Huh Seoan, Siripuram Ramamurthy, Lee Robert H, Turkin Vladimir V, O'Neill Derek, Hamm Thomas M, Heckman Charles J, Manuel Marin
Department of Physiology, Northwestern University, Feinberg School of Medicine, Chicago, Illinois.
Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, Georgia.
J Neurophysiol. 2017 Jul 1;118(1):93-102. doi: 10.1152/jn.00045.2017. Epub 2017 Mar 29.
The majority of studies on the electrical properties of neurons are carried out in rodents, and in particular in mice. However, the minute size of this animal compared with humans potentially limits the relevance of the resulting insights. To be able to extrapolate results obtained in a small animal such as a rodent, one needs to have proper knowledge of the rules governing how electrical properties of neurons scale with the size of the animal. Generally speaking, electrical resistances of neurons increase as cell size decreases, and thus maintenance of equal depolarization across cells of different sizes requires the underlying currents to decrease in proportion to the size decrease. Thus it would generally be expected that voltage-sensitive currents are smaller in smaller animals. In this study, we used in vivo preparations to record electrical properties of spinal motoneurons in deeply anesthetized adult mice and cats. We found that PICs do not scale with size, but instead are constant in their amplitudes across these species. This constancy, coupled with the threefold differences in electrical resistances, means that PICs contribute a threefold larger depolarization in the mouse than in the cat. As a consequence, motoneuronal firing rate sharply increases as animal size decreases. These differences in firing rates are likely essential in allowing different species to control muscles with widely different contraction speeds (smaller animals have faster muscle fibers). Thus from our results we have identified a possible new mechanism for how electrical properties are tuned to match mechanical properties within the motor output system. The small size of the mouse warrants concern over whether the properties of their neurons are a scaled version of those in larger animals or instead have unique features. Comparison of spinal motoneurons in mice to cats showed unique features. Firing rates in the mouse were much higher, in large part due to relatively larger persistent inward currents. These differences likely reflect adaptations for controlling much faster muscle fibers in mouse than cat.
大多数关于神经元电特性的研究是在啮齿动物，特别是小鼠身上进行的。然而，与人类相比，这种动物体型微小，这可能会限制由此得出的见解的相关性。为了能够推断在诸如啮齿动物这样的小动物身上获得的结果，人们需要对神经元电特性如何随动物体型缩放的相关规则有适当的了解。一般来说，神经元的电阻随着细胞大小的减小而增加，因此要在不同大小的细胞间维持相等的去极化，就需要使潜在电流按大小减小的比例相应减小。因此，通常可以预期，较小动物的电压敏感电流也较小。在本研究中，我们采用在体标本记录深度麻醉的成年小鼠和猫脊髓运动神经元的电特性。我们发现，持续性内向电流（PICs）并不随体型缩放，而是在这些物种中其幅度保持恒定。这种恒定性，再加上电阻三倍的差异，意味着PICs在小鼠中产生的去极化比在猫中大三倍。结果，随着动物体型减小，运动神经元的放电频率急剧增加。这些放电频率的差异可能对于不同物种能够以差异很大的收缩速度控制肌肉至关重要（较小的动物具有更快的肌纤维）。因此，从我们的结果中，我们确定了一种可能的新机制，即运动输出系统内的电特性是如何被调节以匹配机械特性的。小鼠体型小，这引发了人们对其神经元特性是较大动物神经元特性的缩放版本还是具有独特特征的担忧。小鼠与猫的脊髓运动神经元比较显示出独特特征。小鼠的放电频率要高得多，这在很大程度上归因于相对较大的持续性内向电流。这些差异可能反映了小鼠相比于猫在控制快得多的肌纤维方面的适应性。
