mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...

, cardiac electrical activity originates from specialized

myocytes A muscle cell, also known as a myocyte, is a mature contractile cell in the muscle of an animal. In humans and other vertebrates there are three types: skeletal, smooth, and cardiac (cardiomyocytes). A skeletal muscle cell is long and threadli ...

of the

sinoatrial node The sinoatrial node (also known as the sinuatrial node, SA node, sinus node or Keith–Flack node) is an ellipse, oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of Cell (biology), cells known as pa ...

(SAN) which generate spontaneous and rhythmic

action potentials An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific cell rapidly rises and falls. ...

(AP). The unique functional aspect of this type of myocyte is the absence of a stable

resting potential The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential. The re ...

during diastole. Electrical discharge from this

cardiomyocyte Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of ...

may be characterized by a slow smooth transition from the Maximum Diastolic Potential (MDP, -70 mV) to the threshold (-40 mV) for the initiation of a new AP event. The voltage region encompassed by this transition is commonly known as pacemaker phase, or slow diastolic depolarization or phase 4. The duration of this slow diastolic depolarization (pacemaker phase) thus governs the cardiac chronotropism. It is also important to point out that the modulation of the cardiac rate by the

autonomic nervous system The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...

also acts on this phase. Sympathetic stimuli induce the acceleration of rate by increasing the slope of the pacemaker phase, while parasympathetic activation exerts the opposite action. 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 the 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 current The pacemaker current (I''f'', or IK''f'', also called 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 of the heart an ...

(I_f) as one of the most important.Bucchi A, Baruscotti M, Robinson RB, DiFrancesco D. (2007) Modulation of rate by autonomic agonists in SAN cells involves changes in diastolic depolarization and the pacemaker current. J Mol Cell Cardiol. Jul;43(1):39-48. There is now substantial evidence that also sarcoplasmic reticulum (SR) Ca²⁺ -transients participate in the generation of the diastolic depolarization via a process involving the Na–Ca exchanger.

References

See also