Neuromuscular Junction, Animation · Alila Medical Media
讲解神经肌肉接头的结构与动作电位生成过程，适合练习神经生物学相关知识点。
A skeletal muscle contracts only when stimulated by nerve impulses from a motor neuron.
骨骼肌只有在受到运动神经元发出的神经冲动刺激时才会发生收缩。
Without innervation, muscles become paralyzed and eventually atrophied.
如果没有神经支配，肌肉就会瘫痪，最终还会发生萎缩。
The axon of a motor neuron usually gives out many branches, supplying multiple muscle fibers.
运动神经元的轴突通常会分出许多分支，为多条肌肉纤维提供神经冲动。
These fibers contract in unison when activated, and constitute a motor unit.
当这些纤维被激活时，它们会同时收缩，从而形成一个运动单位（motor unit）。
A motor unit can contain anywhere from just a few muscle fibers, to thousands of them.
一个运动单位（motor unit）可以包含少数几根肌肉纤维，也可以包含数千根肌肉纤维。
Small motor units are found in muscles that require finer control, for example, muscles that are responsible for subtle movements of the eyes.
那些需要更精细控制的肌肉中，会存在较小的运动单位（即负责肌肉运动的神经细胞群）；例如，控制眼球细微运动的肌肉就属于这类肌肉。
Large motor units are found in larger muscles that require strength, such as muscles of the arms and legs.
大肌群单位存在于那些需要强大力量的较大肌肉中，例如手臂和腿部的肌肉。
Muscle fibers of the same motor unit are usually not clustered together, but scattered throughout the muscle.
同一运动单位的肌肉纤维通常不会聚集在一起，而是分散在整个肌肉中。
This way, stimulation of a motor unit has a slight but uniform effect on the whole muscle, and not causing a small localized twitch.
这样一来，对运动单位的刺激会对整个肌肉产生轻微但均匀的影响，而不会导致局部性的肌肉抽搐。
The strength of a muscle contraction is determined by the number of motor units that are activated at one time.
肌肉收缩的强度取决于同一时间被激活的运动单位（motor units）的数量。
Even at rest, most muscles are in
即使在休息状态下，大多数肌肉也处于“活跃”状态（即仍在进行微小的收缩或放松运动）。
a partial contraction state, called muscle tonus, which is maintained by alternating activation of a small number of motor units.
这是一种部分收缩的状态，被称为“肌肉张力”（muscle tone）；这种状态是通过交替激活少量的运动单位（motor units）来维持的。
The connection between a motor neuron and
运动神经元与……之间的连接
a muscle fiber is called a neuromuscular
肌纤维被称为神经肌肉纤维（neuromuscular fiber）。
junction, which is basically a chemical synapse between the nerve terminal and a specialized area of muscle cell membrane called the motor end-plate.
这种连接结构本质上属于一种化学突触（即神经末梢与肌肉细胞膜上特定区域之间的信号传递机制），该特定区域被称为“运动终板”（motor end-plate）。
When an action potential reaches the nerve terminal, it causes the release of the neurotransmitter acetylcholine into the synaptic space.
当动作电位到达神经末梢时，会促使神经递质乙酰胆碱释放到突触间隙中。
Acetylcholine then binds to nicotinic receptors on the end-plate.
随后，乙酰胆碱会与末板（end-plate）上的烟碱受体结合。
Nicotinic receptors are ligand-gated ion channels.
尼古丁受体属于配体门控型离子通道（ligand-gated ion channels）。
Upon binding to acetylcholine, they open to allow sodium to enter the cells, depolarizing the cell membrane, producing the so-called end-plate potential.
当这些离子与乙酰胆碱结合后，它们会打开通道，使钠离子进入细胞内部，从而导致细胞膜去极化，从而产生所谓的“终板电位”（end-plate potential）。
An action potential is generated in the
动作电位是在……中产生的。
muscle cell only when the end-plate potential reaches the threshold required to activate voltage-gated sodium channels located outside the end-plate,
肌细胞只有在终板电位达到激活位于终板外部的电压门控钠通道所需的阈值时才会发生反应。
in the neighboring membrane.
在相邻的膜中。
When activated, these channels allow faster influx of sodium, further depolarizing and eventually reversing the polarity of the cell membrane.
当这些通道被激活后，钠离子会更快地流入细胞内，从而导致细胞膜进一步去极化，并最终改变细胞膜的极性。
At this point, voltage-gated potassium channels open for potassium to move out, quickly returning membrane voltage to its original resting value.
此时，电压门控钾通道会被打开，钾离子会从中流出，从而使膜电位迅速恢复到其正常的静息状态。
Once generated, the action potential spreads like a wave thanks to similar voltage-gated ion channels located throughout the muscle fiber.
一旦动作电位产生，它就会像波浪一样在肌肉纤维中传播——这是因为肌肉纤维中分布着许多结构相似的电压门控离子通道。
The action potential also runs deep into the fiber via T-tubules, to reach the sarcoplasmic reticulum.
动作电位还会通过 T-管深入纤维内部，最终抵达肌浆网。
Here, it activates voltage-gated calcium channels, releasing calcium from the sarcoplasmic reticulum into the cytosol of muscle cells.
在这里，电压门控钙通道被激活，从而将钙离子从肌浆网释放到肌肉细胞的细胞质中。
Calcium then sets off muscle contraction by the “sliding filament mechanism”.
钙离子通过“滑行丝机制”引发肌肉收缩。
This mechanism is described in another video.
这一机制在另一个视频中有详细说明。
Another important component of the neuromuscular junction is the enzyme acetylcholinesterase.
神经肌肉接头（neuromuscular junction）的另一个重要组成部分是乙酰胆碱酯酶（acetylcholinesterase）这种酶。
This enzyme removes all acetylcholine molecules that do not immediately bind with a receptor and those that are done activating a receptor.
这种酶会清除所有未能立即与受体结合的乙酰胆碱分子，以及那些已经激活了受体的乙酰胆碱分子。
The enzyme action essentially terminates synaptic activation, giving the muscle time to relax, and thus preventing continuous contraction that would result in muscle spasms.
这种酶的作用实质上终止了突触的激活过程，使肌肉有时间放松，从而避免了持续的收缩——因为持续的收缩会导致肌肉痉挛。
Substances that cause muscle weakness or paralysis
会导致肌肉无力或瘫痪的物质
do so by interfering with the function of neuromuscular junction: - Botulinum toxin prevents acetylcholine release from the presynaptic side of the junction.
它们通过干扰神经肌肉接头（neuromuscular junction）的功能来实现这一目的：肉毒杆菌毒素会阻止乙酰胆碱从神经突触前末端释放出来。
- Some other toxins attach to nicotinic receptor, blocking acetylcholine from binding, but do not open the ion channel.
还有一些其他毒素会附着在尼古丁受体上，从而阻止乙酰胆碱与其结合，但这些毒素并不会打开离子通道。
- Certain drugs lodge into the channel of nicotinic receptor, blocking the passage of sodium.
某些药物会滞留在尼古丁受体的通道中，从而阻断钠离子的通过。
All these substances prevent activation of muscle cells and cause flaccid paralysis.
所有这些物质都会抑制肌肉细胞的活性，从而导致肌肉松弛性麻痹（即肌肉无法正常收缩）。
On the other hand, some pesticides inhibit acetylcholinesterase, preventing degradation of acetylcholine, causing continuous activation of muscles.
另一方面，某些杀虫剂会抑制乙酰胆碱酯酶的活性，从而阻止乙酰胆碱的降解，导致肌肉持续处于兴奋状态。
That’s how they induce muscle spasms and cause spastic paralysis.
他们就是通过这种方式诱发肌肉痉挛，从而导致痉挛性瘫痪的。