Peter A. Kirkwood

Does a C3-C4 propriospinal system transmit corticospinal excitation in the primate? An investigation in the macaque monkey.

1 Synaptic responses to electrical stimulation of the contralateral pyramidal tract were measured in intracellular recordings from 206 upper limb motoneurones in ten chloralose‐anaesthetized macaque monkeys. The objective was to search for evidence of a disynaptic excitatory pathway via C3‐C4 propriospinal interneurones similar to that in the cat. 2 In monkeys with intact spinal cords, only a small proportion of motoneurones (19 %) responded with late EPSPs to repetitive stimulation of the pyramid; only 3 % had segmental latencies that were appropriate for a disynaptic pathway. 3 From previous studies in the cat, it was expected that a lesion to the dorsolateral funiculus (DLF) at C5 would interrupt the corticospinal input to the spinal segments supplying upper limb muscles, whilst leaving intact excitation transmitted via a C3‐C4 propriospinal system, the descending axons of which travel in the ventral part of the funiculus. In five of the monkeys a lesion was made to the DLF at C5 which spared the ventrolateral columns. It severely reduced the monosynaptic EPSPs and disynaptic IPSPs evoked from the pyramidal tract that were present in the intact monkey spinal cord, and which might have masked the presence of disynaptic EPSPs. However, even after the lesion the proportion of motoneurones with such late EPSPs was still low (18 %); 14 % of motoneurones had EPSPs within the disynaptic range. 4 In addition, some EPSPs with relatively long segmental latencies (> 1·1 ms) were recorded before and after the C5 lesions, but since these effects could be evoked by single stimuli, had stable latencies and did not facilitate with repetitive shocks, it is likely that they represent monosynaptic EPSPs evoked by slowly conducting corticospinal fibres which survived the lesions. 5 In seven of the monkeys motoneurone responses to stimulation of the ipsilateral lateral reticular nucleus (LRN) were also tested. Most motoneurones showed EPSPs with short latencies (1·2‐2·5 ms) and other properties characteristic of monosynaptic activation. This is consistent with the presence of collaterals of C3‐C4 propriospinal neurones to the LRN, as demonstrated in the cat. 6 These short‐latency EPSPs evoked from the LRN were just as common before (77 %) as after (75 %) the C5 lesion. They had small amplitudes both before (mean ± s.d. 1·1 ± 0·59 mV) and after (1·2 ± 0·72 mV) the lesion. Unlike the situation in the cat, only a small proportion (16 %) of motoneurones activated from the LRN showed late EPSPs after repetitive stimulation of the pyramid. 7 The results provide little evidence for significant corticospinal excitation of motoneurones via a system of C3‐C4 propriospinal neurones in the monkey. The general absence of responses mediated by such a system in the macaque, under experimental conditions similar to those in which they are seen in the cat, show that extrapolation of results from the cat to the primate should be made with considerable caution.

Restoration of hand function in a rat model of repair of brachial plexus injury

The incurability of spinal cord injury and subcortical strokes is due to the inability of nerve fibres to regenerate. One of the clearest clinical situations where failure of regeneration leads to a permanent functional deficit is avulsion of the brachial plexus. In current practice, surgical re-implantation of avulsed spinal roots provides a degree of motor recovery, but the patients neither recover sensation nor the use of the hand. In the present rat study, we show that transplantation of cultured adult olfactory ensheathing cells restores the sensory input needed for a complex, goal-directed fore-paw function and re-establishes synaptic transmission to the spinal grey matter and cuneate nucleus by providing a bridge for regeneration of severed dorsal root fibres into the spinal cord. Success in a first application of human olfactory ensheathing cells in clinical brachial plexus injury would open the way to the wider field of brain and spinal cord injuries.

The respiratory drive to thoracic motoneurones in the cat and its relation to the connections from expiratory bulbospinal neurones

The descending control of respiratory‐related motoneurones in the thoracic spinal cord remains the subject of some debate. In this study, direct connections from expiratory bulbospinal neurones to identified motoneurones were investigated using spike‐triggered averaging and the strengths of connection revealed were related to the presence and size of central respiratory drive potentials in the same motoneurones. Intracellular recordings were made from motoneurones in segments T5–T9 of the spinal cord of anaesthetized cats. Spike‐triggered averaging from expiratory bulbospinal neurones in the caudal medulla revealed monosynaptic EPSPs in all groups of motoneurones, with the strongest connections to expiratory motoneurones with axons in the internal intercostal nerve. In the latter, connection strength was similar irrespective of the target muscle (e.g. external abdominal oblique or internal intercostal) and the EPSP amplitude was positively correlated with the amplitude of the central respiratory drive potential of the motoneurone. For this group, EPSPs were found in 45/83 bulbospinal neurone/motoneurone pairs, with a mean amplitude of 40.5 μV. The overall strength of the connection supports previous measurements made by cross‐correlation, but is about 10 times stronger than that reported in the only previous similar survey to use spike‐triggered averaging. Calculations are presented to suggest that this input alone is sufficient to account for all the expiratory depolarization seen in the recorded motoneurones. However, extra sources of input, or amplification of this one, are likely to be necessary to produce a useful motoneurone output.

Patterns of expiratory and inspiratory activation for thoracic motoneurones in the anaesthetized and the decerebrate rat

The nervous control of expiratory muscles is less well understood than that of the inspiratory muscles, particularly in the rat. The patterns of respiratory discharges in adult rats were therefore investigated for the muscles of the caudal intercostal spaces, with hypercapnia and under either anaesthesia or decerebration. With neuromuscular blockade and artificial ventilation, efferent discharges were present for both inspiration and expiration in both external and internal intercostal nerves. This was also the case for proximal internal intercostal nerve branches that innervate only internal intercostal and subcostalis muscles. If active, this region of muscle in other species is always expiratory. Here, inspiratory bursts were almost always present. The expiratory activity appeared only gradually and intermittently, when the anaesthesia was allowed to lighten or as the pre‐decerebration anaesthesia wore off. The intermittent appearance is interpreted as the coupling of a slow medullary expiratory oscillator with a faster inspiratory one. The patterns of nerve discharges, in particular the inspiratory or biphasic activation of the internal and subcostalis layers, were confirmed by observations of equivalent patterns of EMG discharges in spontaneously breathing preparations, using denervation procedures to identify which muscles generated the signals. Some motor units were recruited in both inspiratory and expiratory bursts. These patterns of activity have not previously been described and have implications both for the functional role of multiple respiratory oscillators in the adult and for the mechanical actions of the muscles of the caudal intercostal spaces, including subcostalis, which is a partly bisegmental muscle.

Multiple phases of excitation and inhibition in central respiratory drive potentials of thoracic motoneurones in the rat

Intracellular recordings were made from motoneurones with axons in the intercostal nerves of T9 or T10 in adult rats, with neuromuscular blockade and artificial ventilation, under hypercapnia and under either anaesthesia or decerebration. In nearly all motoneurones, central respiratory drive potentials (CRDPs) were seen, which included an excitatory wave in inspiration, in expiration, or in both of these. This was the case both for motoneurones with axons in the internal intercostal nerve (n= 81) and for those with axons in the external intercostal nerve (n= 5). In the decerebrates, motoneurones with purely inspiratory CRDPs were rare (1/44), but those excited in both phases (showing biphasic CRDPs) were common (22/44). For about one‐third of biphasic CRDPs (11/30), the inspiratory depolarization was seen to reverse to a hyperpolarization when the motoneurone was depolarized, which was interpreted as indicating concurrent inhibition and excitation during this phase. A few motoneurones were seen where depolarization revealed signs of inhibition in both phases. The results confirm the novel observations of biphasic excitation in individual intercostal nerve branches, EMG sites and motor units reported in a companion paper. They also provide new insights into the functional roles of inhibition in motoneurones physiologically activated in natural rhythmic behaviours.

Responses of single corticospinal neurons to intracortical stimulation of primary motor and premotor cortex in the anesthetized macaque monkey

The responses of individual primate corticospinal neurons to localized electrical stimulation of primary motor (M1) and of ventral premotor cortex (area F5) are poorly documented. To rectify this and to study interactions between responses from these areas, we recorded corticospinal axons, identified by pyramidal tract stimulation, in the cervical spinal cord of three chloralose-anesthetized macaque monkeys. Single stimuli (≤400 μA) were delivered to the hand area of M1 or F5 through intracortical microwire arrays. Only 14/112 (13%) axons showed responses to M1 stimuli that indicated direct intracortical activation of corticospinal neurons (D-responses); no D-responses were seen from F5. In contrast, 62 axons (55%) exhibited consistent later responses to M1 stimulation, corresponding to indirect activation (I-responses), showing that single-pulse intracortical stimulation of motor areas can result in trans-synaptic activation of a high proportion of the corticospinal output. A combined latency histogram of all axon responses was nonperiodic, clearly different from the periodic surface-recorded corticospinal volleys. This was readily explained by correcting for conduction velocities of individual axons. D-responding axons, taken as originating in neurons close to the M1 stimulating electrodes, showed more I-responses from M1 than those without a D-response, and 8/10 of these axons also responded to F5 stimulation. Altogether, 33% of tested axons responded to F5 stimulation, most of which also showed I-responses from M1. These excitatory effects are in keeping with facilitation of hand muscles evoked from F5 being relayed via M1. This was further demonstrated by facilitation of test responses from M1 by conditioning F5 stimuli.

Voltage-dependent amplification of synaptic inputs in respiratory motoneurones

•  The processes whereby various excitatory and inhibitory inputs are integrated in spinal motoneurones during naturally occurring motor acts are not well understood, largely because there are amplifying mechanisms within the motoneurone that can control the effective strengths of the inputs. •  Knowledge of these processes is important in understanding conditions such as motoneurone disease, or the spasticity that can follow spinal cord injury or stroke •  Respiration is a natural motor act that continues normally under experimental conditions, and this study investigated, for the first time, the likely amplifying processes at work in respiratory motoneurones. •  In phrenic motoneurones, which control the most important respiratory muscle, the diaphragm, we found that the mechanism most favoured by investigations in other motoneurones, the activation of persistent inward currents via calcium channels, appears to make a very small contribution. Instead, modulation of synaptic currents (through NMDA channels) appears to be more important.

Connections between expiratory bulbospinal neurons and expiratory motoneurons in thoracic and upper lumbar segments of the spinal cord

Cross-correlation of neural discharges was used to investigate the connections between expiratory bulbospinal neurons (EBSNs) in the caudal medulla and expiratory motoneurons innervating thoracic and abdominal muscles in anesthetized cats. Peaks were seen in the cross-correlation histograms for around half of the EBSN-nerve pairs for the following: at T8, the nerve branches innervating internal intercostal muscle and external abdominal oblique muscle and a more distal branch of the internal intercostal nerve; and at L1, a nerve branch innervating internal abdominal oblique muscle and a more distal branch of the ventral ramus. Fewer peaks were seen for the L1 nerve innervating external abdominal oblique, but a paucity of presumed α-motoneuron discharges could explain the rarity of the peaks in this instance. Taking into account individual EBSN conduction times to T8 and to L1, as well as peripheral conduction times, nearly all of the peaks were interpreted as representing monosynaptic connections. Individual EBSNs showed connections at both T8 and L1, but without any discernible pattern. The overall strength of the monosynaptic connection from EBSNs at L1 was found to be very similar to that at T8, which was previously argued to be substantial and responsible for the temporal patterns of expiratory motoneuron discharges. However, we argue that other inputs are required to create the stereotyped spatial patterns of discharges in the thoracic and abdominal musculature.

Electrophysiological actions of the rubrospinal tract in the anaesthetised rat.

The rubrospinal tract (RST) of the rat is widely used in studies of regeneration and plasticity, but the electrophysiology of its spinal actions has not been described. In anaesthetised rats with neuromuscular blockade, a tungsten microelectrode was located in the region of the red nucleus (RN) by combining stereotaxis with recording of antidromic potentials evoked from the contralateral spinal cord. Single stimuli through this electrode typically elicited two descending volleys in the contralateral dorsolateral funiculus (DLF) separated by about 1 ms, and one volley recorded from the ipsilateral DLF. Latencies of the ipsilateral and the early contralateral volley were similar. The activation of these volleys depended on the location of the stimulation site in or near the RN. Evidence is adduced to show that: (a) the late contralateral volley is carried by fibres of RST neurones, synaptically activated; (b) the early contralateral volley is mostly carried by RST fibres stimulated directly; (c) the ipsilateral volley is sometimes carried by RST fibres from the RN on the side contralateral to the stimulus; (d) otherwise, either early volley may derive from fibres in other tracts. Synaptic potentials related to the volleys were recorded within the cervical enlargement and their distribution plotted on cross-sections of the spinal cord. These measurements suggest that the great majority of RST terminations are on interneurones in the intermediate region contralateral to the RN. Direct synaptic actions on motoneurones are likely to be weak. Stimulation parameters appropriate for specific activation of the RST in future studies are suggested.

Specificity in monosynaptic and disynaptic bulbospinal connections to thoracic motoneurones in the rat.

•  In the rat, unlike in other species, motoneurones of both the internal intercostal nerve and the external intercostal nerve show a phase of excitation in expiration. •  This study investigated the pathways transmitting this excitation from the medulla. •  Direct (monosynaptic) excitation was found from individual expiratory neurones in the medulla to internal intercostal nerve motoneurones, but only indirect (disynaptic) excitation was found from the same neurones to the motoneurones of the external intercostal nerve. •  This is the first demonstration of two separate pathways from individual long descending fibres specific to two different sets of motoneurones. •  This specificity could be useful in studying plasticity or regeneration in thoracic segments in investigations of mechanisms involved in spinal cord injury and repair.