CORTICAL NEURONS WITH PARTICULAR REFERENCE TO THE APICAL DENDRITES
CORTICAL NEURONS WITH PARTICULAR REFERENCE TO THE APICAL DENDRITES
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DOI：
10.1101/sqb.1952.017.01.019
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发表时间：
1952-01-01
期刊：
影响因子：
--
通讯作者：
CHANG, HT
中科院分区：
文献类型：
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作者：
CHANG, HT
实验有证据表明，直接电刺激皮层表面诱发的皮层电位的第一分量代表皮层锥体的顶树突的活动。通过对皮层树突电位的研究，发现了一些生理特性：（1）神经冲动沿沿着树突传播的传导速度约为2 m/s;秒临时工。超过30摄氏度。传导率随温度的升高而急剧下降。下降到28摄氏度以下。枝晶在22 ℃时停止导电。50 ℃的高温会不可逆地损害顶端树突的功能。(2)顶树突的兴奋性曲线与周围神经和脊髓的兴奋性曲线相似，只是时间过程更长。(3)树突比轴突更容易受到O2缺乏的影响，但在缺氧时树突功能衰竭之前，轴突放电停止。它被解释为是由于在轴突-体细胞连接区域建立的阻塞。阻断的确切部位位于轴突起始段的远端，该段始终没有髓鞘，是轴突最薄的部分。大脑皮层中没有郎维叶结节;然而，皮层中纤维的髓鞘不是连续的管。在侧枝循环的起源处，它们总是被无髓鞘的伸展所打断。皮质的无髓纤维多于有髓纤维。髓鞘形成不能被视为皮质神经元功能成熟的指标。在发育过程中，皮层神经元最后一个成熟的结构是芽。芽是突触前冲动的感受器。它们在顶端树突处具有最高密度，并且在细胞体上不存在。在功能上，树突上的芽构成了一个机械屏障，阻止突触结直接到达树突的茎，因此它们作为突触兴奋的限制因素。由于胚芽极细长的柄具有高欧姆电阻，它们也会延迟和减弱突触兴奋。皮层神经元之间的突触关系可分为两类：小泡周围突触和腺样体周围突触。根据突触刺激作为局部过程的原理，在启动突触后放电时，小泡周围突触的活动是最有效的，而正常情况下的腺旁突触只能产生电紧张性变化，从而改变神经元的兴奋性状态。在感觉系统的神经核中，需要传入信息的忠实和迅速的中继的突触结，在本质上大多是球体周围的。脊髓运动神经元和网状结构中的大细胞接受大量的旁突突触结。在大脑皮层中，大锥体接受来自胼胝体传入、联合纤维、锥体轴突的上行侧支等的旁腺突触结，以及来自不同皮层层中的自体细胞的短轴突的小泡周围突触结。因此，可以得出结论，在第五层的大锥体细胞可能无法直接激活的传入纤维没有援助的皮质internuncials。
Exptl. evidences indicate that the 1st component of the cortical potential evoked by direct electrical stimulation of the cortical surface represents activity of the apical dendrites of cortical pyramids. From studies on the cortical dendritic potential, some physiological properties have been found. (1) The conduction velocity of nerve impulses propagating along dendrites is about 2 m./sec. at temps. above 30[degree]C. Rate of conduction decreases sharply as the temp. drops below 28[degree]C. Dendrites cease to conduct at 22[degree]C. Heat at 50[degree]C impairs irreversibly the function of apical dendrites. (2) The excitability curve of apical dendrites is similar to that of the peripheral nerve and the spinal cord, except that the time course is much more drawn out. (3) Dendrites are more susceptible to O2 deficiency than axons, yet axonal discharges cease to take place before the functional failure of dendrites during anoxia. It is interpreted as being due to a blockage established at the region of axon-soma junction. The exact site of blockage is placed at the distal end of the initial segment of axon, which is always free of myelin sheath and is the thinnest part of the axon. There are no nodes of Ranvier in the cerebral cortex; nevertheless, myelin sheaths of the fibers in the cortex are not continuous tubes. They are always interrupted by unmyelinated stretches at the origin of collaterals. Cortex has more unmyelinated than myelinated fibers. Myelination cannot be regarded as an index of functional maturation of cortical neurons. The last structure of cortical neuron to become mature during development is the gemmules. The gemmules are the receptive apparatus for presynaptic impulses. They have the highest density at the apical dendrites and are absent on the cell body. Functionally, gemmules on the dendrites constitute a mechanical barrier preventing synaptic knobs from reaching the stem of dendrites directly, and hence they serve as a limiting factor for synaptic excitation. They also delay and attenuate synaptic excitation because of the high ohmic resistance of the gemmules'' extremely slender stalks. The synaptic relations between cortical neurons can be classified into 2 categories: the pericorpuscular synapsis and the paradendritic synapsis. According to the principle of synaptic stimulation as a local process, activity of the pericorpuscular synapsis is most effective in initiating a postsynaptic discharge, while the paradendritic synapsis under normal conditions can only create electrotonic changes so as to modify the state of excitability of a neuron. The synaptic knobs in the nuclei in the sensory system, where a faithful and prompt relay of afferent messages is required, are mostly pericorpuscular in nature. Spinal motoneurons and large cells in the reticular formation receive a great number of paradendritic synaptic knobs. In the cerebral cortex large pyramids receive paradendritic synaptic knobs from callosal afferents, association fibers, ascending collaterals of the pyramidal axons, etc., and pericorpuscular synaptic knobs from short axons of autochthonous cells in different cortical layers. It is therefore concluded that large pyramidal cells in the 5th layer probably cannot be activated directly by afferent fibers without aid of cortical internuncials.