Synaptic Mechanism Of Receptive Field, Integration Field And Morphology Of Cat's Striate Cortical Neurons

The synaptic properties of classical receptive field (CRF) and integration field (IF) of cat's striate cortical neurons and their morphological features were investigated using the whole cell recording and intracellular staining techniques in vivo. In part I of this thesis, a new approach of the whole cell recording method developed by our laboratory is discussed. The core of this method is to remain the dura intact. To ensure the penetration of glass microelectrode, dura mater was softened by collagenase. A continuous positive pressure was applied during penetration to keep electrode clean of tissue debris. When the tip of the electrode was finally resting on the cell membrane, the positive pressure was released and a small negative pressure was applied to the electrode for the formation of a seal. This method can effectively minimize cardiopulmonary pulsations of the brain and remain the intracellular recordings and injections stable for an hour or two without performing a pneumothorax preparation. Cortical neurons had well-preserved morphological state and normal electrophysiological properties after dura mater was treated by the enzyme.In part II, the effects of intracortical excitatory and inhibitory circuitries on the orientation selectivity were studied using the in vivo whole cell recording technique. Under this condition, the resting membrane potentials of most neurons were between -30 and -60 mV, and the EPSPs, IPSPs and action potentials (AP) were clearly observed. When a sinusoidal luminance grating moving across the simple cell's receptive field, all the cells tested responded with periodic fluctuations in their membrane potentials at the temporal frequency of the visual stimulus. Thefluctuations were mostly due to periodic depolarization of membrane potential resulted from the summed EPSPs, which were occasionally interrupted by individual IPSPs. Orientation tuning of the cells can be deduced from the input signals, i.e., the amplitude of EPSPs and from the output signals, the firing frequency of AP, respectively. By comparing the tuning width of the input and output tuning curves, it is shown that the orientation selectivity is sharpened after being processed by the intracortical connection. To explore the mechanisms of this sharpening process, we analysed the transfer function of cortical neurons. An exponential transfer characteristic was observed between the synaptic input and the cell's output. This non-linear transformation of cortical neurons may underlie the sharpening of orientation tuning properties. This mechanism was also involved in the sharpening of the other selectivities of visual attributes. On the other hand, the amplitude of summed IPSPs was also modulated by the visual stimulation. In most cases, the IPSPs reached maximum at the optimal orientation, implying an effect of enhancing temporal and spatial precision.In part III, the synaptic plasticity of orientation and direction selectivity of simple cells in primary visual cortex was studied. We use hyperpolarize and depolarize current to change the membrane potential and discharge activity of the cells, paired with selected orientation drifting grating stimulation. The subsequent orientation tuning properties were examined. We found the optimal orientation and direction shifted not only related to the polarity of the current paired with training orientation. In part IV, synaptic mechanisms of the interaction between the CRF and IF were investigated using the whole cell recording technique in vivo. The probability of depolarization and hyperpolarization of membrane potential was counted when CRF and/or IF were stimulated by the drifting sinusoidal gratings. When an optimal grating moved across the facilitatory both the CRF and IF, the amplitude and frequency of EPSPs increased and the membrane potential depolarized. The mechanisms and components of the changes in membrane potential may differ in different cells. Similarly, stimulating the inhibitory IF induced increase of the amplitude and frequency of IPSPs or supp...