更改

===不应期===

===不应期===

The absolute refractory period is largely responsible for the unidirectional propagation of action potentials along axons.{{sfn|Purves|Augustine|Fitzpatrick|Hall|2008|p=56}} At any given moment, the patch of axon behind the actively spiking part is refractory, but the patch in front, not having been activated recently, is capable of being stimulated by the depolarization from the action potential.

==动作电位的传播==

 

 

==Propagation动作电位的传播==

The action potential generated at the axon hillock propagates as a wave along the axon.{{sfn|Bullock|Orkand|Grinnell|1977|pp=160–164}} The currents flowing inwards at a point on the axon during an action potential spread out along the axon, and depolarize the adjacent sections of its membrane. If sufficiently strong, this depolarization provokes a similar action potential at the neighboring membrane patches. This basic mechanism was demonstrated by [[Alan Lloyd Hodgkin]] in 1937. After crushing or cooling nerve segments and thus blocking the action potentials, he showed that an action potential arriving on one side of the block could provoke another action potential on the other, provided that the blocked segment was sufficiently short.<ref group="lower-alpha" name=":1">{{cite journal | vauthors = Hodgkin AL | title = Evidence for electrical transmission in nerve: Part I | journal = The Journal of Physiology | volume = 90 | issue = 2 | pages = 183–210 | date = July 1937 | pmid = 16994885 | pmc = 1395060 | doi = 10.1113/jphysiol.1937.sp003507 | author-link = Alan Lloyd Hodgkin }}<br />* {{cite journal | vauthors = Hodgkin AL | title = Evidence for electrical transmission in nerve: Part II | journal = The Journal of Physiology | volume = 90 | issue = 2 | pages = 211–32 | date = July 1937 | pmid = 16994886 | pmc = 1395062 | doi = 10.1113/jphysiol.1937.sp003508 | author-link = Alan Lloyd Hodgkin }}</ref>

The action potential generated at the axon hillock propagates as a wave along the axon.{{sfn|Bullock|Orkand|Grinnell|1977|pp=160–164}} The currents flowing inwards at a point on the axon during an action potential spread out along the axon, and depolarize the adjacent sections of its membrane. If sufficiently strong, this depolarization provokes a similar action potential at the neighboring membrane patches. This basic mechanism was demonstrated by [[Alan Lloyd Hodgkin]] in 1937. After crushing or cooling nerve segments and thus blocking the action potentials, he showed that an action potential arriving on one side of the block could provoke another action potential on the other, provided that the blocked segment was sufficiently short.<ref group="lower-alpha" name=":1">{{cite journal | vauthors = Hodgkin AL | title = Evidence for electrical transmission in nerve: Part I | journal = The Journal of Physiology | volume = 90 | issue = 2 | pages = 183–210 | date = July 1937 | pmid = 16994885 | pmc = 1395060 | doi = 10.1113/jphysiol.1937.sp003507 | author-link = Alan Lloyd Hodgkin }}<br />* {{cite journal | vauthors = Hodgkin AL | title = Evidence for electrical transmission in nerve: Part II | journal = The Journal of Physiology | volume = 90 | issue = 2 | pages = 211–32 | date = July 1937 | pmid = 16994886 | pmc = 1395062 | doi = 10.1113/jphysiol.1937.sp003508 | author-link = Alan Lloyd Hodgkin }}</ref>

轴丘产生的动作电位沿轴突传播。当动作电位沿轴突扩散时，电流在轴突上的某一点向内流动，并使其膜的相邻部分去极化。如果足够强的话，这种去极化会在相邻的膜片上激发类似的动作电位。这一基本机制在1937年由艾伦·劳埃德·霍奇金 Alan Lloyd Hodgkin 证明。在挤压或冷却神经节段，从而阻断动作电位后，他表明，动作电位到达阻滞的一侧可以激发另一侧的动作电位，只要阻滞的节段足够短。<ref name=":1" group="lower-alpha" />

轴丘产生的动作电位沿轴突传播。当动作电位沿轴突扩散时，电流在轴突上的某一点向内流动，并使其膜的相邻部分去极化。如果足够强的话，这种去极化会在相邻的膜片上激发类似的动作电位。这一基本机制在1937年由艾伦·劳埃德·霍奇金 Alan Lloyd Hodgkin 证明。在挤压或冷却神经节段，从而阻断动作电位后，他表明，动作电位到达阻滞的一侧可以激发另一侧的动作电位，只要阻滞的节段足够短。<ref name=":1" group="lower-alpha" />

Once an action potential has occurred at a patch of membrane, the membrane patch needs time to recover before it can fire again. At the molecular level, this ''absolute refractory period'' corresponds to the time required for the voltage-activated sodium channels to recover from inactivation, i.e., to return to their closed state.{{sfn|Stevens|1966|pp=19–20}} There are many types of voltage-activated potassium channels in neurons. Some of them inactivate fast （A-type currents） and some of them inactivate slowly or not inactivate at all; this variability guarantees that there will be always an available source of current for repolarization, even if some of the potassium channels are inactivated because of preceding depolarization. On the other hand, all neuronal voltage-activated sodium channels inactivate within several milliseconds during strong depolarization, thus making following depolarization impossible until a substantial fraction of sodium channels have returned to their closed state. Although it limits the frequency of firing,{{sfn|Stevens|1966|pp=21–23}} the absolute refractory period ensures that the action potential moves in only one direction along an axon.{{sfn|Purves|Augustine|Fitzpatrick|Hall|2008|p=56}} The currents flowing in due to an action potential spread out in both directions along the axon.{{sfn|Bullock|Orkand|Grinnell|1977|pp=161–164}} However, only the unfired part of the axon can respond with an action potential; the part that has just fired is unresponsive until the action potential is safely out of range and cannot restimulate that part. In the usual [[orthodromic conduction]], the action potential propagates from the axon hillock towards the synaptic knobs （the axonal termini）; propagation in the opposite direction—known as [[antidromic conduction]]—is very rare.{{sfn|Bullock|Orkand|Grinnell|1977|p=509}} However, if a laboratory axon is stimulated in its middle, both halves of the axon are "fresh", i.e., unfired; then two action potentials will be generated, one traveling towards the axon hillock and the other traveling towards the synaptic knobs.

Once an action potential has occurred at a patch of membrane, the membrane patch needs time to recover before it can fire again. At the molecular level, this ''absolute refractory period'' corresponds to the time required for the voltage-activated sodium channels to recover from inactivation, i.e., to return to their closed state.{{sfn|Stevens|1966|pp=19–20}} There are many types of voltage-activated potassium channels in neurons. Some of them inactivate fast （A-type currents） and some of them inactivate slowly or not inactivate at all; this variability guarantees that there will be always an available source of current for repolarization, even if some of the potassium channels are inactivated because of preceding depolarization. On the other hand, all neuronal voltage-activated sodium channels inactivate within several milliseconds during strong depolarization, thus making following depolarization impossible until a substantial fraction of sodium channels have returned to their closed state. Although it limits the frequency of firing,{{sfn|Stevens|1966|pp=21–23}} the absolute refractory period ensures that the action potential moves in only one direction along an axon.{{sfn|Purves|Augustine|Fitzpatrick|Hall|2008|p=56}} The currents flowing in due to an action potential spread out in both directions along the axon.{{sfn|Bullock|Orkand|Grinnell|1977|pp=161–164}} However, only the unfired part of the axon can respond with an action potential; the part that has just fired is unresponsive until the action potential is safely out of range and cannot restimulate that part. In the usual [[orthodromic conduction]], the action potential propagates from the axon hillock towards the synaptic knobs（the axonal termini）; propagation in the opposite direction—known as [[antidromic conduction]]—is very rare.{{sfn|Bullock|Orkand|Grinnell|1977|p=509}} However, if a laboratory axon is stimulated in its middle, both halves of the axon are "fresh", i.e., unfired; then two action potentials will be generated, one traveling towards the axon hillock and the other traveling towards the synaptic knobs.

一旦膜片上的一个动作电位发生了，膜片需要时间恢复才能再次激活。在分子水平上，这个绝对不应期相当于电压激活的钠离子通道从失活状态恢复到关闭状态所需的时间。神经元中存在多种类型的电压激活钾通道。其中一些快速失活（A 型电流），一些慢速失活或根本不失活；这种变异性保证了总有可用的电流来源复极化，即使一些钾离子通道由于先前的去极化作用而失活。另一方面，在强去极化过程中，所有神经元电压激活钠通道在几毫秒内失活，从而使去极化不可能发生，直到相当一部分的钠通道恢复到它们的关闭状态。虽然它限制了放电的频率，但绝对不应期电位确保了动作电位沿轴突只向一个方向移动。由于动作电位的作用，电流沿轴突向两个方向扩散。然而，只有轴突未激活的部分才能作出动作电位的反应；刚刚激活的部分是没有反应的，直到动作电位安全地超出范围，不能再次激活该部分。在通常的正向传导中，动作电位从轴丘向突触扣结（轴突终端）传导，向相反方向传导的现象非常罕见。然而，如果一个实验室的轴突在它的中间被刺激，两半的轴突都是“新鲜的”，也就是说，没有被刺激，那么两个动作电位就会产生，一个朝向轴突小丘，另一个朝向突触扣结。

一旦膜片上的一个动作电位发生了，膜片需要时间恢复才能再次激活。在分子水平上，这个绝对不应期相当于电压激活的钠离子通道从失活状态恢复到关闭状态所需的时间。神经元中存在多种类型的电压激活钾通道。其中一些快速失活（A 型电流），一些慢速失活或根本不失活；这种变异性保证了总有可用的电流来源复极化，即使一些钾离子通道由于先前的去极化作用而失活。另一方面，在强去极化过程中，所有神经元电压激活钠通道在几毫秒内失活，从而使去极化不可能发生，直到相当一部分的钠通道恢复到它们的关闭状态。虽然它限制了放电的频率，但绝对不应期电位确保了动作电位沿轴突只向一个方向移动。由于动作电位的作用，电流沿轴突向两个方向扩散。然而，只有轴突未激活的部分才能作出动作电位的反应；刚刚激活的部分是没有反应的，直到动作电位安全地超出范围，不能再次激活该部分。在通常的正向传导中，动作电位从轴丘向突触扣结（轴突终端）传导，向相反方向传导的现象非常罕见。然而，如果一个实验室的轴突在它的中间被刺激，两半的轴突都是“新鲜的”，也就是说，没有被刺激，那么两个动作电位就会产生，一个朝向轴突小丘，另一个朝向突触扣结。

Myelin has two important advantages: fast conduction speed and energy efficiency. For axons larger than a minimum diameter （roughly 1 [[micrometre]]）, myelination increases the [[conduction velocity]] of an action potential, typically tenfold.<ref name="hartline_2007" group=lower-alpha /> Conversely, for a given conduction velocity, myelinated fibers are smaller than their unmyelinated counterparts. For example, action potentials move at roughly the same speed （25&nbsp;m/s） in a myelinated frog axon and an unmyelinated [[squid giant axon]], but the frog axon has a roughly 30-fold smaller diameter and 1000-fold smaller cross-sectional area. Also, since the ionic currents are confined to the nodes of Ranvier, far fewer ions "leak" across the membrane, saving metabolic energy. This saving is a significant [[natural selection|selective advantage]], since the human nervous system uses approximately 20% of the body's metabolic energy.<ref name="hartline_2007" group=lower-alpha>{{cite journal | vauthors = Hartline DK, Colman DR | title = Rapid conduction and the evolution of giant axons and myelinated fibers | journal = Current Biology | volume = 17 | issue = 1 | pages = R29-35 | date = January 2007 | pmid = 17208176 | doi = 10.1016/j.cub.2006.11.042 | s2cid = 10033356 | doi-access = free }}</ref>

Myelin has two important advantages: fast conduction speed and energy efficiency. For axons larger than a minimum diameter （roughly 1 [[micrometre]]）, myelination increases the [[conduction velocity]] of an action potential, typically tenfold.<ref name="hartline_2007" group=lower-alpha /> Conversely, for a given conduction velocity, myelinated fibers are smaller than their unmyelinated counterparts. For example, action potentials move at roughly the same speed （25&nbsp;m/s） in a myelinated frog axon and an unmyelinated [[squid giant axon]], but the frog axon has a roughly 30-fold smaller diameter and 1000-fold smaller cross-sectional area. Also, since the ionic currents are confined to the nodes of Ranvier, far fewer ions "leak" across the membrane, saving metabolic energy. This saving is a significant [[natural selection|selective advantage]], since the human nervous system uses approximately 20% of the body's metabolic energy.<ref name="hartline_2007" group=lower-alpha>{{cite journal | vauthors = Hartline DK, Colman DR | title = Rapid conduction and the evolution of giant axons and myelinated fibers | journal = Current Biology | volume = 17 | issue = 1 | pages = R29-35 | date = January 2007 | pmid = 17208176 | doi = 10.1016/j.cub.2006.11.042 | s2cid = 10033356 | doi-access = free }}</ref>

髓鞘具有两个重要的优点：传导速度快和能量效率高。对于大于最小直径（大约1微米）的轴突，髓鞘形成增加了动作电位的传导速度，通常是原来的十倍.<ref name="hartline_2007" group="lower-alpha" /> Co。相反，相同的传导速度，有髓纤维比无髓纤维小。例如，在有髓青蛙轴突和无髓青蛙轴突中，动作电位的移动速度大致相同（25米/秒），但是青蛙轴突的直径要小30倍，横截面积要小1000倍。此外，由于离子电流仅限于郎飞结，跨膜“泄漏”的离子要少得多，从而节省了新陈代谢能量。这种节省是一个重大的选择优势，考虑到人类神经系统消耗大约20% 的身体代谢能量.<ref name="hartline_2007" group="lower-alpha" />。

髓鞘具有两个重要的优点：传导速度快和能量效率高。对于大于最小直径（大约1微米）的轴突，髓鞘形成增加了动作电位的传导速度，通常是原来的十倍.<ref name="hartline_2007" group="lower-alpha" /> Co。相反，相同的传导速度，有髓纤维比无髓纤维小。例如，在有髓青蛙轴突和无髓青蛙轴突中，动作电位的移动速度大致相同（25米/秒），但是青蛙轴突的直径要小30倍，横截面积要小 1000 倍。此外，离子电流仅限于郎飞结，跨膜“泄漏”的离子要少得多，从而节省了新陈代谢能量。这种节省是一个重大的选择优势，考虑到人类神经系统消耗大约 20% 的身体代谢能量。<ref name="hartline_2007" group="lower-alpha" />

The length of axons' myelinated segments is important to the success of saltatory conduction. They should be as long as possible to maximize the speed of conduction, but not so long that the arriving signal is too weak to provoke an action potential at the next node of Ranvier. In nature, myelinated segments are generally long enough for the passively propagated signal to travel for at least two nodes while retaining enough amplitude to fire an action potential at the second or third node. Thus, the [[safety factor]] of saltatory conduction is high, allowing transmission to bypass nodes in case of injury. However, action potentials may end prematurely in certain places where the safety factor is low, even in unmyelinated neurons; a common example is the branch point of an axon, where it divides into two axons.{{sfn|Bullock|Orkand|Grinnell|1977|p=163}}

The length of axons' myelinated segments is important to the success of saltatory conduction. They should be as long as possible to maximize the speed of conduction, but not so long that the arriving signal is too weak to provoke an action potential at the next node of Ranvier. In nature, myelinated segments are generally long enough for the passively propagated signal to travel for at least two nodes while retaining enough amplitude to fire an action potential at the second or third node. Thus, the [[safety factor]] of saltatory conduction is high, allowing transmission to bypass nodes in case of injury. However, action potentials may end prematurely in certain places where the safety factor is low, even in unmyelinated neurons; a common example is the branch point of an axon, where it divides into two axons.{{sfn|Bullock|Orkand|Grinnell|1977|p=163}}