The role of the mesencephalic locomotor region (MLR) in initiating and controlling the power of swimming was studied in semi-intact preparations of larval and adult sea lampreys. The brain and the rostral portion of the spinal cord were exposed in vitro, while the intact caudal two-thirds of the body swam freely in the Ringer's-containing chamber. Electrical microstimulation (2-10 Hz; 0. 1-5.0 microA) within a small periventricular region in the caudal mesencephalon elicited well-coordinated and controlled swimming that began within a few seconds after the onset of stimulation and lasted throughout the stimulation period. Swimming stopped several seconds after the end of stimulation. The power of swimming, expressed by the strength of the muscle contractions and the frequency and the amplitude of the lateral displacement of the body or tail, increased as the intensity or frequency of the stimulating current were increased. Micro-injection of AMPA, an excitatory amino acid agonist, into the MLR also elicited active swimming. Electrical stimulation of the MLR elicited large EPSPs in reticulospinal neurons (RS) of the middle rhombencephalic reticular nucleus (MRRN), which also displayed rhythmic activity during swimming. The retrograde tracer cobalt-lysine was injected into the MRRN and neurons (dia. 10-20 microm) were labelled in the MLR, indicating that this region projects to the rhombencephalic reticular formation. Taken together, the present results indicate that, as higher vertebrates, lampreys possess a specific mesencephalic region that controls locomotion, and the effects onto the spinal cord are relayed by brainstem RS neurons.

1. The impulse activity of single neurones in the motor cortex (MC) was recorded extracellularly, using movable varnish-insulated tungsten microelectrodes, in six adult, freely moving cats. Neuronal activity was recorded while the cats walked on a flat floor, as they stepped over a series of barriers, and as they walked on the flat rungs of a horizontal ladder. The mean discharge rate (mR) and the depth of frequency modulation (dM) in each cell were estimated over 10-100 steps. 2. The activity of ninety-eight MC cells (Including thirteen pyramidal tract neurones (PTNs)) was recorded during stepping over barriers 25 cm apart. The mR in 66% and the dM in 61% of these cells changed by more than 20% during locomotion with barriers compared to locomotion on the flat (an increase was more often the case). 3. The activity of nine cells was recorded during stepping over barriers 12 cm apart, and the activity of twenty-seven cells (including five PTNs) during walking with barriers only 6 cm apart. The mR in 67% and in 59% of the cells, respectively, and the dM in 56% and in 67% of the cells, respectively, were greater in these locomotor tasks than during locomotion on the flat. 4. The activity of twenty cells was recorded during walking and compared in experiments with different distances between barriers. The mR in 50% and the dM in 75% of the neurones progressively increased when the distance between successive barriers was diminished. 5. The discharge rates of thirteen cells were compared in two different locomotor tasks: (i) when the cat stepped over barriers requiring hyperflexion of the limbs and (ii) when it walked on the flat with loads attached to the distal forelimbs causing a hyperactivity of flexor muscles. The activity of nine cells was different during stepping over the barriers compared to locomotion with loadings on the forelimbs. 6. The activity of 108 cells (twenty-four PTNs) was recorded during walking along a horizontal ladder with flat rungs. The mR of 61% and the dM of 72% of cells changed by more than 20% during locomotion on the ladder compared with that on the flat (most often they increased). 7. The position of the peak rate relative to the step cycle did not differ in the majority of cells (in 78-91% depending on the task) during locomotion on the flat, with the barriers or on the ladder.(ABSTRACT TRUNCATED AT 400 WORDS)

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Many suspected inhibitory interneurons (SINs) of primary somatosensory cortex (S1) receive a potent monosynaptic thalamic input (thalamocortical SINs, SINstc). It has been proposed that nearly all such SINstc of a S1 barrel column (BC) receive excitatory synaptic input from each member of a subpopulation of neurons within the topographically aligned ventrobasal (VB) thalamic barreloid. Such a divergent and convergent network leads to several testable predictions: sharply synchronous activity should occur between SINstc of a BC, sharp synchrony should not occur between SINstc of neighboring BCs, and sharp synchrony should not occur between SINs or other neurons of the same BC that do not receive potent monosynaptic thalamic input. These predictions were tested by cross-correlating the activity of SINstc of the same and neighboring BCs. Correlations among descending corticofugal neurons of layer 5 (CF-5 neurons, identified by antidromic activation) and other neurons that receive little or no monosynaptic VB input also were examined. SINs were identified by a high-frequency (>600 Hz) burst of three or more spikes elicited by VB stimulation and had action potentials of short duration. SINstc were further differentiated by short synaptic latencies to electrical stimulation of VB thalamus (<1.7 ms) and to peripheral stimulation (<7.5 ms). The above predictions were confirmed fully. 1) Sharp synchrony (+/-1 ms) was seen between all SINstc recorded within the same BC (a mean of 4.26% of the spikes of each SINtc were synchronized sharply with the spikes of the paired SINtc). Sharp synchrony was not dependent on peripheral stimulation, was not oscillatory, and survived general anesthesia. Sharp synchrony was superimposed on a broader synchrony, with a time course of tens of milliseconds. 2) Little or no sharp synchrony was seen when CF-5 neurons were paired with SINstc or other neurons of the same BC. 3) Little or no sharp synchrony was seen when SINstc were paired with other SINstc located in neighboring BCs. Intracellular recordings obtained from three SINs in the fully awake state supported the assertion that SINs are GABAergic interneurons. Each of these cells met our extracellular criteria for identification as a SIN, each had a spike of short duration (0.4-0.5 ms), and each responded to a depolarizing current pulse with a nonadapting train of action potentials. These results support the proposed network linking VB barreloid neurons with SINstc within the topographically aligned BC. We suggest that sharp synchrony among SINstc results in highly synchronous inhibitory postsynpatic potentials (IPSPs)in the target neurons of these cells and that these summated IPSPs may be especially effective when excitatory drive to target cells is weak and asynchronous.