Ahmad A, Spear P D
Department of Psychology, University of Wisconsin-Madison 53706.
J Comp Neurol. 1993 Aug 22;334(4):631-43. doi: 10.1002/cne.903340410.
Visual abilities decline during aging, and many of these declines are due to neural changes in the retina or brain. We have begun studies of the monkey visual system to investigate the location and nature of these changes as well as to answer general questions about the effects of aging on neural structure and function. We began with the dorsal lateral geniculate nucleus (LGN) because it is the main structure through which visual information passes on the way to cortex and because the parallel parvicellular and magnocellular pathways are most easily identified and studied in the LGN. In the present experiment, we determined the sizes, densities, and numbers of LGN neurons in young-adult (5 to 12.5 years) and old (23 to 27.5 years) rhesus monkeys. The measures were corrected for tissue shrinkage, and stereological procedures were used that yield unbiased estimates. In young-adult monkeys, neurons densities were lower in the magnocellular layers (about 14,000/mm3) than in the parvicellular layers (23,000/mm3). Neuron density increased about 28% from anterior to posterior in both types of layers. There was an average of approximately 1,267,000 neurons in the parvicellular layers and 148,000 neurons in the magnocellular layers; however, there was substantial variability (1.9-fold) among five brains studied. Aging produced a statistically significant decrease in neuron density in both the magnocellular (29% average decrease) and parvicellular (41% average decrease) layers. However, there was no significant loss of neurons. Rather, the density decrease was due to a small (nonsignificant) decrease in the number of neurons combined with a small (nonsignificant) increase in LGN volume. The increase in LGN volume was due to a significant increase in neuron soma-size and proportional increase in the volume of glial cells, blood vessels, and neuropil. These results, together with those of other studies, suggest that the effects of aging on the primate visual pathway from retina through striate cortex are relatively subtle. It is possible that the major neural changes occur more centrally. Alternatively, individual differences in the effect of aging may require much larger samples or prior screening to observe consistent changes.
视觉能力在衰老过程中会下降，其中许多下降是由于视网膜或大脑的神经变化所致。我们已经开始对猴子视觉系统进行研究，以探究这些变化的位置和性质，并回答有关衰老对神经结构和功能影响的一般性问题。我们从背外侧膝状体核（LGN）开始研究，因为它是视觉信息传递到皮层的主要结构，而且在LGN中最容易识别和研究平行的小细胞和大细胞通路。在本实验中，我们测定了年轻成年（5至12.5岁）和老年（23至27.5岁）恒河猴LGN神经元的大小、密度和数量。这些测量值已针对组织收缩进行了校正，并采用了能得出无偏估计的体视学方法。在年轻成年猴子中，大细胞层的神经元密度（约14,000个/mm³）低于小细胞层（23,000个/mm³）。两种类型的层中，神经元密度从前向后均增加约28%。小细胞层平均约有1,267,000个神经元，大细胞层平均有148,000个神经元；然而，在所研究的五个大脑中存在很大的变异性（1.9倍）。衰老导致大细胞层（平均下降29%）和小细胞层（平均下降41%）的神经元密度出现统计学上的显著下降。然而，神经元数量没有显著减少。相反，密度下降是由于神经元数量的小幅（不显著）减少以及LGN体积的小幅（不显著）增加所致。LGN体积的增加是由于神经元胞体大小的显著增加以及神经胶质细胞、血管和神经纤维体积的成比例增加。这些结果与其他研究结果一起表明，衰老对从视网膜到纹状皮层的灵长类视觉通路的影响相对微妙。可能主要的神经变化发生在更中枢的部位。或者，衰老影响的个体差异可能需要更大的样本量或预先筛选才能观察到一致的变化。
