Cramer K S, Van Essen D C
Division of Biology 216-76, California Institute of Technology, Pasadena 91125, USA.
Dev Biol. 1995 Sep;171(1):16-26. doi: 10.1006/dbio.1995.1256.
In adult mammalian skeletal muscles, fast and slow muscle fibers are selectively innervated by single inputs from corresponding motor neuron types, giving rise to fast and slow motor units. At birth, however, muscle fibers are polyinnervated, and connections between motor neurons and muscle fibers are not as specific as those found in adults. Excess synapses are removed during the first few postnatal weeks. In addition to changes in the degree of polyinnervation, motor unit types undergo maturation in their contractile properties. In this study, we have investigated the maturation of motor unit types during postnatal synapse elimination in the rabbit soleus muscle. The ratio of twitch tension to tetanic tension in a motor unit is an indication of its contractile type. Our results indicate that during synapse elimination, the twitch/tetanus ratios for fast motor units increase while those for slow motor units decrease. The ratio of motor unit tension at polyinnervated ages to that at singly innervated ages has previously been used to estimate the degree of polyinnervation for fast versus slow muscle fibers. We found that twitch and tetanic tension yield conflicting estimates of polyinnervation. This discrepancy was resolved on the basis of intracellular recordings of endplate potentials. Using latencies to endplate potentials as an indicator of muscle fiber type, we found that fast and slow muscle fibers are polyinnervated to a similar extent during both early and intermediate stages of synapse elimination, suggesting that specific tension, and not polyinnervation, changes differently in fast versus slow muscle fibers. These changes are consistent with those we found in twitch/tetanus ratios. Furthermore, these intracellular recordings suggest a high degree of specificity at birth, which is further refined during synapse elimination.
在成年哺乳动物的骨骼肌中，快肌纤维和慢肌纤维分别由相应类型的运动神经元的单一输入选择性地支配，从而形成快运动单位和慢运动单位。然而，在出生时，肌纤维是多神经支配的，运动神经元与肌纤维之间的连接不像在成年动物中那样特异。多余的突触在出生后的头几周内被消除。除了多神经支配程度的变化外，运动单位类型在收缩特性方面也会经历成熟过程。在本研究中，我们调查了兔比目鱼肌出生后突触消除过程中运动单位类型的成熟情况。运动单位中强直收缩张力与单收缩张力的比值是其收缩类型的一个指标。我们的结果表明，在突触消除过程中，快运动单位的单收缩/强直收缩比值增加，而慢运动单位的该比值降低。此前，多神经支配时期运动单位的张力与单神经支配时期运动单位的张力之比已被用于估计快肌纤维和慢肌纤维的多神经支配程度。我们发现，单收缩张力和强直收缩张力对多神经支配程度的估计相互矛盾。基于终板电位的细胞内记录解决了这一差异。以终板电位的潜伏期作为肌纤维类型的指标，我们发现，在突触消除的早期和中期，快肌纤维和慢肌纤维的多神经支配程度相似，这表明快肌纤维和慢肌纤维的比张力而非多神经支配程度变化不同。这些变化与我们在单收缩/强直收缩比值中发现的变化一致。此外，这些细胞内记录表明出生时存在高度特异性，在突触消除过程中这种特异性会进一步细化。
