A re-evaluation of the afterhyperpolarization mechanism in dorsal spinocerebellar tract neurons

Comparison of response properties of dorsal and ventral spinocerebellar tract neurons to a physiological stimulus

The response characteristics of dorsal spinocerebellar tract (DSCT) neurons and ventral spinocerebellar tract (VSCT) neurons to the cutaneous inputs applied to footpads were studied in the cat. Three different wave forms were used: step displacement of varying amplitudes (0.1-3.5 mm); constant amplitude ramps with different slopes (5-120 mm/s); and constant amplitude sinusoidal displacements of varying frequencies (1-20 Hz). Both DSCT and VSCT neurons responded phasically to cutaneous stimuli of different wave forms. The phasic responses were related to both the amplitude and velocity of the peripheral stimulus. However, the responses of DSCT neurons were graded over only a very narrow, low range of stimulus intensities, whereas the responses of VSCT neurons were graded over a larger range of skin indentation up to 3 mm. Only the DSCT neurons exhibited some length sensitivity to ramp stimuli, and only DSCT neurons were activated repetitively by periodic stimuli. These results suggest both DSCT and VSCT can transmit exteroceptive information but respond selectively to different features of these stimuli.

Afterhyperpolarization mechanism in the dorsal spinocerebellar tract cells of the cat

1. The longlasting afterhyperpolarization (a.h.p.) following single or short trains of spikes in dorsal spinocerebellar tract (DSCT) neurons of the cat has been studied with intracellular recording techniques. 2. The a.h.p. amplitude was found to be potential dependent, increasing with depolarization and decreasing with hyperpolarization of the membrane. With large membrane hyperpolarization, the a.h.p. could be reversed in direction, the estimated reversal level being around 30 mV more negative than the threshold potential for spike initiation. The a.h.p. amplitude was also little affected by Cl- ions injected into the cell. 3. The a.h.p. was associated with an increase in the membrane conductance, as measured with short current pulses. The major part of the conductance change was related to the a.h.p. itself and not secondary to the hyperpolarization, i.e. to an anomalous rectification. A conductance change was also found when the membrane potential was polarized close to the a.h.p. reversal level. There was a clear correlation between the a.h.p. amplitude and the measured conductance changes. 4. It is concluded that the a.h.p. in DSCT neurones, as in spinal motoneurones, is caused primarily by an increase in membrane conductance to potassium ions. 5. The time course of the conductance change underlying the a.h.p. was calculated from the a.h.p. voltage and a mathematical expression describing this time course is given. The properties of the a.h.p. in DSCT cells are compared with those in spinal motoneurones and the functional significance of the differences is discussed briefly.

The relationship of afterhyperpolarizations of extraocular motoneurons to membrane potential

The effect of membrane potential displacements caused by external current injection on the amplitudes of two antidromically evoked afterhyperpolarizations (AHP1 and AHP2) was measured in extraocular motoneurons. AHP1 had a reversal potential of-70mV and an average compensation for displacements from the reversal potential ("compensation gain") of -0.36. AHP2 had a reversal potential of-80mV and a compensation gain of -0.08. Measurements of input resistance of these motoneurons demonstrate a 30-50% decrease at depolarized membrane potentials. This rectification could account for the apparent lack of summation of AHP2. The functional role of AHP1 and AHP2 in the regulation of discharge frequencies of extraocular motoneurons during nromal eye movements is discussed.

Synaptic action on Clarke's column neurones in relation to afferent terminal size

1. Excitatory post-synaptic potentials (e.p.s.p.s) were recorded intracellularly from Clarke's column neurones (DSCT neurones) of the cat in response to adequate stimuli applied to a variety of sensory receptors.2. The amplitude of e.p.s.p.s so produced varied from less than 0.2 mV to more than 2-3 mV. The amplitude distribution of e.p.s.p.s suggested that the mean number of ;quanta' of transmitter released by one impulse varied widely from one fibre to another arising from a given type of sensory receptor.3. The average amplitude of e.p.s.p.s evoked by single afferent impulses was significantly smaller for cutaneous inputs than for muscle or joint inputs. However, synaptic action on DSCT neurones produced by different sensory inputs was equally greater, on the average, than that on spinal motoneurones.4. Both small and large e.p.s.p.s in DSCT neurones failed to increase in amplitude during post-synaptic hyperpolarization applied through the cell body. This failure could not be attributed to possible anomalous rectification in the post-synaptic membrane.5. Small and large e.p.s.p.s were comparable in half-decay time, but there was a positive correlation between amplitude and time-to-peak of e.p.s.p.s. It is suggested that the locations of synapses responsible for small and large e.p.s.p.s are intermingled on the dendrites and that large e.p.s.p.s are associated with a longer duration of transmitter action than small e.p.s.p.s.6. Degenerating terminals of primary afferent fibres on DSCT neurones and motoneurones were examined with the electron microscope after chronic section of the dorsal roots.7. Dendritic degenerating terminals showed no significant difference in size between motoneurones and DSCT neurones. Degenerating ;giant' terminals were found on DSCT neurones, but they were located only on or very close to the cell body.8. It is concluded that the major factor responsible for a large number of ;quanta' of transmitter released at synapses on DSCT neurones is the number of multiple synaptic contacts formed by one afferent fibre rather than the size of individual synapses.