) Ca²⁺-transients participate to the generation of the diastolic depolarization via a process involving the Na–Ca exchanger. The rhythmic activity of some

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

like the

pre-Bötzinger complex The preBötzinger complex, sometimes written pre-Bötzinger complex (preBötC), is a functionally and anatomically specialized site in the ventral-lateral region of the lower medulla oblongata (i.e., lower brainstem). The preBötC is part of the ven ...

is modulated by neurotransmitters and neuropeptides, and such modulatory connectivity gives to the neurons the necessary plasticity to generating distinctive, state-dependent rhythmic patterns that depend on pacemaker potentials.

Pacemakers

he heart has several pacemakers, each which fires at its own intrinsic rate: * ''SA node:'' 60–100 bpm * ''Atrioventricular node(AVN):'' 40–60 bpm * ''Purkinje fibres:'' 20–40 bpm The potentials will normally travel in order
SA node → Atrioventricular node → Purkinje fibres Normally, all the foci will end up firing at the SA node rate, not their intrinsic rate in a phenomenon known as overdrive-suppression. Thus, in the normal, healthy heart, only the SA node intrinsic rate is observable.

Pathology

However, in pathological conditions, the intrinsic rate becomes apparent. Consider a heart attack which damages the region of the heart between the SA node and the AV node. SA node → , block, AV node → Purkinje fibres The other foci will not see the SA node firing; however, they will see the atrial foci. The heart will now beat at the intrinsic rate of the AV node.

Induction

The firing of the pacemaker cells is induced electrically by reaching the threshold potential of the cell membrane. The

is the potential an excitable cell membrane, such as a

myocyte A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscl ...

, must reach in order to induce an action potential. This

depolarization In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is esse ...

is caused by very small net inward currents of calcium ions across the cell membrane, which gives rise to the action potential.

Bio-pacemakers

Bio-pacemakers are the outcome of a rapidly emerging field of research into a replacement for the

electronic pacemaker An artificial cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural cardiac pacemaker) or pacemaker is a medical device that generates electrical impulses delivered by electrodes to the chambers of the heart eith ...

. The bio-pacemaker turns quiescent myocardial cells (e.g. atrial cells) into pacemaker cells. This is achieved by making the cells express a gene which creates a pacemaker current.

References

{{DEFAULTSORT:Pacemaker Potential Cardiac electrophysiology Graded potentials

Pacemakers

Pathology

Induction

Bio-pacemakers

See also

References