Stimuli and local response
For a cell to be excited, a stimulus must act upon it. Stimuli are divided into subthreshold, threshold, and suprathreshold.
With a weak (subthreshold) stimulus, the resting potential decreases only slightly at the electrode application site. A local response (LR) occurs. Its main feature is direct proportionality: the stronger the stimulus, the higher the response amplitude.
If the stimulation reaches the threshold value, the local response reaches the critical depolarization level (CDL). It is at this moment that the local change turns into a full-fledged action potential.
Dynamics and phases of the action potential
The development of excitation includes several successive stages displayed on the graph:
- Prespike: slow partial depolarization corresponding to the local response.
- Depolarization (spike, peak): a sharp voltage surge. The charge shifts from the critical level (-50 mV) to positive values (e.g., +30 mV). This charge reversal phenomenon is called inversion or overshoot (the inner surface of the membrane becomes positive).
- Repolarization: voltage drop and return to the resting level. At the end of this phase, a negative after-potential (NAP) — trace depolarization — may be observed.
- Trace hyperpolarization: the charge drops below the initial -70 mV. This is designated as a positive after-potential (PAP).
The duration and shape of these phases depend on the tissue. In neurons and skeletal myocytes, the spike is very short, and the presence of after-potentials is variable.
Ionic mechanisms and channel function
All potential changes are driven by membrane proteins shaped like tubules — ion channels.
- Resting state: The membrane potential is -70 mV. Potassium channels are closed. Sodium channels are also closed but ready for action: their activation m-gates are shut, and inactivation h-gates are open. There is more potassium inside the cell and more sodium outside.
- Onset of stimulation (LR): m-gates begin to open slowly, but the inward sodium current is negligible.
- Depolarization phase: Upon reaching the threshold (-50 mV), all m-gates open. An avalanche-like, passive influx of Na⁺ ions begins down the concentration gradient.
- Repolarization: The incoming positive charge causes the h-gates to close (inactivation of sodium channels). Simultaneously, the 'charge receptors' of the potassium pores respond by opening. K⁺ ions rush outward, restoring the negative charge inside.
- Hyperpolarization and restoration: Since potassium channels close slowly, potassium efflux continues (sometimes supplemented by Cl⁻ influx), driving the charge below -70 mV. The initial balance is finally restored by Na⁺,K⁺-ATPase, which actively pumps the ions back. Sodium channels reset (closing m-gates and opening h-gates), and the cell is ready for a new impulse.
All-or-none law
This law reflects the constancy of the action potential amplitude. Its essence is simple:
- 对于阈下刺激，细胞不产生冲动（“无”）。
- 对于阈刺激和阈上刺激，会产生该细胞所能达到的最大幅度的动作电位（“全”）。
单个结构遵循该定律：神经元、单个神经纤维和肌纤维、平滑肌以及心脏组织。然而，整条神经干或骨骼肌不遵循该定律。这是因为它们由许多兴奋阈值各不相同的纤维组成。
简述：主题相关问答
局部反应与动作电位有什么区别？
局部反应在阈下刺激作用时产生，而动作电位在阈刺激或阈上刺激作用时产生。与动作电位不同，局部反应不能传导至较远距离（以电紧张性衰减方式传导并迅速消退），不遵循“全或无”定律（其幅度与刺激强度呈渐进式相关），具有总和能力，且没有不应期。
| 特征 | 局部反应 | Action potential |
|---|---|---|
| 刺激 | 阈下刺激 | 阈刺激/阈上刺激 |
| 传导 | 不传导 / 具衰减性传导 | 不衰减传导 |
| 总和 | 可行 | 不可能 |
| 定律 | 强度（渐进性） | “全或无” |
| 不应期 | 无反应 | 有 |
动作电位在有髓神经纤维和无髓神经纤维上的传导速度是多少？
传导速度取决于是否存在髓鞘及纤维直径：有髓纤维的传导速度比无髓纤维快，粗纤维比细纤维快。在有髓纤维中传导是呈跳跃式的，因此兴奋的传播速度比连续传导时更高。
- 有髓纤维：包括 A 类和 B 类纤维。最快的是 Aα 类纤维，速度约为 100 m/s；Aδ 类纤维传导速度为 4–30 m/s；降压神经的有髓纤维传导速度可达 10 m/s。
- 无髓纤维：包括 C 类纤维。它们速度最慢：约为 1 m/s；C 类纤维的速度也可为 0.5–2 m/s。
什么是绝对不应期？它对应于动作电位的哪一个时相？
绝对不应期是指膜完全不可兴奋的时期，此时无法产生新的动作电位；在此期间刺激阈值趋于无穷大，兴奋性等于零。这与钠通道的暂时性失活有关；绝对不应期是由大多数 Na⁺ 通道的失活引起的。
绝对不应期对应于峰电位时期：去极化相，并且根据关于肌肉收缩的文献资料，也对应于动作电位的初始复极化相。
如何记忆静息状态下钠通道的门控：M（m 门）——没门（关闭，阻挡进入），而 H（h 门）——好开（开启）。
课堂与考试常见问题
钠钾泵在锋电位产生过程中起什么作用？
泵本身不参与去极化和复极化的快速时相。它的任务是在动作电位结束后恢复初始的离子浓度。
为什么受到强刺激时，动作电位振幅不增加？
根据“全或无”定律，早在阈刺激时，所有可用的钠通道就会开放，产生最大的离子电流。细胞无法做出更强烈的反应。
该主题的其他拓展内容
- 心肌细胞和平滑肌中动作电位过程的特点
- 氯离子在形成后超级化中的作用
- 钾通道中电敏电荷受体的作用机制
- 持续开放的钾漏通道与静息电位的维持
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