Controlled locomotion in the mesencephalic cat: distribution of facilitatory and inhibitory regions within pontine tegmentum

Abstract

1. The contribution of postural tonus to controlled locomotion elicited by the stimulation of mesencephalic locomotor region (MLR) was studied in the acute precolicular-postmammillary decerebrate (mesencephalic) cat. 2. A microelectrode was placed in the unilateral MLR and another was placed systematically at 1-mm increments throughout the pons (H--4 to H--1O) at level ranging from P2 to P11 dorsoventrally and mediolaterally from 0 to L or R6. Depending on the general condition of the animal, stimuli through this second electrode were delivered preceding, succeeding, or simultaneous with the MLR stimulation. 3. Stimulation of the ventral part of the caudal tegmental field (P3 to P9, H--6 to H--8) increased extensor tonus of the hindlimbs, as assessed by recording muscle activity. Concomitant stimulation of this region converted MLR-elicited hindlimb stepping to coordinated four-legged locomotion and also elicited locomotion even when stimulation of the MLR alone failed to elicit locomotion. Stimulation of this ventral tegmental region alone at a larger stimulus intensity elicited spastic locomotor movements associated with a substantial increase in extensor tonus. 4. Stimulation of the lateral tegmented field surrounding the pontine locomotor region (PLR) also facilitated MLR effects, but had a relatively weaker facilitatory effect on postural tonus then stimulation of the ventrocaudal tegmental field. PLR stimulation alone was also ineffective when postural tonus was not well maintained. 5 Stimulation of the dorsal part of caudal tegmental field (P3 to P9, H--4 to H--6) in its midline dramatically decreased extensor tonus of the hindlimbs. MLR-elicited controlled locomotion was completely suppressed during concomitant stimulation of this inhibitory region. 6. These results indicated clearly that the degree of existing postural tonus greatly affects MLR-elicited locomotor movements and that an increase in postural tonus and an activation of the spinal stepping generator are not separate phenomenia.

Locomotion produced in mesencephalic cats by injections of putative transmitter substances and antagonists into the medial reticular formation and the pontomedullary locomotor strip

“…The locations of the cell bodies in the lower brain stem that are capable of initiating locomotion correspond to the MLR fiber terminations reported previously (Steeves and Jordan, 1984) and to the site where cooling to block synapse transmission reversibly abolished locomotion produced by electrical stimulation of the classical MLR (Shefchyk et al, 1984). The present study extends the MRF region previously shown to be capable of generating locomotion when electrically stimulated (Mori et al, 1978(Mori et al, , 1980Garcia-Rill and Skinner, 1987) and shows that an area from the midline to L2 can be stimulated electrically to induce locomotion. This confirms the previous finding by Mori et al (1978) that midline stimulation is effective for production of locomotion, but it is in contrast to the report by Garcia-Rill and Skinner (1987) who claim that midline stimulation does not produce locomotion.…”

“…The present study extends the MRF region previously shown to be capable of generating locomotion when electrically stimulated (Mori et al, 1978(Mori et al, , 1980Garcia-Rill and Skinner, 1987) and shows that an area from the midline to L2 can be stimulated electrically to induce locomotion. This confirms the previous finding by Mori et al (1978) that midline stimulation is effective for production of locomotion, but it is in contrast to the report by Garcia-Rill and Skinner (1987) who claim that midline stimulation does not produce locomotion. The fact that the electrical threshold for the initiation of locomotion by stimulation of the MRF is higher than that observed for the MLR (Mori et al, 1980) or PLS (this study) suggests that a larger volume of tissue must be stimulated to produce locomotion, possibly due to a more diffuse distribution of neurons within the reticular formation.…”

“…2). This region is more extensive than the P3-P9 MRF regions previously shown to be capable of producing locomotion when electrically stimulated (Mori et al, 1978(Mori et al, , 1980. The effective PLS sites included the lateral tegmental field (FTL) medial and ventral to the spinal nucleus of the fifth nerve as previously described (Mori et al, 1977;Shik and Yagodnitsyn, 1977) but also included the spinal nucleus of the fifth nerve (5SM, BP).…”

“…This has been suggested to explain the effects of ventral tegmental field stimulation described by coworkers (1978, 1980) which produces increased hindlimb extensor activity and locomotion when electrically stimulated. Postural adjustments prior to the onset of locomotion have previously been reported during stimulation of the MLR (Mori et al, 1980;Garcia-Rill et al, 1985) and its reticular formation projection sites (Mori et al, 1978(Mori et al, , 1980(Mori et al, , 1982. Thus, while the MLR relays its descending command signal for locomotion, it may also influence parallel postural systems also located in the MRF.…”

“…As summarized by Jordan (1986) reticulospinal cells in the MRF then produce locomotion by pathways traveling in the ventral lateral funiculus (VLF) of the spinal cord. Electrical stimulation of the MRF can produce locomotion (Mori et al, 1978(Mori et al, , 1980Garcia-Rill and Skinner, 1987) and cells in this region show increased metabolic activity during MLR-evoked fictive locomotion . The direct evidence implicating the involvement of cells in the MRF in the mediation of locomotion includes the observation that cooling restricted areas in the MRF to block synaptic transmission can reversibly abolish both spontaneous and MLR-evoked locomotion (Shefchyk et al, 1984) and the finding that injections of cholinergic agonists or substance P into the MRF can elicit locomotion (Garcia-Rill and Skinner, 1987).…”