Merrill E G, Lipski J
J Neurophysiol. 1987 Jun;57(6):1837-53. doi: 10.1152/jn.1987.57.6.1837.
The investigation examined the synaptic input from medullary respiratory neurons in the nucleus retroambigualis (NRA) to external (EIM) and internal (IIM) intercostal motoneurons. Antidromic mapping revealed that 112/117 (96%) tested NRA units had axons descending into thoracic spinal cord with extensive arborizations at many thoracic segments, mainly contralaterally. The conduction velocities ranged from 10 to 105 m X s-1. The descending projections did not appear to be somatotopically arranged. Cross-correlation of the spike trains of NRA inspiratory units with the discharge of external intercostal nerves (performed usually with 4 contralateral nerves) showed significant narrow peaks only in 5 out of 40 averages. Of the 25 trigger units tested for the thoracic projection in this series of experiments, 24 were antidromically activated. Intracellular recordings were made from 52 IIMs [mean membrane potential 65.3 mV, central respiratory drive potentials (CRDPs) greater than 1 mV present in 23/52] and 53 EIM (mean membrane potential 54.3 mV, CRDPs in 31/53). During the depolarizing phase of the CRDPs, synaptic noise with frequent and apparently unitary EPSPs with amplitudes in excess of 1 mV was observed. Spike-triggered averages of synaptic noise were computed for 153 pairings between 137 NRA neurons and 105 contralateral intercostal motoneurons. Only four PSPs were revealed: two monosynaptic EPSPs between expiratory NRA units and IIMs and two probably disynaptic EPSPs between inspiratory NRA units and EIMs. When advancing the microelectrode down to the motoneuron pools, frequent recordings were made from interneurons with spontaneous respiratory discharge (inspiratory or expiratory) located dorsal and medial to the motor nuclei. The interneurons could be excited following stimulation of segmental afferents. It is concluded that monosynaptic connections between respiratory NRA neurons and intercostal motoneurons are rare (connectivity no more than approximately 4%). Segmental interneurons, interposed between the majority of descending respiratory axons and intercostal motoneurons, are likely to produce large unitary EPSPs and, thus, short-term synchronization in the discharge of intercostal motoneurons as observed by others.
该研究检测了延髓疑后核（NRA）中髓质呼吸神经元至肋间外肌运动神经元（EIM）和肋间内肌运动神经元（IIM）的突触输入。逆向映射显示，117个被检测的NRA单位中有112个（96%）的轴突下行至胸段脊髓，并在多个胸段广泛分支，主要在对侧。传导速度范围为10至105 m·s⁻¹。下行投射似乎没有按躯体定位排列。NRA吸气单位的峰电位序列与肋间外神经放电的互相关（通常对4条对侧神经进行）显示，在40次平均中只有5次出现显著的窄峰。在这一系列实验中，对25个用于检测胸段投射的触发单位进行检测，其中24个被逆向激活。对52个IIM进行了细胞内记录[平均膜电位65.3 mV，52个中有23个存在大于1 mV的中枢呼吸驱动电位（CRDPs）]以及53个EIM（平均膜电位54.3 mV，53个中有31个存在CRDPs）。在CRDPs的去极化阶段，观察到具有频繁且明显单一的兴奋性突触后电位（EPSPs）的突触噪声，其幅度超过1 mV。计算了137个NRA神经元与105个对侧肋间运动神经元之间153对配对的突触噪声的峰电位触发平均值。仅发现了4个突触后电位：呼气NRA单位与IIM之间有2个单突触EPSPs，吸气NRA单位与EIM之间有2个可能的双突触EPSPs。当将微电极向下推进到运动神经元池时，经常记录到位于运动核背侧和内侧的具有自发呼吸放电（吸气或呼气）的中间神经元。刺激节段性传入神经后，这些中间神经元可被兴奋。得出的结论是，呼吸NRA神经元与肋间运动神经元之间的单突触连接很少见（连接性不超过约4%）。介于大多数下行呼吸轴突与肋间运动神经元之间的节段性中间神经元可能会产生大的单一EPSPs，从而如其他人所观察到的那样，在肋间运动神经元的放电中产生短期同步。
