Primary motor cortex reorganization in a long-term monkey amputee

The primary motor cortex (M1) was mapped with intracortical microstimulation (ICMS) in a 15 year-old macaque whose right upper extremity was amputated at the shoulder joint prior to 2 years of age. Movements of the right shoulder girdle and stump were evoked by ICMS throughout the left M1 upper extremity region. The size of the left M1 upper extremity region contralateral to the amputated arm was not appreciably different from the size of the right upper extremity region contralateral to the intact arm. Long stimulus trains and/or higher stimulus currents were needed to evoke detectable movements at significantly more loci in the left than in the right M1 upper extremity region. These observations would be consistent with unmasking of a high threshold representation of shoulder musculature that normally exists throughout the central core of the upper extremity region, where it underlies a lower threshold representation of the distal forelimb. Alternatively, invasion of the de-efferented distal forelimb core by surrounding shoulder representation may have occurred. Differences between the limited M1 reorganization observed in the present study and the more extensive reorganization of S1 observed in other studies may reflect fundamental differences between M1 and S1, and/or differences in the extent of de-efferentation versus deafferentation.     

Functional imaging reveals numerous fields in the monkey auditory cortex

Anatomical studies propose that the primate auditory cortex contains more fields than have actually been functionally confirmed or described. Spatially resolved functional magnetic resonance imaging (fMRI) with carefully designed acoustical stimulation could be ideally suited to extend our understanding of the processing within these fields. However, after numerous experiments in humans, many auditory fields remain poorly characterized. Imaging the macaque monkey is of particular interest as these species have a richer set of anatomical and neurophysiological data to clarify the source of the imaged activity. We functionally mapped the auditory cortex of behaving and of anesthetized macaque monkeys with high resolution fMRI. By optimizing our imaging and stimulation procedures, we obtained robust activity throughout auditory cortex using tonal and band-passed noise sounds. Then, by varying the frequency content of the sounds, spatially specific activity patterns were observed over this region. As a result, the activity patterns could be assigned to many auditory cortical fields, including those whose functional properties were previously undescribed. The results provide an extensive functional tessellation of the macaque auditory cortex and suggest that 11 fields contain neurons tuned for the frequency of sounds. This study provides functional support for a model where three fields in primary auditory cortex are surrounded by eight neighboring “belt” fields in non-primary auditory cortex. The findings can now guide neurophysiological recordings in the monkey to expand our understanding of the processing within these fields. Additionally, this work will improve fMRI investigations of the human auditory cortex.     

Dynamics of functional reciprocal connections between cerebral motor cortex neurons under stimulation in cats

Activity was recorded from neurons belonging to the representation of the forelimb in the motor cortex (sulcus cruciatus, L 7–9 mm) using multiple multi-channel/barrel electrodes during acute experiments on cats. Cross-correlation analysis of impulse trains was adopted to investigate dynamics of interneuronal connections during passive flexion and electrical stimulation of the limb contralateral to the recording site. It was found that neither passive bending nor electrical stimulation of the limb leads to a significant increase in the total number of direct relationships between cortical neurons. At the same time, passive flexion does produce a considerable decrease in the number of instances of both inputs operating in neighboring neurons (50–100 µm apart) and an increase in cells located further (between 100 and 400 µm) apart. Some increase in the number of direct inhibitory interactions between neighboring neurons was observed during electrical stimulation.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Nentskii Institute of Experimental Biology, Warsaw, Poland. Center of Experimental and Clinical Medicine, Warsaw, Poland. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 73–80, January–February, 1991.     

Electrical characteristics of neurons in the cat motor cortex

Electrical characteristics of motor cortical neurons were studied in acute experiments on immobilized cats. Values of the input resistances varied from units to tens of megohms (mean 11.11±3.93 MΩ). The threshold current is a hyperbolic function of input resistance of the corresponding neurons and negative correlation was found between the axonal conduction velocity and input resistance. The time constant (τ₀) of the membrane was 7.1±3.46 msec. A time constant τ₁, of 1.65±0.36 msec, could also be distinguished in some neurons. Electrotonic lengths of dendrites of the cortical neurons were calculated by the use of Rall's model: mean 3.66±0.94 (in units of length constant).     

Clustered organization of neurons with similar extra-receptive field properties in the primary visual cortex

The primary visual cortex is organized into clusters of cells having similar classical receptive field (CRF) properties. Nonclassical, extra-receptive fields (ERFs) can either inhibit or facilitate the response elicited by stimulation within the CRF. Here, we report that in the primary visual cortex of cat, neurons with similar inhibitory or facilitatory ERF properties are also grouped into clusters. These clusters are randomly distributed in all cortical layers, with no detectable relationship with orientation and ocular dominance columns. This functional organization of neurons with respect to ERF properties may allow an efficient processing of global visual information.     

Roles for ephrins in positionally selective synaptogenesis between motor neurons and muscle fibers

Motor axons form topographic maps on muscles: rostral motor pools innervate rostral muscles, and rostral portions of motor pools innervate rostral fibers within their targets. Here, we implicate A subfamily ephrins in this topographic mapping. First, developing muscles express all five of the ephrin-A genes. Second, rostrally and caudally derived motor axons differ in sensitivity to outgrowth inhibition by ephrin-A5. Third, the topographic map of motor axons on the gluteus muscle is degraded in transgenic mice that overexpress ephrin-A5 in muscles. Fourth, topographic mapping is impaired in muscles of mutant mice lacking ephrin-A2 plus ephrin-A5. Thus, ephrins mediate or modulate positionally selective synapse formation. In addition, the rostrocaudal position of at least one motor pool is altered in ephrin-A5 mutant mice, indicating that ephrins affect nerve-muscle matching by intraspinal as well as intramuscular mechanisms.     

Representation of others' action by neurons in monkey medial frontal cortex

Successful social interaction depends on not only the ability to identify with others but also the ability to distinguish between aspects of self and others. Although there is considerable knowledge of a shared neural substrate between self-action and others' action, it remains unknown where and how in the brain the action of others is uniquely represented. Exploring such agent-specific neural codes is important because one's action and intention can differ between individuals. Moreover, the assignment of social agency breaks down in a range of mental disorders. Here, using two monkeys monitoring each other's action for adaptive behavioral planning, we show that the medial frontal cortex (MFC) contains a group of neurons that selectively encode others' action. These neurons, observed in both dominant and submissive monkeys, were significantly more prevalent in the dorsomedial convexity region of the MFC including the pre-supplementary motor area than in the cingulate sulcus region of the MFC including the rostral cingulate motor area. Further tests revealed that the difference in neuronal activity was not due to gaze direction or muscular activity. We suggest that the MFC is involved in self-other differentiation in the domain of motor action and provides a fundamental neural signal for social learning.     

Auditory modulation of the inferior temporal cortex neurons in rhesus monkey

We performed a systematic study to check whether neurons in the area TE (the anterior part of inferotemporal cortex) in rhesus monkey, regarded as the last stage of the ventral visual pathway, could be modulated by auditory stimuli. Two fixating rhesus monkeys were presented with visual, auditory or combined audiovisual stimuli while neuronal responses were recorded. We have found that the visually sensitive neurons are also modulated by audiovisual stimuli. This modulation is manifested as the change of response rate. Our results have shown also that the visual neurons were responsive to the sole auditory stimuli. Therefore, the concept of inferotemporal cortex unimodality in information processing should be re-evaluated.     

GABA in developing rat skeletal muscle and motor neurons

Protoplasma - In recent years, considerable evidence is accumulated pointing to participation of gamma-aminobutyric acid (GABA) in intercellular signaling in the peripheral nervous system,...     

Synchrony between circular and longitudinal muscle contractions during peristalsis in normal subjects

The current understanding is that longitudinal muscle contraction begins before and outlasts circular muscle contraction during esophageal peristalsis in normal subjects. The goal of our study was to reassess the relationship between the contractility of two muscle layers using novel ways to look at the muscle contraction. We studied normal subjects using synchronized high-frequency ultrasound imaging and manometry. Swallow-induced peristalsis was recorded at 5 and 10 cm above the lower esophageal sphincter (LES). Ultrasound (US) images were analyzed for muscle cross-sectional area (CSA) and circularity index of the esophagus during various phases of esophageal contraction. A plot of the M mode US image, muscle CSA, and esophageal circularity index was developed to assess the temporal correlation between various parameters. The muscle CSA wave began before and lasted longer than the contraction pressure wave at both 5 and 10 cm above the LES. M mode US images revealed that the onset of muscle CSA wave was temporally aligned with the onset of lumen collapse. The peak muscle CSA occurred in close proximity with the peak pressure wave. The esophagus started to become more circular (decrease in circularity index) with the onset of the muscle CSA wave. The circularity index and muscle CSA returned to the baseline at approximately the same time. In conclusion, the onset of lumen collapse and return of circularity index of the esophagus are likely to be the true markers of the onset and end of circular muscle contraction. Circular and longitudinal muscle layers of the esophagus contract in a precise synchronous fashion during peristalsis in normal subjects.     

Penis-retractor muscle of Aplysia: excitatory motor neurons

Cobalt axonal iontophoresis and intracellular recordings were used to identify a cluster of several motor neurons innervating the penis-retractor muscle of Aplysia. Intracellularly recorded motor neuron action potentials elicited direct, one-for-one, constant latency excitatory junctional potentials (ejps) in individual muscle fibers. The axons of motor neurons could be recorded extracellularly in the penis-retractor nerve and stimulation of the nerve backfired the motor neurons. Perfusion of the ganglion, the muscle, or both with solutions of either increased Mg++/decreased Ca++ or increased Ca++ sea water indicated that the presumed motor neuron impaled was not a sensory cell and that interneurons were not intercalated in the pathway. Innervation of muscle fibers was found to be functionally polyneuronal and diffuse. The ejps were found to undergo marked facilitation with repetitive motor-neuron stimulation. The motor neurons were isolated in a distinct cluster in the right pedal ganglion. Their electrical activity was characterized by spontaneous irregular action potentials and a moderate input of postsynaptic potentials.