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===Pacemaker potentials===

===Pacemaker potentials===

{{Main|Pacemaker potential}}

{{Main|Pacemaker potential}}

 

[[文件:Pacemaker potential.svg.png|替代=|缩略图|In [[pacemaker potential]]s, the cell spontaneously depolarizes (straight line with upward slope) until it fires an action 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.|链接=Special:FilePath/Pacemaker_potential.svg]]

 

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]].

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]].

==Other cell types 其他细胞类型==

==Other cell types 其他细胞类型==

===Cardiac action potentials 心肌动作电位===

===Cardiac action potentials 心肌动作电位===

[[Image:Ventricular myocyte action potential.svg|thumb|right|220px|Phases of a cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing eflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive [[calcium]] channels.|链接=Special:FilePath/Ventricular_myocyte_action_potential.svg.png]]

[[Image:Ventricular myocyte action potential.svg|thumb|220px|[[文件:Ventricular myocyte action potential.svg.png|缩略图]]Phases of a cardiac action potential. The sharp rise in voltage ("0") corresponds to the influx of sodium ions, whereas the two decays ("1" and "3", respectively) correspond to the sodium-channel inactivation and the repolarizing eflux of potassium ions. The characteristic plateau ("2") results from the opening of voltage-sensitive [[calcium]] channels.|链接=Special:FilePath/Ventricular_myocyte_action_potential.svg.png|替代=]]

The cardiac action potential differs from the neuronal action potential by having an extended plateau, in which the membrane is held at a high voltage for a few hundred milliseconds prior to being repolarized by the potassium current as usual.<ref name=Kleber group=lower-alpha /> This plateau is due to the action of slower [[calcium]] channels opening and holding the membrane voltage near their equilibrium potential even after the sodium channels have inactivated.

The cardiac action potential differs from the neuronal action potential by having an extended plateau, in which the membrane is held at a high voltage for a few hundred milliseconds prior to being repolarized by the potassium current as usual.<ref name=Kleber group=lower-alpha /> This plateau is due to the action of slower [[calcium]] channels opening and holding the membrane voltage near their equilibrium potential even after the sodium channels have inactivated.