Polarization of primary afferent terminals of lumbosacral cord elicited by the activity of spinal locomotor generator.

The changes of electrical polarization of primary afferent terminals in the lumbosacral spinal cord have been investigated on immobilized, decorticated and spinal cats during fictive locomotion. Fictive locomotion was spontaneous or provoked by stimulation of either dorsal root or dorsal funiculi in the lumbar segments. The activation of the locomotor generator and appearance of fictive locomotion were always associated with a sustained dorsal root hyperpolarization. On the background of this positivity periodic negative dorsal root potential oscillations appeared synchronously with efferent discharges in motor hind-limb nerves. These periodic waves of primary afferent depolarization occurred in phase in different ipsilateral lumbosacral segments. On the contralateral side the periodic changes in dorsal root potential were out of phase during fictive stepping and in phase during fictive galloping. The use of Wall's technique has shown that tonic and periodic changes in dorsal root potential reflect the changes occurring in polarization of central terminals of cutaneous and muscle (Ia and Ib) groups of afferent fibres. It is concluded that the level of electrical polarization of primary afferent terminals is determined directly by the activity of the spinal locomotor generator; activation of the generator is followed by hyperpolarization of primary afferent terminals. By so modulating the polarization of afferent terminals, the locomotor generator can perform tonic and phase-dependent selection of afferent information.

New locomotor regions of the brainstem revealed by means of electrical stimulation.

New sites in the brainstem eliciting treadmill locomotion have been revealed in decerebrated cats by electrical stimulation. These are the cochlear nuclei, cuneate nucleus, spinocerebellar tracts, and substantia grisea centralis at the level of red nuclei which lie outside the known locomotor regions. Participation of neurons and fibers forming ascending sensory tracts (medial and lateral lemniscus) is of special interest in initiation of locomotion. Collaterals of these tracts pass through the hypothalamic and mesencephalic locomotor regions and may contribute largely to initiation of locomotor generators. Hypotheses about the leading role of non-specific afferent activation of the brainstem reticular formation in initiation of locomotion are put forward.

Afferent and efferent connections of brainstem locomotor regions: study by means of horseradish peroxidase transport technique.

Afferent and efferent connections of the hypothalamic and mesencephalic locomotor regions and also the bulbar locomotor strip were studied in cat using retrograde (horseradish peroxidase) transport technique. To study the sources of afferent projections, the enzyme microinjections were performed exactly into the same brain sites eliciting treadmill locomotion by means of electrical stimulation. When studying efferent projections horseradish peroxidase labeled neurons were revealed within locomotor regions after enzyme microinjections into different brain structures. Experimental data have shown that the hypothalamic and mesencephalic locomotor regions have mutual afferent and efferent projections with numerous brain areas including interconnections. Apart from the entopeduncular nucleus, the great number of different sensory nuclei are noted: among the sources of afferent projections are the nucleus tractus spinalis nervi trigemini, nucleus cuneatus, nucleus tractus solitarius and vestibular nuclei. In addition, after horseradish peroxidase injection into the mesencephalic locomotor region labeled neurons were found in the cochlear nuclei. Direct descending neuronal projections of the hypothalamic and mesencephalic locomotor regions are distributed mainly in the ipsilateral brainstem. Only a few of them reach the lumbar spinal cord. The most marked efferent projections of given regions are those to the brainstem reticular formation. After horseradish peroxidase injection into a functionally identified bulbar locomotor strip, labeled neurons were revealed in different stem regions mainly caudal to the enzyme injection site. The existence of a locomotor strip as an independent structural formation is called into question. When studying the locomotor region connections, the structural heterogeneity of these regions is revealed. Transitory fibers of ascending tracts are presumably within their limits side by side with neurons. The role of these fibers in stepping initiation by electrical stimulation of locomotor regions remains uncertain.