T. Hongo

1. Glass microelectrodes were inserted into the lateral vestibular nucleus of Deiters, and potential changes were recorded both extracellularly and intracellularly under stimulation of the spinal cord.2. When the ipsilateral C3 segment was stimulated at its ventrolateral surface, negative field potentials of several millivolts were recorded from the area which was histologically identified as Deiters' nucleus. These field potentials were presumed to be caused by the antidromic activation of Deiters' neurones through the vestibulospinal tract.3. The antidromic field potential of Deiters' nucleus showed different distributions in the ventrodorsal direction for activation at the C3 and L1 levels, which is in keeping with the histologically determined somatotopical arrange-ment.4. Within the nucleus of Deiters specified by the field potentials, a total of 134 units were impaled with microelectrode. They were identified as Deiters' neurones by the characteristics of the antidromic spikes, their afterhyperpolarization and by occurrence of synaptic noise and PSPs.5. In sixteen selected neurones the resting potential was -55 to -75 mV and the spike height 60 to 100 mV (mean, 80.8 mV). The duration of the spike was 0.4 to 0.6 msec (mean, 0.47 msec).6. The conduction velocity along the vestibulospinal fibres was determined in fifty Deiters' neurones which were activated antidromically both from the C3 and L1 segmental levels. It ranged from 24 to 140 m/sec, the peak frequency being at 90 to 100 m/sec.7. The falling phase of the spike, after it crossed the base line, reversed to an afterhyperpolarization which reached its summit at 0.7 to 8.8 msec (mean, 2.4 msec), and diminished within 33 to 100 msec (mean, 49 msec). These values are smaller than those for cat spinal motoneurones. As a unique feature in Deiters' neurones the afterhyperpolarization did not show a temporal summation when it was evoked successively at short intervals.8. The stimulation of the ipsilateral C3 segment just subthreshold for the axon of the impaled cells induced EPSPs monosynaptically in 15.4% and IPSPs polysynaptically in 7.7% of the examined neurones. The recurrent axon collaterals might be responsible for these IPSPs, but the possibility was not excluded for the other pathways.9. Stimulation at the C3 segment supramaximal for the antidromic field potential of Deiters' nucleus produced monosynaptic EPSP in 50% and polysynaptic IPSP in 28.6%. It was suggested that the monosynaptic EPSP was produced through the ascending tract fibres in the spinal cord.10. In view of the fast time course of the action potential and of rarity of the possible recurrent inhibition, it was postulated that Deiters' neurones would be grouped with the fast type of lumbosacral motoneurones.

Abstract The supraspinal and reflex control of γ‐motoneurones has been studied with intra‐ and extracellular recording from lumbosacral γ‐motoneurones in the cat. Monosynaptic EPSPs were recorded in some γ‐motoneurones on stimulation of the brain stem. These effects were evoked from the Deiters' nucleus and from fibres descending in the medial longitudinal fascicle probably originating in the pontine reticular formation. Previous investigations have revealed monosynaptic connections to α‐motoneurones from these regions and our results suggest parallel effects to α‐ and part of the γ‐motoneurone population supplying one muscle. Indirect evidence suggests that this monosynaptic effect is exerted only on static γ‐motoneurones, thus implying a linkage between the descending monosynaptic control of α‐ and static γ‐motoneurones via these pathways. The reflex effects to γ‐motoneurones have been studied with graded electrical stimulation of ipsi‐ and contralateral hindlimb nerves. Five γ‐motoneurones, presumably all belonging to extensor motor nuclei, were found to receive IPSPs from group I afferents and it is suggested that only static γ‐motoneurones are influenced. It has not been possible to decide if these IPSPs are evoked from Ib or Ia afferents. The reflex effects from group II and III muscular afferents, joint and cutaneous afferents seem to conform to the effects evoked in α‐motoneurones from these afferents.

A reconstruction was made of the intramedullary trajectory of 23 physiologically identified Ia afferents from cat hind limb muscles (medial gastrocnemius, soleus, plantaris, flexor digitorum-hallucis longus, and hamstring). The afferents were stained by intra-axonally injected HRP. The axons of these afferents were traced over distances of 5.8 mm to 15.7 mm rostrocaudally. In the dorsal funiculus fibers from all the muscles showed a similar course and similarly bifurcated into an ascending and a descending branch. The mean diameters of stem axons, ascending branches, and descending branches were 6.6 micrometer, 5.8 micrometer, and 3.0 micrometer, respectively. Within the analyzed lengths of the spinal cord five to eleven collaterals were given off from the two branches. The distances between adjacent collaterals of the ascending and descending branches averaged 1200 micrometer and 790 micrometer, respectively. The collaterals as a rule passed through the medial half of the dorsal horn before they entered the deeper parts of the gray matter. The terminal distribution areas common to all Ia collaterals were: (1) the medial half of the base of the dorsal horn, mainly lamina VI: (2) lamina VII; and (3) lamina IX. The numbers of terminals were largest in lamina IX and smallest in lamina VII. The density of terminals in lamina IX was highest in the homonymous motor cell column. The terminal distribution areas of adjacent collaterals showed no overlap in the sagittal plane. Terminal branches carried one bouton terminal and up to six boutons en passage with an average of 1.8 terminals per terminal branch. Apparent axosomatic and axodendritic contacts were seen on small-sized and medium-sized neurons in laminae V-VI, medium-sized neurons in lamina VII, and large neurons in lamina IX. One motoneurons was contacted by an average of 3.3 terminals. In addition to the common features, Ia collaterals of various muscles of origin showed some differences in their trajectories in the ventral horn, and in their terminations in the gray matter.