mammals Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

, cardiac electrical activity originates from specialized

myocytes 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 ...

of 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 ...

(SAN) which generate spontaneous and rhythmic

action potentials 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, c ...

(AP). The unique functional aspect of this type of myocyte is the absence of a stable resting potential during diastole. Electrical discharge from this

cardiomyocyte Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that ...

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), formerly referred to as the vegetative nervous system, is a division of the peripheral nervous system that supplies viscera, internal organs, smooth muscle and glands. The autonomic nervous system is a control ...

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 (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 ...

(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