Restoring locomotion after spinal-cord injury has been a difficult problem to solve with traditional functional electrical stimulation (FES) systems. Intraspinal microstimulation (ISMS) is a novel approach to FES that takes advantage of spinal-cord locomotor circuits by stimulating in the spinal cord directly. Previous studies in spinal-cord intact cats showed near normal recruitment order, reduced fatigue, and functional, synergistic movements induced by stimulation through a few microwires implanted over a 3-cm region in the lumbosacral cord. The present study sought to test the feasibility of ISMS for restoring locomotion after complete spinal-cord transection. In four adult male cats, the spinal cord was severed at T10, T11, or T12. Two to four weeks later, 30 wires (30 microm, stainless steel) were implanted, under anesthesia, in both sides of the lumbosacral cord. The cats were then decerebrated. Stimulus pulses (40-50 Hz, 200 micros, biphasic) with amplitudes ranging from 1-4x threshold (threshold = 32 +/- 19 microA) were delivered through each unipolar electrode. Kinetics, kinematics, and electromyographic (EMG) measurements were obtained with the cats suspended over a stationary treadmill with embedded force platforms for the hindlimbs. Phasic, interleaved stimulation through electrodes generating flexor or extensor movements produced bilateral weight-bearing stepping of the hindlimbs with ample foot clearance during swing. Minimal changes in kinematics and little fatigue were seen during episodes of 40 consecutive steps. The results indicate that ISMS is a promising technique for restoring locomotion after injury.

It is commonly accepted that locomotor-related neuronal circuitry resides in the lumbosacral spinal cord. Pharmacological agents, epidural electrical stimulation, and sensory stimulation can be used to activate these instrinsic networks in in vitro neonatal rat and in vivo cat preparations. In this study, we investigated the use of low-level tonic intraspinal microstimulation (ISMS) as a means of activating spinal locomotor networks in adult cats with complete spinal transections. Trains of low-amplitude electrical pulses were delivered to the spinal cord via groups of fine microwires implanted in the ventral horns of the lumbosacral enlargement. In contrast to published reports, tonic ISMS applied through microwires in the caudal regions of the lumbosacral enlargement (L7-S1) was more effective in eliciting alternating movements in the hindlimbs than stimulation in the rostral regions. Possible mechanisms of action of tonic ISMS include depolarization of locally oscillating networks in the lumbosacral cord, backfiring of primary afferents, or activation of propriospinal neurons.

In order to study the visual thalamocortical connections in the sheep, horseradish peroxidase (0.3--0.5 microliter of a 30% solution) has been injected in the gyri marginalis, ectomarginalis medius pars medialis, ectomarginalis medius pars lateralis and ectosylvius caudalis. The results show that: (1) the dorsal lateral geniculate nucleus (LGNd) projects to the former three gyri. Dorsal parts of the LGNd project to caudal areas, whereas its ventral parts project to rostral areas of these gyri; medial parts of the LGNd project to the gyrus ectomarginalis medius pars lateralis, while lateral parts project to the gyrus marginalis; (2) the medial interlaminar nucleus (MIN) or pars geniculata pulvinaris of Rose ('42b) projects to the caudal part of the gyrus marginalis and to the gyrus ectomarginalis medius pars lateralis; (3) the pulvinar proper of Rose (PUL) projects to the caudal part of the gyrus ectosylvius caudalis whereas the rostral part of this gyrus receives input from the medial geniculate body. In relation to Rose's cytoarchitectonic study of the cortex of sheep ('42a) the present study has shown that the LGNd projects to both the area striata (gyrus marginalis + gyrus ectomarginalis medius pars medialis) and area occipitalis (gyrus ectomarginalis medius pars lateralis) of Rose, that the gyrus marginalis and the area occipitals receive a second projection (from the MIN), and that the PUL projects beyond the area occipitalis to the area parietalis of Rose.

Secondary trigeminocerebellar connections have been studied with HRP histochemistry in 25 sheep. The results indicate that almost all of the cerebellar cortex except flocculus, ventral paraflocculus and lobules I-IV receives bilateral (mostly ipsilateral) fibers from the trigeminal nuclei. A topographical organization of trigeminocerebellar fibers is present. The mesencephalic tract nucleus projects to the anterior lobe, the simple lobule (HVI), lobules VI, VIII, and the dorsal paraflocculus. The ventral group of the princeps and spinal tract (mainly IDV) nuclei projects to all lobules studied in vermis and hemispheres. More dorsal parts of these nuclei have a more restricted projection field including the vermal lobules VI, VII, and IX and the hemisphere. Cells within and ventral to the motor nucleus of the trigeminal nerve were found labeled after injections into the anterior lobe, the simple lobule, and lobule IX. Labeled cells in the region of the nucleus ovalis and close to the solitary tract project to the simple and paramedian lobule and lobule IX.