The role of the corticomotor system in precision finger movements
The ability to perform precise and relatively independent movements of the fingers is an important feature of the primate's motor repetoire. A substantial amount of evidence suggests a special role for the hand area of the motor cortex in the execution of these movements, and in particular, thoses cells which make direct cortico-motoneuronal connections with motoneurones innervating the hand muscles. The axons of these corticomotoneuronal (CM) cells form part of the pyramidal tract. The subject of this study is the influence of these CM cells upon the activity of hand muscles in the conscious monkey performing a precision grip task between thumb and index fingers. Three aspects of the cortico-motoneuronal connection have been investigated; 1. The effect of discharges in individual CM cells on the gross e.m.g. activity of intrinsic hand and forearm muscles, and the distribution of these effects over various muscles. 2. The effect of individual CM cell discharge on the single motor unit activity in the intrinsic thumb muscles. 3. The significance of differences in firing frequency of individual CM cells upon their modulation of gross e.m.g. The spike-triggered averaging technique was employed to study the overall effect of identified pyramidal tract neurones (PTN) on gross e.m.g. activity. Rectified gross e.m.g. was averaged with respect to the discharges of single PTNs. The influence of the triggering cortical cell was revealed in the averages as a transient increase in muscle activity at the appropriate time after the cell had fired. This effect is called post-spike facilitation (PSF). The occurance of PSF in the spike-triggered average was taken as evidence that the triggering cells made a direct, presumably monosynaptic, connection with the motoneurones of the muscle in question. Quantitative and qualitative aspects of PSF are discussed. The activity of 5 to 10 intrinsic hand and forearm muscles were averaged with respect to individual PTNs. In this way the PSF distribution over different muscles, produced by one cortical cell, could be determined. It was established that CM cells produce PSF in a relatively restricted number of muscles (2-3 out of 10 tested). The impact of CM cells on the discharge activity of single motor units was studied by cross-correlating both spike trains. To overcome the considerable sampling problem, the occurrence of PSF in the spike-triggered gross e.m.g. average was used as a criterion for further cross-correlation analysis. The cross-correlogram peaks half-width and onset latency were suggestive for monosynaptic excitation of the motoneurones. Different approaches are discussed to estimate the strength of these connections and to relate these strengths to the number of synpatic boutons that the CM cell makes upon its target motoneurones. In terms of the number of extra discharges fired by the single motor unit in response to CM cell firing, the effects can be described as weak (5-20 extra motor unit spikes per 1000 CM cell discharges). It was possible to study the connectivity of single CM cells with different motor units of the same muscle by simultaneously recording several (2-5) motor units. It was found that if a CM cell showed a correlogram peak with one of the concurrently sampled motor units, most other motor units sampled from the same muscle would give correlogram peaks with the same CM cell. This finding is suggestive for a rather diffuse collateralization of the CM cell axons within the motoneurone pool of the target muscle. Finally the influence of different firing frequencies of CM cells on the form and strength of PSF effects was investigated. A striking result was that discharges preceded by long interspike intervals (> 50ms)could still exert quite powerful excitatory effects. The results of this series of experiments further elucidate the nature of the corticomotoneuronal connection and give an impression of the facilitatory capacity of single CM cells on motoneurons innervating the hand muscles.