Shortland P, Fitzgerald M
Department of Anatomy, University College London.
Eur J Neurosci. 1994 Jan 1;6(1):75-86. doi: 10.1111/j.1460-9568.1994.tb00249.x.
Previous studies have shown that following neonatal peripheral nerve injury, adjacent intact myelinated and unmyelinated primary afferents sprout into the central denervated terminal area. The present study investigates this in more detail and goes further, to study the fate of the central terminals of the surviving axotomized primary afferent neurons. Bulk labelling of the sciatic and saphenous nerves with horseradish peroxidase conjugated to choleragenoid (B-HRP), to label the A fibres, or wheatgerm agglutinin (WGA-HRP), to label C fibres were employed to investigate the central consequences of sciatic nerve section and ligation on the day of birth, in adult rats. Bulk labelling of the axotomized sciatic or intact saphenous nerve with either tracer and comparison with contralateral controls revealed alterations to the terminal field. The intact saphenous nerve terminal field expanded caudally from mid L4 to the L4-L5 boundary when labelled with WGA-HRP and to the sacral cord when labelled with B-HRP. Labelling the axotomized sciatic nerve with either tracer revealed little change in the overall somatotopic organization of central terminals, although labelling was less intense compared to control nerves and more variable with WGA-HRP. Invasion of the substantia gelatinosa (SG) by axotomized A fibres was observed in segments L3-5, into the area occupied by axotomized C fibres. This area was also invaded by intact saphenous A fibres in the L4-5 segments. These results demonstrate that following neonatal nerve section: (i) axotomized primary afferents are able to retain a 'normal' somatotopic map in the rostrocaudal plane; (ii) both A and C fibres from adjacent intact nerves sprout into the denervated territory, but A fibres sprout further caudally; (iii) axotomized A fibres and invading intact A fibres both sprout dorsally into denervated SG. As a result, there is considerable overlap between nerve territories in denervated spinal cord, suggesting that competition for laminar termination sites exists between A and C fibres and also between axotomized and intact primary afferents.
以往研究表明，新生儿外周神经损伤后，相邻完整的有髓和无髓初级传入神经会向中枢去神经支配的终末区域生长。本研究对此进行了更详细的调查，并进一步研究了存活的轴突切断初级传入神经元中枢终末的命运。在成年大鼠出生当天，采用与霍乱毒素结合的辣根过氧化物酶（B-HRP）对坐骨神经和隐神经进行大量标记以标记A纤维，或采用小麦胚芽凝集素（WGA-HRP）标记C纤维，以研究坐骨神经切断和结扎的中枢后果。用这两种示踪剂对轴突切断的坐骨神经或完整的隐神经进行大量标记，并与对侧对照进行比较，结果显示终末区域发生了改变。当用WGA-HRP标记时，完整隐神经的终末区域从L4中部向尾侧扩展至L4-L5边界，而用B-HRP标记时则扩展至骶髓。用这两种示踪剂标记轴突切断的坐骨神经，结果显示中枢终末的整体躯体定位组织变化不大，尽管与对照神经相比标记强度较低，且WGA-HRP标记的变化更大。在L3-5节段观察到轴突切断的A纤维侵入胶状质（SG），侵入到轴突切断的C纤维所占据的区域。在L4-5节段，完整的隐神经A纤维也侵入了该区域。这些结果表明，新生儿神经切断后：（i）轴突切断的初级传入神经能够在头尾平面上保留“正常”的躯体定位图谱；（ii）来自相邻完整神经的A纤维和C纤维都会向去神经支配区域生长，但A纤维向更尾侧生长；（iii）轴突切断的A纤维和侵入的完整A纤维都会背侧生长到去神经支配的SG。因此，去神经支配脊髓中的神经区域之间存在相当大的重叠，这表明A纤维和C纤维之间以及轴突切断的和完整的初级传入神经之间存在对层状终末位点的竞争。
