| Research Abstract |
In lightly anesthetized rats, the caudate-putamen (CP), globus pallidus (GP), entopeduncular nucleus (Ep) and substantia nigra pars reticulata (SNr) were stereotaxically mapped at 500-mum intervals using a microstimulation technique in conjunction with histological identification of the stimulating sites. For comparison, microstimulation also was carried out in the internal capsule (ic) or the cerebral peduncle (cpd). A stainless steel microelectrode was lowered in 99-mum steps, and a 125-ms duration train of 40, cathodal pulses (333 Hz, 40-muA pulse currents) was delivered at each step once every 2.5s. For later histological reconstruction, iron was electrolytically deposited from the electrode tip in the selected loci. Microexcitable zones (MZs) for jaw muscle activity were concentrated 2.5-3.5mm lateral to the midline with a corresponding posterior spread of 3mm from the anterior commissure level. The qreatest microexcitability was found in the ic or the cpd, whereas the least was o
… More
bserved in the CP.The EMG activity of characteristic discharge pattern was evoked exclusively from the jaw-opening digastric muscles with an ipsilateral predominance by microstimulation of an MZ within a specific component of the basal ganglia such as CP,GP,or Ep.
A 60-80 mum tip diameter glass injection pippete, containing an insulated elgiloy filament for microstimulation, was filled with 0.05M GABA or 0.05M muscimol HBr in 0.9% saline. The injection pippete was guided to the right Ep by observing of the characteristic spontaneously firing motor unit activity (approx.10 Hz) from the ipsilataral digastristic muscle following the pippete tip penetrated the Ep. The motor unit increased its firing rate to 40-50 Hz in response to the above-mentioned microstimulation current delivered through the elgiloy filament. 0.1 mul of GABA or muscimol solution was ejected for 1 s with the aid of Pneumatic PicoPump (World Precision Instruments, Inc., U.S.A) attached to the pipette shank with polyethylene tubing. Immediatelyafter the injection, the digastric motor unit increased its firing rate to the same range as in the microstimulation current and the hyperactivity continued for several tens s. Both chemicals were effective at the same degree.
In the monkey, the output neurons in the Gpi (internal globus pallidus) /SNr discharge at mean rates of greater than 50 spikes/s and provide GABAergic inhibitory output to the target neurons in the thalamus, superior colliculus, and pedunclopontine region, where they must be subjected to tonic inhibition in the awake, resting animals (Hikosaka and Wurtz, 1985a, b). The interpretation that a pause in SNr activity leads to the saccade is based on the observation that local injection into SNr of muscimol, a GABA agonist that inhibits SNr neurous, results in spontaneous saccadestoward the contralateral visual field (Hikosaka and Wurtz, 1985a, b). In light of the aforementioned interpretation, we considered that the appearance of single motor unit activity in the digastric muscles is due to a pause in the output neuron activity caused by mechanical damage following intrusion of the microelectrode tip into the Ep. Similarly, microstimulation of the Ep might produce the pause in discharge of different output neurons and result in the phasic burst in the activity of the digastric muscle. We have demonstrated that hyperactivity of the jaw-opening muscles induced by stimulation of the rat basal ganglia is due to to the mechanism of a disinhibition of jaw-opening motoneurons. Less
|
