Editing Action potential (section)

===Pacemaker potentials===
{{Main|Pacemaker potential}}

[[Image:Pacemaker potential.svg|thumb|right|In [[pacemaker potential]]s, the cell spontaneously depolarizes (straight line with upward slope) until it fires an action potential.|alt=A plot of action potential (mV) vs time. The membrane potential is initially −60 mV, rise relatively slowly to the threshold potential of −40 mV, and then quickly spikes at a potential of +10 mV, after which it rapidly returns to the starting −60 mV potential. The cycle is then repeated.]]

In sensory neurons, action potentials result from an external stimulus. However, some excitable cells require no such stimulus to fire: They spontaneously depolarize their axon hillock and fire action potentials at a regular rate, like an internal clock.{{sfn|Junge|1981|pp=115–132}} The voltage traces of such cells are known as [[pacemaker potential]]s.{{sfn|Bullock|Orkand|Grinnell|1977|pp=152–153}} The [[cardiac pacemaker]] cells of the [[sinoatrial node]] in the [[heart]] provide a good example.<ref name="noble_1960" group=lower-alpha >{{cite journal | vauthors = Noble D | title = Cardiac action and pacemaker potentials based on the Hodgkin-Huxley equations | journal = Nature | volume = 188 | issue = 4749 | pages = 495–7 | date = November 1960 | pmid = 13729365 | doi = 10.1038/188495b0 | bibcode = 1960Natur.188..495N | s2cid = 4147174 }}</ref> Although such pacemaker potentials have a [[neural oscillation|natural rhythm]], it can be adjusted by external stimuli; for instance, [[heart rate]] can be altered by pharmaceuticals as well as signals from the [[sympathetic nervous system|sympathetic]] and [[parasympathetic nervous system|parasympathetic]] nerves.{{sfn|Bullock|Orkand|Grinnell|1977|pp=444–445}} The external stimuli do not cause the cell's repetitive firing, but merely alter its timing.{{sfn|Bullock|Orkand|Grinnell|1977|pp=152–153}} In some cases, the regulation of frequency can be more complex, leading to patterns of action potentials, such as [[bursting]].{{cn|date=May 2024}}