Journal of neurophysiology
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We examined the efficacy of several patterns of low-frequency stimulation for producing long-term depression (LTD) or depotentiation in the hippocampal fiber pathway to the prefrontal cortex in the anesthetized rat. Field potentials elicited by stimulation of the CA1/subicular region of the ventral hippocampus were recorded in the prelimbic area of the prefrontal cortex. We found no evidence that low-frequency trains (0.5-1 Hz), consisting of either single pulses, paired pulses (35-ms interpulse interval), or two-pulse bursts (5-ms interval), produce LTD in the prefrontal cortex. ⋯ Depotentiation is reversible and can be induced >2 h after the induction of LTP. Repeated trains failed to decrease the prefrontal cortex response below the original, unpotentiated level. These findings demonstrate the existence of a depotentiation mechanism that is capable of exerting powerful control over ongoing or recently induced synaptic plasticity in hippocampocortical connections in vivo.
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Comparative Study Clinical Trial
Differential control of reciprocal inhibition during walking versus postural and voluntary motor tasks in humans.
Experiments were done to determine whether the strength of reciprocal inhibition from ankle flexors to extensors can be controlled independently of the level of ongoing motor activity in a task-dependent manner. In this paper we use the term reciprocal inhibition in the functional sense--inhibition of the antagonist(s) during activity of the agonist(s)--without reference to specific neural pathways that may be involved. The strength of reciprocal inhibition of the soleus alpha-motoneurons was determined by measuring the amplitude of the H reflex during voluntary, postural, and locomotor tasks requiring activity of the ankle flexor tibialis anterior (TA). ⋯ In this task, as in the OLS task, the H reflex began to decrease during the reaction time before the onset of TA EMG activity. We conclude that the strength of reciprocal inhibition of the soleus alpha-motoneuron pool can thus be controlled independently of the level of motor activity in the ankle flexors. The strength of the inhibition of the antagonist(s) depends on the task, and for each task the strength of the inhibition is not necessarily proportional to the level of motor activity in the agonist(s). (ABSTRACT TRUNCATED)
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Clinical Trial
Cerebral processing of acute skin and muscle pain in humans.
The human cerebral processing of noxious input from skin and muscle was compared with the use of positron emission tomography with intravenous H2(15)O to detect changes in regional cerebral blood flow (rCBF) as an indicator of neuronal activity. During each of eight scans, 11 normal subjects rated the intensity of stimuli delivered to the nondominant (left) forearm on a scale ranging from 0 to 100 with 70 as pain threshold. Cutaneous pain was produced with a high-energy CO2 laser stimulator. ⋯ Direct statistical comparisons between innocuous cutaneous and intramuscular stimulation with the use of Z scores and VOI analyses showed no reliable differences between these two forms of noxious stimulation, indicating a substantial overlap in brain activation pattern. The comparison of noxious cutaneous and intramuscular stimulation indicated more activation in the premotor cortex, SII, and prefrontal cortex with cutaneous stimulation, but these differences did not reach statistical significance. The similar cerebral activation patterns suggest that the perceived differences between acute skin and muscle pain are mediated by differences in the intensity and temporospatial pattern of neuronal activity within similar sets of forebrain structures.
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We employed microelectrode recording techniques to study the sensitivity of individual neurons in the striate cortex of anesthetized and paralyzed monkeys to relative interocular image disparities and to determine the effects of basic stimulus parameters on these cortical binocular interactions. The visual stimuli were drifting sine wave gratings. After the optimal stimulus orientation, spatial frequency, and direction of stimulus movement were found, the cells' disparity tuning characteristics were determined by measuring responses as a function of the relative interocular spatial phase of dichoptic grating pairs. ⋯ Overall, the results show that sensitivity to interocular spatial phase disparities is a common property of striate neurons. A cell's disparity tuning characteristics appear to largely reflect its monocular receptive field properties and the interocular balance between excitatory and inhibitory inputs. However, distinct functional classes of cortical neurons could not be discriminated on the basis of disparity sensitivity alone.
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When moving objects with a precision grip, fingertip forces normal to the object surface (grip force) change in parallel with forces tangential to the object (load force). We investigated whether voluntary wrist actions can affect grip force independent of load force, because the extrinsic finger muscles cross the wrist. Grip force increased with wrist angular speed during wrist motion in the horizontal plane, and was much larger than the increased tangential load at the fingertips or the reaction forces from linear acceleration of the test object. ⋯ For example, when transporting grasped objects, upper limb accelerations simultaneously produce inertial torques at the wrist that must be resisted, and inertial loads at the fingertips from the object that must be offset by increased grip force. The muscle coactivation described here would cause similarly timed pulses in the wrist force and grip force. However, grip-load coupling from this mechanism would not contribute much to grasp stability when small wrist forces are required, such as for slow movements or when the object's total resistive load is small.