Nanoconnectomic upper bound on the variability of synaptic plasticity.
Nanoconnectomic upper bound on the variability of synaptic plasticity.
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DOI：
10.7554/elife.10778
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发表时间：
2015-11-30
期刊：
影响因子：
7.7
通讯作者：
Sejnowski TJ
中科院分区：
文献类型：
--
作者：
Bartol TM;Bromer C;Kinney J;Chirillo MA;Bourne JN;Harris KM;Sejnowski TJ
计算机中的信息是通过可以存储和恢复的比特数来量化的。关于大脑的一个重要问题是，通过突触的可塑性，一个突触可以存储多少信息，这取决于概率突触活动的历史。突触的大小和功效之间的强相关性使我们能够估计突触可塑性的可变性。在海马体神经元的EM重建中，我们发现单个轴突在相同的树突上进行两个或多个突触接触，具有共同的突触前和突触后活动历史。这两对的脊柱头部和颈部直径，而不是颈部长度，在大小上几乎相同。我们发现至少有26个可区分的突触强度，对应于每个突触存储4.7比特的信息。由于突触激活的随机可变性，观察到的精度需要在几分钟内平均活动。DOI: http://dx.doi.org/10.7554/eLife.10778.001人类大脑的记忆容量是多少？计算机内存中的存储容量以位为单位测量，每个位的值可以是0或1。在大脑中，信息以突触强度的形式存储，突触强度是衡量一个神经元的活动对与之相连的另一个神经元的影响程度。不同强度的数量可以用比特来衡量。因此，大脑的总存储容量取决于突触的数量和可区分的突触强度的数量。在结构上，神经元由胞体组成，胞体通过索状轴突影响其他神经元。细胞体上有许多被称为树突的短分支，树突上覆盖着微小的突起，或称为“刺”。大多数兴奋性突触形成于一个神经元的轴突和另一个神经元的树突棘之间。当突触两侧的两个神经元同时活跃时，突触就会变得更强，这是一种记忆形式。树突棘也变得更大，以容纳支持更强突触所需的额外分子机制。一些轴突与相同的树突形成两个或更多的突触，但在不同的树突棘上。这些突触的强度应该是相同的，因为它们将经历相同的神经活动历史。Bartol等人使用了一种称为连续切片电子显微镜的技术，对大脑的一部分进行了三维重建，从而可以比较这些突触形成的树突棘的大小。结果显示，脊椎头部的突触区域和体积几乎相同。这种显著的相似性可以用来估计单个突触可以存储的信息量，因为树突棘的大小和它们的突触可以作为突触强度的代表。对一小块脑组织的测量揭示了26种不同的树突棘大小，每一种都与不同的突触强度有关。这个数字转化为每个突触大约4.7位信息的存储容量。这一估计明显高于以前的建议。这意味着大脑的总记忆容量——拥有数万亿个突触——可能被低估了一个数量级。要证实这种可能性，还需要在大脑的同一区域和其他区域进行额外的测量。DOI: http://dx.doi.org/10.7554/eLife.10778.002
Information in a computer is quantified by the number of bits that can be stored and recovered. An important question about the brain is how much information can be stored at a synapse through synaptic plasticity, which depends on the history of probabilistic synaptic activity. The strong correlation between size and efficacy of a synapse allowed us to estimate the variability of synaptic plasticity. In an EM reconstruction of hippocampal neuropil we found single axons making two or more synaptic contacts onto the same dendrites, having shared histories of presynaptic and postsynaptic activity. The spine heads and neck diameters, but not neck lengths, of these pairs were nearly identical in size. We found that there is a minimum of 26 distinguishable synaptic strengths, corresponding to storing 4.7 bits of information at each synapse. Because of stochastic variability of synaptic activation the observed precision requires averaging activity over several minutes. DOI: http://dx.doi.org/10.7554/eLife.10778.001 What is the memory capacity of a human brain? The storage capacity in a computer memory is measured in bits, each of which can have a value of 0 or 1. In the brain, information is stored in the form of synaptic strength, a measure of how strongly activity in one neuron influences another neuron to which it is connected. The number of different strengths can be measured in bits. The total storage capacity of the brain therefore depends on both the number of synapses and the number of distinguishable synaptic strengths. Structurally, neurons consist of a cell body that influences other neurons through a cable-like axon. The cell body bears numerous short branches called dendrites, which are covered in tiny protrusions, or “spines”. Most excitatory synapses are formed between the axon of one neuron and a dendritic spine on another. When two neurons on either side of a synapse are active simultaneously, that synapse becomes stronger, a form of memory. The dendritic spine also becomes larger to accommodate the extra molecular machinery needed to support a stronger synapse. Some axons form two or more synapses with the same dendrite, but on different dendritic spines. These synapses should be the same strength because they will have experienced the same history of neural activity. Bartol et al. used a technique called serial section electron microscopy to create a 3D reconstruction of part of the brain that allowed the sizes of the dendritic spines these synapses form on to be compared. This revealed that the synaptic areas and volumes of the spine heads were nearly identical. This remarkable similarity can be used to estimate the number of bits of information that a single synapse can store, since the size of dendritic spines and their synapses can be used as proxies for synaptic strength. Measurements in a small cube of brain tissue revealed 26 different dendritic spine sizes, each associated with a distinct synaptic strength. This number translates into a storage capacity of roughly 4.7 bits of information per synapse. This estimate is markedly higher than previous suggestions. It implies that the total memory capacity of the brain – with its many trillions of synapses – may have been underestimated by an order of magnitude. Additional measurements in the same and other brain regions are needed to confirm this possibility. DOI: http://dx.doi.org/10.7554/eLife.10778.002