There is currently much interest in the synchronisation of neural discharge and the potential role it may play in information coding within the nervous system. We describe some recent results from investigations of synchronisation within the motor system. Local field potentials (LFPs) and identified pyramidal tract neurones (PTNs) were recorded from the primary motor cortex of monkeys trained to perform a precision grip task. The LFPs showed bursts of oscillatory activity at 20-30 Hz, which were coherent with the rectified electromyographs (EMG) of contralateral hand and forearm muscles. This oscillatory synchronisation showed a highly specific task dependence, being present only during the part of the task when the animal maintained a steady grip and not during the movement phases before or after it. PTNs were phase-locked to LFP oscillations, implying that at least part of the coherence between cortical activity and EMG was mediated by corticospinal fibres. The phase locking of the PTNs to LFP oscillations produced task-dependent oscillatory synchronisation between PTN pairs, as assessed by the single-unit cross-correlation histogram. Recordings were also made from normal human subjects performing a precision grip similar to that used in the monkey recordings. Pairs of EMGs recorded from intrinsic hand and forearm muscles showed 20-30 Hz coherence, which modulated during task performance, being present only during periods of steady contraction. We suggest that these changes in EMG-EMG synchronisation reflect changing levels of synchronous drive from the corticospinal system. The generation of oscillations in the cortex is discussed in the light of results from a model of local cortical circuits. Other modelling work has shown that synchrony in the corticospinal inputs could act to recruit motoneurones more efficiently, producing more output force from a muscle than asynchronous inputs firing at the same mean rate. A speculative hypothesis is presented on the role of synchronous oscillations in the motor system, which is consistent with experimental observations to date.

Baker, Stuart N., Elizabeth M. Pinches, and Roger N. Lemon. Synchronization in monkey motor cortex during a precision grip task. II. Effect of oscillatory activity on corticospinal output. J Neurophysiol 89: 1941Neurophysiol 89: -1953Neurophysiol 89: , 2003 10.1152/jn.00832.2002. Recordings from primary motor cortex (M1) during periods of steady contraction show oscillatory activity; these oscillations are coherent with the activity of contralateral muscles. We investigated synchronization of corticospinal output neurons with the oscillations, which could provide the pathway for their transmission to the spinal motoneurons. One hundred seventy-six antidromically identified pyramidal tract neurons (PTNs) were recorded from M1 in three macaque monkeys trained to perform a precision grip task. Local field potentials (LFP) were simultaneously recorded. All analysis was confined to the hold period of the task, where our previous work has shown that there is the strongest oscillatory activity. Coherence was calculated between LFP and PTN discharge. Significant coherence was seen in three bands, with frequencies of 10 -14, 17-31, and 34 -44 Hz. Coherence values were low, with the majority of PTN-LFP coherences having a peak lower than 0.05. The phase of coherence was approximately Ϫ/2 radians for each band (with LFP polarity defined as negative upward), although there was some dispersion of phase across the population of PTNs. Coherence was also calculated between pairs of PTNs that had been simultaneously recorded. Where there was significant coherence, it was also generally smaller than 0.05. The phase of PTN-PTN coherence clustered around zero radians. A computer model was constructed to assist the interpretation of the experimental results. It simulated an integrate-and-fire neuron responding to synaptic inputs. A fraction of the synaptic inputs was synchronized with a simulated LFP; the remainder were uncorrelated with it. The model showed that coherence between the LFP and the output spike train considerably underestimated the fraction of synchronized inputs. Additionally, for a given fraction of synchronized inputs, coherence was smaller for high-compared with low-frequency bins. Cell discharge rate also influenced the spike-LFP coherence: coherence was higher for simulations in which the cell discharged at a faster rate. Thus although levels of PTN-LFP coherence seen experimentally were low, a considerable proportion of the input to the PTN must be synchronized with the global oscillatory activity recorded by the LFP. The low LFP-PTN coherences do however indicate that cortical oscillations are transmitted with only low fidelity in the discharge of a single PTN. Using further computer simulations, it was demonstrated that a small population of PTNs could encode the cortical oscillatory signal effectively, since the action of averaging across the population improves the signal:noise ratio. The oscillations will therefore be effectively transmitted to spinal motoneurons, and this has important consequences for ...

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In the developing retinotectal projection, retinal axon arbor structure changes rapidly within the target tectal neuropil at stages when the visual system functions to process visual information. In vivo imaging of single retinotectal axon arbors shows that up to 50% of the arbor branch length can be restructured within 8 h and short branchtips have average lifetimes of 10 min. To determine if presynaptic sites are restricted to the relatively stable part of the arbor or if they are also located on the more dynamic portions of the arbor, punctate staining of synaptic vesicle proteins (SVP) synapsin 1 and synaptophysin was mapped within individual retinal axons using double-label confocal immunocytochemistry. We report that SVP puncta were distributed throughout the retinotectal axon arbor. Notably, short branchtips, which are known to be extremely dynamic, contain the presynaptic machinery necessary for synaptic transmission. These data support a model in which activity-dependent mechanisms can influence presynaptic axon arbor morphology by modifying the rate of dynamic rearrangements of axonal branchtips.

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