Supraspinal control of slow and fast spinal motoneurons of the cat

Recruitment order among motoneurons from different motor nuclei

Recruitment order among motoneurons from different motor nuclei. The principles by which motoneurons (MNs) innervating different multiple muscles are organized into activity are not known. Here we test the hypothesis that coactivated MNs belonging to different muscles in the decerebrate cat are recruited in accordance with the size principle, i.e., that MNs with slow conduction velocity (CV) are recruited before MNs with higher CV. We studied MN recruitment in two muscle pairs, the lateral gastrocnemius (LG) and medial gastrocnemius (MG) muscles, and the MG and posterior biceps femoris (PBF) muscles because these pairs are coactivated reliably in stretch and cutaneous reflexes, respectively. For 29/34 MG-LG pairs of MNs, the MN with lower CV was recruited first either in all trials (548/548 trials for 22 pairs) or in most trials (225/246 trials for 7 pairs), whether the MG or the LG MN in a pair was recruited first. Intertrial variability in the force thresholds of MG and LG MNs recruited by stretch was relatively low (coefficient of variation = 18% on average). Finally, punctate stimulation of the skin over the heel recruited 4/4 pairs of MG-LG MNs in order by CV. By all of these measures, recruitment order is as consistent among MNs from these two ankle muscles as it is for MNs supplying the MG muscle alone. For MG-PBF pairings, the MN with lower CV was recruited first in the majority of trials for 13/24 pairs and in reverse order for 9/24 pairs. The recruitment sequence of coactive MNs supplying the MG and PBF muscles was, therefore, random with respect to axonal conduction velocity and not organized as predicted by the size principle. Taken together, these findings demonstrate for the first time, that the size principle can extend beyond the boundaries of a single muscle but does not coordinate all coactive muscles in a limb.

Alternate activity in the synergistic muscles during prolonged low-level contractions

The purpose of this study was to investigate the functional interrelationship between synergistic muscle activities during low-level fatiguing contractions. Six human subjects performed static and dynamic contractions at an ankle joint angle of 110 degrees plantar flexion and within the range of 90-110 degrees (anatomic position = 90 degrees) under constant load (10% maximal voluntary contraction) for 210 min. Surface electromyogram records from lateral gastrocnemius (LG), medial gastrocnemius (MG), and soleus (Sol) muscles showed high and silent activities alternately in the three muscles and a complementary and alternate activity between muscles in the time course. In the second half of all exercise times, the number of changes in activity increased significantly (P < 0.05) in each muscle. The ratios of active to silent periods of electromyogram activity were significantly higher (P < 0.05) in MG (4.5 +/- 2.2) and Sol (4.3 +/- 2.8) than in the LG (0.4 +/- 0.1), but no significant differences were observed between MG and Sol. These results suggest that the relative activation of synergistic motor pools are not constant during a low-level fatiguing task.

Integration in descending motor pathways controlling the forelimb in the cat. 14. Differential projection to fast and slow motoneurones from excitatory C3-C4 propriospinal neurones

The projection of C3-C4 propriospinal neurones (PNs) to alpha-motoneurones of forelimb muscles has been analysed with the aid of antidromic stimulation of the ascending branch of the PNs to the lateral reticular nucleus (LRN). A single stimulus of 500 microA applied in the caudo-dorsal part of the LRN evoked a maximal or greater than 90% maximal monosynaptic EPSP in the motoneurones. Systematic mapping of EPSPs evoked by stimulation of 500 microA in and around the LRN revealed that at this strength there was hardly any co-activation of a medial system (Peterson et al. 1979) which evoked small monosynaptic EPSPs with shorter latency and faster time course. The LRN EPSP amplitude was positively correlated with the homonymous group Ia EPSP amplitude, the input resistance and the afterhyperpolarization (AHP) duration. It is therefore postulated that the LRN EPSP amplitude is correlated with motor unit type (Burke 1967, 1968; Burke et al. 1973) with the largest EPSPs in slow (S), the smallest in fast, fatiguable (FF) and possibly intermediate sized in fast, fatigue resistant (FR) units. There was only a small difference in latency of the LRN EPSP in fast and slow motoneurones, while the time course was considerably slower in the latter. It is suggested that slow motoneurones receive projection both from fast and slowly conducting PNs but fast motoneurones mainly from fast PNs. Comparison of the disynaptic pyramidal EPSPs and the LRN EPSPs revealed a positive correlation, but the amplitude ratio pyramidal EPSP: LRN EPSP was smaller in slow than in fast motoneurones. A negative correlation was found between this amplitude ratio and the latency of the disynaptic pyramidal EPSP. It is suggested that this correlation reflects the excitability level in the PNs and that low excitability is due to inhibition of the PNs.

Pyramidal effects in dorsal neck motoneurones of the cat

The effects of contralateral pyramidal stimulation have been investigated with intracellular recording from cat alpha-motoneurones that innervate the dorsal neck musculature. A short train of stimuli evoked three types of synaptic effects: predominant excitation or inhibition and mixed effects characterized chiefly by early excitation followed by inhibition. Latency measurements indicated a minimal disynaptic linkage for excitation and for inhibition. Splenius motoneurones received primarily excitation whereas biventer cervicis-complexus motoneurones received a more varied input characterized by mixed effects or inhibition. Following transection of the pyramid just rostral to the decussation (lower pyramidal lesion) pyramidal stimulation above the lesion still produced disynaptic excitation and longer latency (possibly trisynaptic) inhibition. Pyramidal stimulation just caudal to this transection evoked inhibition with a minimal disynaptic latency, as well as longer latency excitation. The incidence of longer latency excitation was found to be reduced in cats with corticospinal tract transections at the level of the second cervical spinal segment. No post-synaptic potentials were evoked by pyramidal stimulation rostral to a pyramidal transection at the level of the trapezoid body. It is suggested that disynaptic excitation evoked by pyramidal stimulation above the lower pyramidal lesion is mediated by medullary reticulospinal neurones possessing monosynaptic excitatory connexions with neck motoneurones. Longer latency excitation appears to be mediated by neurones that receive corticospinal tract input and are located in the spinal segments containing the neck motoneurones. Disynaptic inhibition is mediated by neurones likely to be situated between the second cervical spinal segment and the level of the lower pyramidal lesion. The results also suggest that the first neurone in the chain mediating longer latency inhibition is located in the brain stem. The differences in pyramidal synaptic input between splenius and biventer cervicis-complexus motoneurones are considered in relation to the roles these muscles may serve in head position control.

Le Cerveau Influence-t-il le Développement Musculaire du Foetus Humain?

Does the human fetal brain influence muscle development? The importance of suprasegmental cerebral influence on developing human fetal muscle is less well understood than the control of histochemical differentiation of muscle by the motor neuron. Muscle biopsies of 21 hypotonic infants and children with nonprogressive congenital lesions of the cerebellum and/or brainstem were studied by histochemical methods. Two neonates who died with severely dysplastic brains and no descending motor tracts had normal muscle. The others, particularly those with cerebellar hypoplasia, had delayed muscle maturation, selective predominance of type I or II muscle fibres or disproportion in fibre sizes. It is concluded that the motor unit is capable of developing normally without suprasegmental influence, but that an abnormal balance of descending impulses may alter histochemical differentiation of fetal muscle. The small ‘subcorticospinal’ pathways arising in the brainstem probably are more influential than the larger corticospinal tract because of later myelination in the latter. The muscle biopsy thus serves to provide evidence of suprasegmental disease in infantile hypotonia.

A morphological study of the axons and recurrent axon collaterals of cat sciatic alpha-motoneurons after intracellular staining with horseradish peroxidase

Utilizing the centrifugal neuronal transport of intracellularly injected horseradish peroxidase (HRP), we have performed a light microscopic (LM) investigation of the intramedullary parts of the axons and axon collaterals of sciatic alpha-motoneurons in the adult cat. The intramedullary parts of the alpha-motor axons had comparatively short internodes (down to 75 microns) and were thinner than reported in earlier studies on the ventral root. Positive correlations were obtained when relating nodal diameters (2.8-7.8 micron) or the mean diameters of the motor axons in the white matter (4.4-9.0 micron) to the diameters of the initial axonal segments (2.3-4.9 micron). Eighty percent of the motor axons gave off one to five collaterals. There was no correlation between the numbers of collaterals and the lengths of the parent motor axons in the gray matter. The branching patterns of the axon collaterals showed considerable variation and the number of end branches from a single collateral ranged between 1 and 39. The rostro-caudal distribution of the collateral end branches was arranged symmetrically within a narrow space (+/- 300 micron) around the origins of the first order collaterals. Outbulgings of the motor axon collaterals, interpreted as synaptic terminals, were found along (59%) or at the ends (41%) of the collateral branches, and were located 200-700 micron away from the origin of the first order collateral. No characteristic LM feature of the outbulgings was distinguished.