Ebner F F, Armstrong-James M A
Neurobiology Section, Brown University, Providence, RI 02912.
Prog Brain Res. 1990;86:129-41. doi: 10.1016/s0079-6123(08)63172-6.
The mechanisms that link sensory inputs in spatially separated regions of cortex can be elucidated by analyzing the mechanisms that generate receptive field properties in cortical neurons under conditions that mimic the waking state; a state when learning, memory and the modification of synaptic strength can be most readily demonstrated. Important advances in understanding receptive field mechanisms in sensory cortex have arisen from studying the precise relationship between the mystacial vibrissae or "whiskers" and their neural representation in separate cortical domains or "barrels". The anatomical precision of whisker projections to barrels permits a unique delineation of thalamocortical and intracortical components of cortical cell responses based on latency and security of response to peripheral receptor stimulation. When recorded in awake animals or even under very light anesthesia, cortical neurons show two components to their response to whisker movement. Neurons in layer IV of a whisker's primary projection zone respond with short latency (7-10 msec) and a high response magnitude (two or more action potentials (spikes) per stimulus). This "Center Receptive Field" (CRF) for layer IV cells is generated in large part by sensory fiber inputs from the thalamus. The CRF is restricted to 1.4 whiskers on average and is the only response detectable when cortical responses are depressed by deep anesthesia. In the "waking state" the same neuron often will respond to deflection of 4-6 surrounding whiskers, but only at longer latency (15-40 msec) and with fewer spikes per stimulus. These more labile responses form an excitatory surround receptive field (SRF). Sensory information that is transduced by individual whiskers and that generates the SRF of a cortical neuron achieves this added response complexity through intracortical mechanisms. The control of the mechanisms that determine the dissemination of sensory information within cortex include: (1) regulating the level of GABAergic inhibition; and (2) potentiation or depression of the response level generated by repeated sensory experience. State-dependent "modulatory" inputs to cortex, such as the noradrenergic and cholinergic fiber system, could regulate the degree of horizontal spread of a sensory input, in part through global changes in the level of inhibition and/or regulating the amplitude of cortical responses, thereby determining the level of associative interactions between sensory inputs.
通过分析在模拟清醒状态的条件下，皮层神经元中产生感受野特性的机制，可阐明将皮层空间分离区域中的感觉输入联系起来的机制；清醒状态是学习、记忆和突触强度改变最易得到证明的状态。在理解感觉皮层感受野机制方面的重要进展，源于对触须或“胡须”与它们在不同皮层区域或“桶状区”中的神经表征之间精确关系的研究。触须向桶状区投射的解剖学精确性，使得基于对周边感受器刺激的反应潜伏期和可靠性，对皮层细胞反应的丘脑皮层和皮层内成分进行独特的描绘成为可能。当在清醒动物甚至非常浅的麻醉状态下记录时，皮层神经元对触须运动的反应显示出两个成分。触须初级投射区第IV层的神经元以短潜伏期（7 - 10毫秒）和高反应幅度（每个刺激产生两个或更多动作电位（峰电位））做出反应。第IV层细胞的这种“中心感受野”（CRF）在很大程度上是由来自丘脑的感觉纤维输入产生的。CRF平均限于1.4根触须，并且是在深度麻醉使皮层反应受到抑制时唯一可检测到的反应。在“清醒状态”下，同一个神经元通常会对4 - 6根周围触须的偏转做出反应，但潜伏期更长（15 - 40毫秒），且每个刺激的峰电位较少。这些更不稳定的反应形成一个兴奋性周边感受野（SRF）。由单个触须转导并产生皮层神经元SRF的感觉信息，通过皮层内机制实现了这种额外的反应复杂性。决定感觉信息在皮层内传播的机制的控制包括：（1）调节GABA能抑制的水平；以及（2）增强或减弱由重复感觉经验产生的反应水平。对皮层的状态依赖性“调制性”输入，如去甲肾上腺素能和胆碱能纤维系统，可以部分地通过抑制水平的全局变化和/或调节皮层反应的幅度，来调节感觉输入的水平扩散程度，从而决定感觉输入之间联想相互作用的水平。
