The meaning of discharge rate: excitation-to-frequency transduction as studied in spinal motoneurones.

In many neurobiological studies, the excitation and inhibition of nerve cells is studied by means of extracellular recordings of impulse discharges. For the interpretation of such data in terms of synaptic effects, it is necessary to know how activating currents give rise to repetitive impulse firing in neurones. In the present contribution, a review will be given of earlier as well as recent findings concerning the excitation-to-frequency transduction in alpha motoneurones of the cat's spinal cord. The transduction properties were investigated by recording the neuronal responses to long-lasting depolarizing currents, injected directly into the cell via an intracellular microelectrode. Such studies have revealed that there is no simple proportionality between the intensity of excitation and the resulting discharge rate. Different motoneurones may show considerable differences with respect to, for instance, (i) the frequency range of maintained firing, (ii) the amount of drop in discharge frequency during a period of constant activation (adaptation). Among spinal motoneurones, there are functionally relevant differences in transduction properties between cells with different kinds of muscle units and motor tasks. To an important extent the transduction properties are determined by the prolonged post-spike permeability changes that give rise to the after-hyperpolarization.