EIDE, E., L. FEDINA, J. JANSEN, A. LUNDBERG and L. VYRLICKY. Properties of Clarke's column neurones. Acta physiol. scand. 1969. 77. 125-144. Mechanisms responsible for the high efficacy of synaptic actkation of the dorsal spinocerebellar tract (DSCT) was investigated with intracellular recording from Clarke's column cells in anesthetized spinal cats. The spike potential is brief (mean duration 0.57 msec) and followed by a delayed depolarization. Our observations suggest a high safety for impulse transmission from the initial segment to the soma and from the soma to the dendrites. The afterhyperpolarization is small ( < 2 mV), shortlasting (30-50 msec) and has a high sensitivity to hyperpolarizing currents. There is only a small conductance change during the afterhyperpolarization. The monosynaptic group I EPSP has a mono-exponential decay, the time course of which is explained by the passive properties of the membrane. I t is postulated that the transmitter action is brief. Possible mechanisms for multiple discharges evoked by group I volleys in DSCT cells are discussed. Injected currents evoke maintained firing and within a wide range there is a linear relation between frequency and current strength (mean slope constant 4.8 imp/sec/mV) . Depolarization produced by muscle stretch and injected current have equivalent effect on the firing rate. The properties of DSCT cells seem well adapted to their function to transfer sensory signals and operate over a wide frequency range

Ito & Oscarsson, 1961; Coombs, Eccles & Fatt, 1955).3. Studied with potassium chloride filled micropipettes. the reversal potential for the group I a IPSP was found to be different from that for the recurrent IPSP when the amount of Cl-diffusing or iontophoretically injected into the motoneurone was small. The amount of difference in reversal potential varied from cell to cell but when present the group I a IPSP reversed to a depolarizing potential more readily than the recurrent IPSP in all cases.4. Interaction between recurrent IPSPs and monosynaptic excitatory post-synaptic potentials (EPSPs) was also studied and the amount of nonlinearity of potential summation was measured.5. The results are consistent with the hypothesis that the terminations of Renshaw cells responsible for the recurrent IPSP are located largely on the proximal dendrites of motoneurones, while the terminations of the interneurones generating the group I a IPSP appear to be closer to or on the cell somata.

EIDE, E., L. FEDINA, J. JANSEN, A. LUNDBERG and L. VYKLICKY. Unitary components in the activation of Clarke's column neurones. Acta physiol. scand. 1969. 77. 145-1 58.I n response to muscle stretch unitary EPSPs were recorded from group I activated dorsal spinocerebellar tract (DSCT) neurones. The size of the different unitary EPSP varied appreciably, the largest was about 5 mV. The rise time and decay times of the unitary EPSPs varied only between 0.2-1 msec and 1-3 msec respectively. The estimate of the number of primary afferent fibres converging on to each second order neurone ranged from 12 to 16. The distribution of amplitudes of individual unitary EPSPs was symmetrical with a coefficient of variation as low as 0.10. This suggests that a large number of quanta are released by each impulse in the primary afferent fibres. Acta physiol. scand. Val. 77: 1-2 10493003.

The effects on transmission in the Ia inhibitory pathway to motoneurones by volleys in ipsilateral primary afferents were investigated with intracellular recording from motoneurones. In spinal cats under chloralose anesthesia Ia IPSPs in flexor as well as extensor motoneurones were regularly facilitated by volleys in cutaneous, high threshold muscle and joint afferents. In decerebrate cats with a low pontine lesion IPSPs were not facilitated from high threshold muscle and joint afferents, although volleys in low threshold cutaneous afferents evoked a facilitation also in this preparation. It is postulated that the Ia inhibitory interneurones receive excitatory actions from the ipsilateral flexor reflex afferents (transmission depressed in the decerebrate state) and through a separate pathway from low threshold cutaneous afferents. The recurrent effects from motor axon collaterals were investigated on inhibitory transmission to motoneurones from different afferents. A strong positive correlation was revealed between recurrent depression of IPSPs evoked from different afferents and facilitation of Ia IPSPs by the same afferent volleys. This suggests that the recurrent depression of IPSPs from different primary afferents depends on an excitatory convergence from them onto the Ia inhibitory inter‐neurones, which mediate at least part of the IPSP evoked in the motoneurone from these afferents.

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