Chand Prem, Jain Neeraj
National Brain Research Centre, Manesar, Haryana, 122051, India.
National Brain Research Centre, Manesar, Haryana, 122051, India
J Neurosci. 2015 Sep 30;35(39):13475-86. doi: 10.1523/JNEUROSCI.2069-15.2015.
Brains of adult monkeys with chronic lesions of dorsal columns of spinal cord at cervical levels undergo large-scale reorganization. Reorganization results in expansion of intact chin inputs, which reactivate neurons in the deafferented hand representation in the primary somatosensory cortex (area 3b), ventroposterior nucleus of the thalamus and cuneate nucleus of the brainstem. A likely contributing mechanism for this large-scale plasticity is sprouting of axons across the hand-face border. Here we determined whether such sprouting takes place in area 3b. We first determined the extent of intrinsic corticocortical connectivity between the hand and the face representations in normal area 3b. Small amounts of neuroanatomical tracers were injected in these representations close to the electrophysiologically determined hand-face border. Locations of the labeled neurons were mapped with respect to the detailed electrophysiological somatotopic maps and histologically determined hand-face border revealed in sections of the flattened cortex stained for myelin. Results show that intracortical projections across the hand-face border are few. In monkeys with chronic unilateral lesions of the dorsal columns and expanded chin representation, connections across the hand-face border were not different compared with normal monkeys. Thalamocortical connections from the hand and face representations in the ventroposterior nucleus to area 3b also remained unaltered after injury. The results show that sprouting of intrinsic connections in area 3b or the thalamocortical inputs does not contribute to large-scale cortical plasticity. Significance statement: Long-term injuries to dorsal spinal cord in adult primates result in large-scale somatotopic reorganization due to which chin inputs expand into the deafferented hand region. Reorganization takes place in multiple cortical areas, and thalamic and medullary nuclei. To what extent this brain reorganization due to dorsal column injuries is related to axonal sprouting is not known. Here we show that reorganization of primary somatosensory area 3b is not accompanied with either an increase in intrinsic cortical connections between the hand and face representations, or any change in thalamocortical inputs to these areas. Axonal sprouting that causes reorganization likely takes place at subthalamic levels.
成年猴子若其颈部脊髓背柱出现慢性损伤，大脑会发生大规模重组。重组会导致完整的颏部输入扩张，从而重新激活初级体感皮层（3b区）、丘脑腹后核以及脑干楔束核中去传入手部表征区域的神经元。这种大规模可塑性的一个可能促成机制是轴突在手-面边界处的萌出。在此，我们确定这种萌出是否发生在3b区。我们首先确定了正常3b区中手部和面部表征之间皮质内固有连接的范围。在这些靠近电生理确定的手-面边界的表征区域内注射少量神经解剖示踪剂。相对于详细的电生理躯体定位图以及在髓鞘染色的扁平皮质切片中组织学确定的手-面边界，对标记神经元的位置进行映射。结果显示，跨越手-面边界的皮质内投射很少。在患有慢性单侧背柱损伤且颏部表征扩张的猴子中，与正常猴子相比，跨越手-面边界的连接并无差异。损伤后，丘脑腹后核中手部和面部表征到3b区的丘脑皮质连接也保持不变。结果表明，3b区内固有连接或丘脑皮质输入的萌出并非大规模皮质可塑性的原因。意义声明：成年灵长类动物脊髓背侧的长期损伤会导致大规模躯体定位重组，颏部输入借此扩展到去传入的手部区域。重组发生在多个皮质区域、丘脑和髓核中。目前尚不清楚这种因背柱损伤导致的大脑重组在多大程度上与轴突发芽有关。在此我们表明，初级体感3b区的重组既不伴随着手部和面部表征之间皮质内固有连接的增加，也不伴随着这些区域丘脑皮质输入的任何变化。导致重组的轴突发芽可能发生在丘脑下水平。
