[Data processing system using PC9800 and Autonics APC204 digitizer].

We have examined the spatial relationship between the mossy fiber and climbing fiber projections to crus IIa in the lateral hemispheres of the rat cerebellum. Experiments were performed in ketamine/xylazine anesthetized rats using extracellular recordings and high-density micromapping techniques. Responses were elicited using small, tactile stimuli applied to the perioral and forelimb regions at a rate of 0.5 Hz. In our first series of experiments we demonstrate that the primary (i.e., strongest) receptive field for a single Purkinje cell's complex spike is similar to the primary receptive field of the granule cells immediately subjacent to that Purkinje cell. In our second series of experiments we demonstrate that the granule cell region most strongly activated by a particular peripheral stimulus is immediately subjacent to the Purkinje cells whose complex spikes are also activated most strongly by the same stimulus. The region of climbing fibers activated by a localized peripheral stimulus is "patchy"; it clearly does not conform to the notion of a continuous microzone. These results support original observations first reported in the 1960s using evoked potential recording techniques that the mossy fiber and climbing fiber pathways converge in cerebellar cortex. However, we extend this earlier work to show that the two pathways converge at the level of single Purkinje cells. Many cerebellar theories assume that mossy fiber and climbing fiber pathways carry information from different peripheral locations or different modalities to cerebellar Purkinje cells. Our results appear to contradict this basic assumption for at least the tactile regions of the lateral hemispheres.

Action Potentials↗

PubMed2001

Role of Ca2+ in membrane excitation and cell motility in Characean cells as a model system.

The Characeae internodal cell is excitable and generates an action potential. The depolarizing phase of the action potential primarily reflects the activation of Cl- channels, which takes place in a Ca2+-dependent manner. Namely, an increase in the Ca2+ influx takes place at the very beginning of the action potential and heightens the Ca2+ concentration in the cytoplasm ([Ca2+]c). The high [Ca2+]c activates Cl- channels at the plasmalemma, resulting in a large depolarization. The high [Ca2+]c also acts as a signal to induce a tonoplast action potential and the instantaneous cessation of cytoplasmic streaming; the tonoplast action potential also is caused by Ca2+-induced activation of Cl- channels at the tonoplast, and the cessation is a result of inhibition of the actin-myosin interaction by Ca2+. When the cytoplasm of the Characeae cell, especially in Nitella flexilis, is hydrated rapidly, [Ca2+], also increases through Ca2+ release from an intracellular store(s). The release may be triggered by the stretching of endomembranes caused by osmotic swelling of the Ca2+ store lumens. Although the origin of Ca2+ is different from that in the case of an action potential, high [Ca2+]c not only induces membrane depolarization through activation of the Cl- channel in a Ca2+-dependent manner but also inhibits cytoplasmic streaming in Characeae.

Action Potentials↗

PubMed2000

Channel sharing in pancreatic beta -cells revisited: enhancement of emergent bursting by noise.

Secretion of insulin by electrically coupled populations of pancreatic beta -cells is governed by bursting electrical activity. Isolated beta -cells, however, exhibit atypical bursting or continuous spike activity. We study bursting as an emergent property of the population, focussing on interactions among the subclass of spiking cells. These are modelled by equipping the fast subsystem with a saddle-node-loop bifurcation, which makes it monostable. Such cells can only spike tonically or remain silent when isolated, but can be induced to burst with weak diffusive coupling. With stronger coupling, the cells revert to tonic spiking. We demonstrate that the addition of noise dramatically increases, via a phenomenon like stochastic resonance, the coupling range over which bursting is seen.

Action Potentials↗