Schwindt P, O'Brien J A, Crill W
Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle 98195-7290, USA.
J Neurophysiol. 1997 May;77(5):2484-98. doi: 10.1152/jn.1997.77.5.2484.
Quantitative aspects of repetitive firing evoked by injected current steps and ramps were studied in layer 5 pyramidal neurons in brain slices of rat sensorimotor cortex to answer the following questions. Do the tonic firing properties of burst-firing and regular-spiking (nonbursting) neurons differ significantly? Does burst firing denote a discrete class of neurons or represent a continuum of firing properties? Is firing rate during the burst of action potentials related to stimulus amplitude? What aspect of the stimulus might the initial firing rate code? How stable are a neuron's firing properties over time? All recorded neurons fired tonically to a long-lasting current above a minimum value, and the tonic firing properties of most neurons were quite similar irrespective of their initial response to a current step. Only a group of high-resistance neurons had significantly different tonic firing properties. When slow current ramps (rising between 0.5 and approximately 20 nA/s) were applied, the relation between firing rate and current during the ramp was very similar to the relation between tonic firing rate and current obtained from long-lasting current steps. Low-resistance cells exhibited three distinct initial responses to a current step: fast adaptation, high-threshold bursts, and low-threshold bursts, observed in 54, 28, and 10% of recorded cells, respectively. High-resistance cells exhibited a distinctive slow adaptation of firing rate. Slowly adapting, fast-adapting (FA), and high-threshold burster (HTB) neurons exhibited no adaptation near the minimum current that evoked repetitive firing (I(o)). FA and HTB cells exhibited two-spike adaptation to a fina tonic firing rate during currents up to 1.6 times I(o). Only a higher current (2.1 times I(o)) evoked a burst in HTB cells, whereas a burst was evoked at I(o) in the low-threshold burster cells. In most cells analyzed, the initial firing rate, whatever its nature, increased monotonically with current step amplitude. The response to fast current ramps indicated that firing rate during adaptation or bursting may code rate of change of current. Repeated measurements during long-duration impalements indicated that both transient and tonic firing properties are stable over time. We discuss how the different tonic firing properties of large and small pyramidal neurons could be more important functionally than the different transient responses (burst/nonburst) of the large neurons. We conclude that the large neurons would perform a better linear transduction of time-varying synaptic current that reaches their somata. We compare the responses evoked by somatically injected current with those evoked by dendritic glutamate iontophoresis in previous studies.
通过注入电流阶跃和斜坡来诱发重复放电，对大鼠感觉运动皮层脑片第5层锥体神经元重复放电的定量方面进行了研究，以回答以下问题。爆发式放电神经元和规则放电（非爆发式）神经元的强直放电特性是否有显著差异？爆发式放电是表示一类离散的神经元，还是代表放电特性的连续统一体？动作电位爆发期间的放电频率与刺激幅度有关吗？初始放电频率可能编码刺激的哪个方面？神经元的放电特性随时间的稳定性如何？所有记录的神经元对高于最小值的持续电流都会产生强直放电，并且大多数神经元的强直放电特性非常相似，无论它们对电流阶跃的初始反应如何。只有一组高电阻神经元具有显著不同的强直放电特性。当施加缓慢的电流斜坡（上升速率在0.5至约20 nA/s之间）时，斜坡期间放电频率与电流之间的关系与从持续电流阶跃获得的强直放电频率与电流之间的关系非常相似。低电阻细胞对电流阶跃表现出三种不同的初始反应：快速适应、高阈值爆发和低阈值爆发，分别在54%、28%和10%的记录细胞中观察到。高电阻细胞表现出独特的放电频率缓慢适应。缓慢适应、快速适应（FA）和高阈值爆发（HTB）神经元在诱发重复放电的最小电流（I(o)）附近没有表现出适应。FA和HTB细胞在电流达到1.6倍I(o)时，对最终的强直放电频率表现出双脉冲适应。只有更高的电流（2.1倍I(o)）会在HTB细胞中诱发爆发，而在低阈值爆发细胞中，在I(o)时就会诱发爆发。在大多数分析的细胞中，无论其性质如何，初始放电频率都随电流阶跃幅度单调增加。对快速电流斜坡的反应表明，适应或爆发期间的放电频率可能编码电流的变化率。在长时间刺入过程中的重复测量表明，瞬态和强直放电特性随时间都是稳定的。我们讨论了大小锥体神经元不同的强直放电特性在功能上可能比大神经元不同的瞬态反应（爆发/非爆发）更重要。我们得出结论，大神经元对到达其胞体的随时间变化的突触电流将进行更好的线性转换。我们将体细胞注入电流诱发的反应与先前研究中树突谷氨酸离子导入诱发的反应进行了比较。
