Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 111

Neurobiologists generally model synaptic inputs to neurons as currents injected into the dendritic tree (refer to Chapter 2) that are transformed into a train of spikes at the cell soma or axon initial segment. If the firing rate were exactly proportional to total synaptic current (above threshold), the relationship between synaptic current and spike rate would be called linear. However, in reality, this relationship is not linear for two main reasons:

 Leakage currents through the membrane tend to shunt away synaptic current more for low, slowly changing current levels than for rapidly changing ones.

 At high spike rates, the refractory period requires disproportionally more synaptic current to increase the spike rate. In fact, as the absolute refractory period is approached, no amount of increased synaptic input current can increase the spike rate.

One interesting result of this second factor is that the relationship between synaptic input current and spike rate in most neurons is more like a logarithmic function than a linear function. Psychophysical laws such as Weber’s and Fechner’s laws have long demonstrated that our perception of magnitude in senses such as sight, sound, and touch also follow a logarithmic relationship, where doubling the stimulus magnitude produces less than a doubling of the sensory perceptual magnitude. Direct experimental comparisons between perception of magnitude and neural firing rates have shown that the logarithmic spike compression underlies the logarithmic perception — that is, the perceptual magnitude follows the neural firing rate, which itself is logarithmically related to the magnitude of the stimulus.