Spikes and Dendritic Responses

Although many investigators consider action potentials to be the most relevant observable consequence of neuronal activation, a number of factors limit their contribution to field and potential distributions measured at the head surface. The transmembrane currents that generate the action potential are radially symmetrical. The field cancellation associated with such distributions prevents their observation at a distance. The action potential itself is biphasic in both time and space: a depolarizing current flows along the axon, ahead of the region of excitation, and a repolarizing current flows in the opposite direction. This so-called quadrupolar current pattern is not readily detected in field measurements at a distance. Finally, the biphasic, temporally transient nature and stochastic timing of action potentials may limit their contribution to time-locked averages typically used in event-related or evoked response paradigms.

A number of lines of evidence suggest that electro-physiological responses observed at the head surface are dominated by postsynaptic dendritic currents. Such currents are typically gated by neurotransmitters, although they may have voltage-dependent characteristics. These channels often have a defined ionic specificity; some ions such as calcium may have powerful and specific effects on the response characteristics of the cell. Potentials set up by postsynaptic ion channels drive passive (i.e., ohmic) conduction along the dendrites.

Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

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