Tobias M L, Kelley D B
Department of Biological Sciences, Columbia University, New York, New York 10027.
J Neurosci. 1988 Jul;8(7):2422-9. doi: 10.1523/JNEUROSCI.08-07-02422.1988.
Sex differences at the laryngeal neuromuscular junction of Xenopus laevis were examined by recording intracellularly from muscle fibers in response to nerve stimulation. Male laryngeal muscle contains 2 physiologically distinct fiber types. Type I fibers generate postsynaptic potentials in response to low-magnitude stimulus pulses and action potentials in response to higher-magnitude stimulus pulses. Type II muscle fibers require repetitive stimulation for action potential production, probably because of facilitation. Subthreshold events in type I and II fibers suggest that these neuromuscular synapses have low safety factor junctions. Female laryngeal muscle contains one fiber type (III), which is physiologically distinct from those found in the male. Type III fibers produce an action potential in response to a single-stimulus pulse of suprathreshold voltage delivered to the laryngeal nerve; subthreshold events were not observed. Iontophoretic injection of Lucifer yellow into a single female muscle fiber resulted in as many as 43 labeled fibers. In males, only one fiber was labeled. Dye-coupling was not observed in adult females treated with the androgenic steroid hormone, testosterone. We have previously reported that laryngeal muscle fibers are recruited throughout a stimulus train presented to the laryngeal nerve in males, but are not recruited in females (Tobias and Kelly, 1987). Sex differences in the frequency of electrophysiological fiber types described here may account for sex differences in fiber recruitment. Synchronous activity of dye-coupled fibers may increase the effectiveness of muscle contraction in females.
通过记录非洲爪蟾喉神经肌肉接头处肌肉纤维对神经刺激的细胞内反应，研究了其性别差异。雄性喉肌包含两种生理上不同的纤维类型。I型纤维在低强度刺激脉冲作用下产生突触后电位，在高强度刺激脉冲作用下产生动作电位。II型肌纤维需要重复刺激才能产生动作电位，这可能是由于易化作用。I型和II型纤维中的阈下事件表明，这些神经肌肉突触具有低安全系数的连接。雌性喉肌包含一种纤维类型（III型），其在生理上与雄性中的纤维不同。III型纤维在向喉神经传递阈上电压的单个刺激脉冲作用下产生动作电位；未观察到阈下事件。将荧光黄离子导入单个雌性肌纤维可导致多达43条纤维被标记。在雄性中，仅一条纤维被标记。在用雄激素类固醇激素睾酮处理的成年雌性中未观察到染料偶联现象。我们之前报道过，在向雄性喉神经施加的一串刺激过程中，喉肌纤维会被募集，但在雌性中则不会被募集（托拜厄斯和凯利，1987年）。此处描述的电生理纤维类型频率的性别差异可能解释了纤维募集的性别差异。染料偶联纤维的同步活动可能会增加雌性肌肉收缩的有效性。
