Direct Cortico-motoneuronal Connections Research Articles

Temporal Course of Transient Direct Corticomotoneuronal Connections during Development in Rodents

We previously observed in rodents that during the 2nd postnatal week corticospinal axons make monosynaptic connections with motoneurons. Prior to that finding, it had been believed that such contacts only occur in higher primates. Although an in vitro electrophysiological study is prerequisite for studying the developmental time course of synaptic connections, the technical difficulty of reliably recording synaptic responses from spinal motoneurons in animals over 2 weeks old hampered the study. Instead, we used retrograde transsynaptic labeling with a genetically modified rabies virus to confirm the presence of direct corticomotoneuronal connections at an early developmental stage and to show that these connections were subsequently eliminated. However, determination of an accurate elimination time course and quantitative evaluation of synaptic connectivity cannot be achieved through viral-tracing experiments.For the present study, we improved the slice preparation procedure and maintenance of slice viability, which enabled us to record postsynaptic responses using the whole cell patch-clamp technique from retrogradely labeled forearm motoneurons up until postnatal week 7. We examined the extent of corticomotoneuronal monosynaptic connections and studied the time course of their accumulation and loss. Positive ratios of monosynaptic corticomotoneuronal EPSCs increased from P6 to P8 and then plateaued (P8–P13: 65%). Thereafter, the monosynaptic connections declined until P21, at which time they were no longer detected. The time course of the falling phase and elimination was confirmed by experiments using optogenetic stimulation. The timing of the elimination fell within the same range (P18-22) estimated in our earlier study using retrograde transsynaptic labeling.

Higher primate-like direct corticomotoneuronal connections are transiently formed in a juvenile subprimate mammal

The corticospinal (CS) tract emerged and evolved in mammals, and is essentially involved in voluntary movement. Over its phylogenesis, CS innervation gradually invaded to the ventral spinal cord, eventually making direct connections with spinal motoneurons (MNs) in higher primates. Despite its importance, our knowledge of the origin of the direct CS-MN connections is limited; in fact, there is controversy as to whether these connections occur in subprimate mammals, such as rodents. Here we studied the retrograde transsynaptic connection between cortical neurons and MNs in mice by labeling the cells with recombinant rabies virus. On postnatal day 14 (P14), we found that CS neurons make direct connections with cervical MNs innervating the forearm muscles. Direct connections were also detected electrophysiologically in whole cell recordings from identified MNs retrogradely-labeled from their target muscles and optogenetic CS stimulation. In contrast, few, if any, lumbar MNs innervating hindlimbs showed direct connections on P18. Moreover, the direct CS-MN connections observed on P14 were later eliminated. The transient CS-MN cells were distributed predominantly in the M1 and S1 areas. These findings provide insight into the ontogeny and phylogeny of the CS projection and appear to settle the controversy about direct CS-MN connections in subprimate mammals.

Skilled reaching and grasping in the rat: lacking effect of corticospinal lesion.

The corticospinal system is a major motor pathway in the control of skilled voluntary movements such as reaching and grasping. It has developed considerably phylogenetically to reach a peak in humans. Because rodents possess advanced forelimb movements that can be used for reaching and grasping food, it is commonly considered that the corticospinal tract (CST) is of major importance for this control also in rodents. A close homology to primate reaching and grasping has been described but with obvious limitations as to independent digit movements, which are lacking in rodents. Nevertheless, it was believed that there are, as in the primate, direct cortico-motoneuronal connections. Later, it was shown that there are no such connections. The fastest excitatory pathway is disynaptic, mediated via cortico-reticulospinal neurons and in the spinal cord the excitation is mainly polysynaptically mediated via segmental interneurons. Earlier behavioral studies have aimed at investigating the role of the CST by using pyramidotomy in the brainstem. However, in addition to interrupting the CST, a pyramidal transection abolishes the input to reticulospinal neurons. It is therefore not possible to conclude if the deficits after pyramidotomy result from interruption of the CST or the input to reticulospinal neurons or both. We have re-investigated the role of the CST by examining the effect of a CST lesion in the C1–C2 spinal segments on the success rate of reaching and grasping. This lesion spares the cortico-reticulospinal pathway. In contrast to investigations using pyramidal transections, the present study did not demonstrate marked deficits in reaching and grasping. We propose that the difference in results can be explained by the intact cortical input to reticulospinal neurons in our study and thus implicate an important role of this pathway in the control of reaching and grasping in the rat.

Spinal inhibition of descending command to soleus motoneurons is removed prior to dorsiflexion

It has recently been demonstrated that soleus motor-evoked potentials (MEPs) are facilitated prior to the onset of dorsiflexion. The purpose of this study was to examine if this could be explained by removal of spinal inhibition of the descending command to soleus motoneurons. To test this, we investigated how afferent inputs from the tibialis anterior muscle modulate the corticospinal activation of soleus spinal motoneurons at rest, during static contraction and prior to movement. MEPs activated by transcranial magnetic stimulation (TMS) and Hoffmann reflexes (H-reflexes), activated by electrical stimulation of the posterior tibial nerve (PTN), were conditioned by prior stimulation of the common peroneal nerve (CPN) at a variety of conditioning-test (CT) intervals. MEPs in the precontracted soleus muscle were inhibited when the TMS pulse was preceded by CPN stimulation with a CT interval of 35 ms, and they were facilitated for CT intervals of 50-55 ms. A similar inhibition of the soleus H-reflex was not observed. To investigate which descending pathways might be responsible for the afferent-evoked inhibition and facilitation, we examined the effect of CPN stimulation on short-latency facilitation (SLF) and long-latency facilitation (LLF) of the soleus H-reflex induced by a subthreshold TMS pulse at different CT intervals. SLF is known to reflect the excitability of the fastest conducting, corticomotoneuronal cells whereas LLF is believed to be caused by more indirect descending pathways. At CT intervals of 40-45 ms, the LLF was significantly more inhibited compared to the SLF when taking the effect on the H-reflex into account. Finally, we investigated how the CPN-induced inhibition and facilitation of the soleus MEP were modulated prior to dorsiflexion. Whereas the late facilitation (CT interval: 55 ms) was similar prior to dorsiflexion and at rest, no inhibition could be evoked at the earlier latency (CT interval: 35 ms) prior to onset of dorsiflexion. The observation that the CPN-induced inhibition of soleus MEPs disappears prior to onset of dorsiflexion may explain why soleus MEPs are facilitated prior to onset of dorsiflexion contraction. A possible mechanism involves the removal of inhibition of the descending command to the motoneurons at a spinal interneuronal level because the inhibition was seen in LLF and not in SLF, and the MEP inhibition was not observed in the H-reflex. The data illustrate that spinal interneuronal pathways modify descending commands to human spinal motoneurons and influence the size of MEPs elicited by TMS.

Premotoneuronal and direct corticomotoneuronal control in the cat and macaque monkey.

The literature on premotoneuronal and direct corticomotoneuronal (CM) control in the cat and macaque monkey is reviewed. The available experimental findings are not in accordance with a recently proposed hypothesis that direct CM connections have "replaced" the premotoneuronal pathways. Instead, we propose that premotoneuronal CM control plays an important role in motor control also in primates and that the direct CM connection has been added during phylogeny.

Striking Differences in Transmission of Corticospinal Excitation to Upper Limb Motoneurons in Two Primate Species

There is considerable debate as to the relative importance, for cortical control of upper limb movements, of direct cortico-motoneuronal (CM) versus indirect, propriospinal transmission of corticospinal excitation to cervical motoneurons. In the cat, which has no CM connections, a significant proportion of corticospinal excitation reaches forelimb motoneurons via a system of C(3)-C(4) propriospinal neurons (PN). In contrast, in the macaque monkey most motoneurons receive direct CM connections, and, under the same experimental conditions as in the cat, there is little evidence for PN transmission. We have investigated corticospinal transmission in the New World squirrel monkey (Saimiri sciureus) because its CM projections are weaker than in the macaque. Intracellular recordings were made from motoneurons identified from the ulnar, median, and deep radial (DR) nerves in four adult squirrel monkeys under chloralose anesthesia and neuromuscular paralysis. Responses to stimulation of the contralateral medullary pyramid were recorded before and after a lesion to the dorsolateral funiculus (DLF) at C(5), designed to interrupt direct corticospinal inputs to the lower cervical segments and unmask PN-mediated effects. This lesion greatly reduced the proportion of motoneurons showing either CM EPSPs or disynaptic IPSPs, but the proportion showing late EPSPs with segmental latencies beyond the monosynaptic range, evoked by repetitive but not single PT stimuli, was unaffected: 23 of 29 motoneurons (79%) before and 32 of 37 (86%) after the lesion; 41% of these late EPSPs had strictly disynaptic latencies after the lesion, only 14% before. These results are in striking contrast to the macaque (late EPSPs in only 18% of motoneurons before a C(5) lesion, 19% after it). Transmission of the late EPSPs via C(3)-C(4) PNs in the squirrel monkey was indicated by their absence after an additional C(2) DLF lesion. Nearly all tested motoneurons also responded with short latency EPSPs to stimulation in the ipsilateral lateral reticular nucleus. By analogy with the cat, these EPSPs probably reflect antidromic activation of ascending collaterals of C(3)-C(4) PNs with monosynaptic connections to motoneurons; the EPSPs were significantly smaller than in the cat but larger than in the macaque. These results suggest that the positive correlation across species between more advanced hand function and the strength of the CM system is accompanied by a negative correlation between hand function and the strength of the PN system. We hypothesize that in primates with more advanced hand function, the CM system effectively replaces PN-mediated control. This would include a contribution to the control of reaching movements, which are said to be specifically under the control of the PN system in the cat, and we speculate that these differences may be related to the degree of dexterity exhibited by the different species. This interpretation of the results predicts that in man, where the CM system is highly developed, the PN system is unlikely to be responsible for significant transmission of cortical commands to upper limb motoneurons.

Corticomotoneuronal connections in the rat: Evidence from double‐labeling of motoneurons and corticospinal axon arborizations

In order to investigate the possibility of direct corticomotoneuronal (CM) connections in the rat, an anterograde-retrograde double-labeling method was developed. Phaseolus vulgaris-leucoagglutinin (PHA-L) anterograde tracing of corticospinal axons was combined with retrograde labeling of spinal motoneurons either by a conjugate of choleragen subunit B with horseradish peroxidase (CB-HRP) or by wheat germ agglutinin (WGA). The location of PHA-L injection unilaterally in the forelimb area of sensorimotor cortex and the CB-HRP or WGA injections in corresponding contralateral wrist or digit extensors or flexors were determined and matched on the basis of movement responses elicited by intracortical microstimulation. Light microscopic observation showed, in addition to the main contralateral dorsal corticospinal tract (CST), the presence of four other CST minor components in the contralateral lateral, ipsilateral ventral, and ipsilateral dorsal funiculi of the cervical spinal white matter and at the base of contralateral dorsal horn of the gray matter, respectively. PHA-L-labeled CST axonal arbors were observed from Rexed's lamina I through lamina X of contralateral spinal gray matter, most extensively in laminae VI and VII; some CST axons reached the zone of motoneuronal somata in lamina IX and a few of them also entered the lateral and occasionally the ventral funiculi, ramifying in the white matter. Between the zones of PHA-L-labeled CST axonal arbors on the one hand and CB-HRP/WGA labeled spinal motoneuronal somata with their extensive dendritic trees on the other, there was a large overlap, covering partly both the gray and the white matter. PHA-L-labeled axonal boutons (en passant or terminaux) were seen to contact the dendrites or even the somata of motoneurons in the gray matter, according to light-microscopic criteria for identification of synaptic contacts. Axodendritic CM contacts were occasionally observed in the lateral funiculus of the white matter as well. In general, only a single contact was observed between an individual PHA-L-labeled CST axon and a given retrogradely labeled motoneuron. In contrast to the common notion that direct CM connections are a specialty of primates, the present morphological data support the presence of direct CM connections also in some other mammals, such as the rat.

Corticobulbar projections to upper and lower facial motoneurons. A study by magnetic transcranial stimulation in man

To investigate the human corticofacial projections, we recorded the compound motor potentials and single motor unit potentials evoked by magnetic transcranial stimulation, in the frontalis and lower facial muscles of healthy subjects. Potentials secondary to activation of the corticobulbar tract were contralateral in lower and bilateral in upper facial muscles. Even though the latency of responses was longer than would be expected for direct cortico-motoneuronal connections, these cannot be excluded either for lower or upper facial motoneurons.

Obligatory bimanual associated movements: Report of a non-familial case in an otherwise normal left-handed boy

Amongst the abnormal synkinesias, bilateral and obligatory associated movements affecting predominantly the hands comprise a group of movement disorders which we have divided into: (1) those occurring as an autosomal dominant hereditary condition; (2) those occurring in the presence of disease of the central nervous system, including congenital abnormalities of the cervical region; and (3) rare, sporadic cases in otherwise healthy individuals. A left-handed patient falling into the last group is reported, in whom detailed neuropsychological assessment was also undertaken. Recalling the normal bimanual synkinesias of childhood, some of the mechanisms implicated in the production of pathological bimanual synkinesias are discussed: the effect of structural lesions within the central nervous system, defects in motor inhibition and the role of sensory afferent information, the part played by the commissures and abnormal motor pathways, the question of handedness, the relationship to mirror writing, and cerebral dominance have all been considered relevant, but from increasing evidence available from experimental work on primates the possible role of direct corticomotoneuronal connections is emphasised.

Corticospinal connections: postnatal development in the rhesus monkey.

The spinal distribution of the corticospinal fibers was studied experimentally in infant rhesus monkeys (Macaca mulatta) by means of the NautaGygax silver impregnation technique. The findings suggest that the bulk of the direct cortico-motoneuronal connections in the rhesus monkey are established postnatally, during at least the first 8 months of life.