THE DESCENDING PATHWAYS TO THE SPINAL CORD, THEIR ANATOMY AND FUNCTION

Descending pathways to the spinal cord in the himé salmon (landlocked red salmon, Oncorhynchus nerka)

Distribution and morphology of the cells of origin of the descending spinal pathways and their axonal courses were studied in the himé salmon, using retrograde labelling with cobaltic lysine and horseradish peroxidase (HRP). Following application of the tracers to the cut end of the spinal cord or injection of the tracers at the 10th to 15th spinal segment, neurons mainly labelled via the axons of passage were distributed in the mesencephalon and the rhombencephalon. Mesencephalic cell groups consisted of the nucleus pretectalis, the nucleus fasciculi longitudinalis medialis, and the nucleus ruber. The former two cell groups sent their axons to the fasciculus longitudinalis medialis. The axons of the nucleus ruber formed a separate loose bundle, the "tractus rubrospinalis." The rhombencephalic cell groups consisted of the rhombencephalic reticular formation, the Mauthner cells (one cell for each side), and the octavolateral area. The rhombencephalic reticular formation could be further subdivided into the nucleus reticularis superior, nucleus reticularis medius, and nucleus reticularis inferior. The axons of these cell groups joined the fasciculus longitudinalis medialis and the "tractus bulbospinalis." The Mauthner cell had two main gigantic dendrites, and its giant axons formed a conspicuous fiber of Mauthner throughout the rhombencephalon down to the spinal cord. The octavolateral area could be subdivided into the nucleus vestibularis magnocellularis, nucleus tangentialis, nucleus vestibularis descendens and nucleus intermedius. The axons of the nucleus vestibularis magnocellularis and nucleus intermedius entered the fasciculus longitudinalis medialis and/or the tractus bulbospinalis. Those of the nucleus vestibularis descendens and nucleus tangentialis formed the "tractus vestibulospinalis". The descending spinal pathways of the himé salmon were compared with those of other fishes and other vertebrates. The significance of these descending spinal pathways in the control of locomotion and sexual behavior is also discussed.

Collateralization of descending pathways from the brainstem to the spinal cord in a lizard, Varanus exanthematicus

With the multiple fluorescent retrograde tracer technique, the collateralization in the spinal cord of descending supraspinal pathways was studied in a lizard, Varanus exanthematicus. Fast Blue (FB) gels were implanted unilaterally in the spinal gray matter of the cervical enlargement and Nuclear Yellow (NY) gels were implanted ipsilaterally in two series of experiments in all spinal funiculi of the lumbar enlargement or in midthoracic spinal segments, respectively. All brainstem nuclei known to project to the spinal cord in reptiles were found to give rise to branching axons that may influence widely separate levels of the spinal cord. The number of double-labeled FB-NY cells varied in these brainstem nuclei from none to half the number of neurons projecting to the cervical enlargement. Highly collateralizing projections (expressed as the percentage of double-labeled neurons, DL) were found to arise from the nucleus raphes inferior, the contralateral nucleus reticularis superior pars lateralis, the contralateral nuclei vestibulares ventromedialis and descendens, and the ipsilateral nucleus reticularis inferior pars ventralis. A lower percentage of DL neurons was noted for the contralateral nucleus ruber and bilaterally for the nucleus reticularis medius and nucleus reticularis inferior. Extensive brainstem projections directed to cervical and high thoracic spinal levels originate from the area lateralis hypothalami, the nucleus of the fasciculus longitudinalis medialis, the contralateral nucleus cerebellaris medialis, and from the nucleus tractus solitarii. Projections preferentially directed to midthoracic or lower levels of the spinal cord were found to arise from the ipsilateral locus coeruleus, the contralateral nucleus reticularis superior pars lateralis, the nucleus reticularis inferior pars ventralis, the nucleus reticularis inferior, and the nucleus raphes inferior. In contrast to findings in mammals, in Varanus exanthematicus the red nucleus, the nucleus vestibularis ventrolateralis, and certain parts of the reticular formation did not display a clear-cut somatotopic organization. In general two different patterns of collateralization can grossly be discerned: a gradual decrease of spinal collaterals caudalward, which can be interpreted as a certain specificity of such projections; and a constant number of collateral nerve fibers throughout the spinal cord that can be interpreted as either a nonspecific or, in contrast, a highly specific system, focussed exclusively on the cervical and lumbar enlargements.

The corticomotoneuronal component of the pyramidal tract: corticomotoneuronal connections and functions in primates

Corticomotoneuronal fibers make up a functional component of the pyramidal tract-corticospinal system which is characteristic of primates. The corticomotoneuronal fibers include large, rapidly conducting axons. They arise from somatotopically arranged areas of precentral cortex and the largest concentration of pyramidal cells of origin in the deep part of lamina V is in area 4. Their influence is exerted contralaterally on the spinal cord, where monosynaptic excitation of spinal motoneurons occurs. Motoneurons innervating distally acting muscles are preferentially excited and marked convergence of corticomotoneuronal influences occurs on these. The excitatory post-synaptic potentials in these motoneurons are characterized by the property of temporal facilitation. Intraspinal divergence of the terminal arborizations of individual corticomotoneuronal fibers could permit the engagement of large populations of motoneurons and also the activation of excitatory and inhibitory interneurons and propriospinal neurons for that region of the spinal cord. Corticomotoneuronal synapses may be located more distally on the dendrites of motoneurons than are the monosynaptic connections from group Ia afferents. The corticomotoneuronal excitation has been demonstrated to be effective in natural functional states when the conscious animal is performing learned movement tasks. Abolition of corticomotoneuronal influences causes a permanent deficit in the fractionation of use of distal muscles and an inability to carry out independent movements of the fingers.

Corticomotoneuronal synapses in the monkey: light microscopic localization upon motoneurons of intrinsic muscles of the hand

Some corticospinal neurons give rise to axons which terminate directly upon motoneurons, thereby establishing corticomotoneuronal connections. The location of corticomotoneuronal synapses upon motoneurons innervating intrinsic muscles of the hand in the monkey was demonstrated by the use of intra-axonal and intracellular horseradish peroxidase (HRP). After a number of corticospinal axons originating from the "hand" area of the precentral gyrus had been injected with HRP in the lateral funiculus at C7-C8, a number of nearby identified intrinsic hand muscle motoneurons were also injected. Connections between corticomotoneuronal fibres and motoneurons were reconstructed from longitudinal parasagittal sections treated by the cobalt-enhanced diaminobenzidine method. Corticospinal axons in the lateral funiculus gave rise to main collaterals which provided an extensive arborization in lamina IX, where it was predominantly longitudinal, and in the adjacent intermediate zone. En passant and single or clustered groups of terminal boutons arose from preterminal branches of these arbors. Seven light-microscopically identified corticomotoneuronal synapses were found. They were located upon the dendrites of recipient motoneurons at 40 micron to 750 micron from the soma and ranged in size from 0.6 X 3.0 micron to 2.4 X 3.6 micron. The results suggest that each main collateral of a corticomotoneuronal axon establishes very few synaptic contacts, and possibly only one, with the dendrites of recipient motoneurons. This small number of contracts per motoneuron is consistent with the small amplitudes of minimal and unitary corticomotoneuronal EPSPs recorded from forelimb and hand motoneurons.

Direct evidence for postsynaptic inhibition in the embryonic chick spinal cord

Motoneurons were recorded intracellularly in the isolated perfused spinal cord of 10 - 16-day chick embryos. Inhibitory postsynaptic potentials (IPSPs) were present in motoneurones of all ages studied and could be evoked by both ventral white column and dorsal root stimulation. IPSPs produced by orthodromic stimulation displayed many features of mature vertebrate motoneuronal IPSPs including the chloride dependence and sensitivity to currents passed through the cell membrane. Strychnine and chloride-free solution produced marked disinhibitory effects in the spinal cord indicating the presence of inhibitory synapses in interneuronal circuits of at least 11-day and older embryos. Possible sources of descending inhibitory influences on motoneurones and some functional aspects are discussed. The results support the hypothesis that the inhibition starts in the embryonic chick spinal cord rather early, before the 10th day of development.

Historical survey. The concept of a sensorimotor cortex: its later history during the twentieth century

This paper continues the historical review on the concept of a sensorimotor cortex into the twentieth century. Paul Flechsig was probably the first to accept this concept after the turn of the century. Like Munk, he believed in an almost reflex-like unity between cortical sensory and motor function. With the help of his myelogenetic technique, Flechsig also demonstrated convincingly in the human brain that the two Rolandic convolutions receive a separate influx of thalamic afferents. Had this finding met with more attention, much experimental work by some of the distinguished later investigators might have proved to be unnecessary. Research has now reached a state when an early plausible explanation of the problem may be confidently expected.

Descending projections from brainstem and sensorimotor cortex to spinal enlargements in the cat. Single and double retrograde tracer studies

Single and double retrograde tracer techniques were employed in cats to investigate: (1) the topographical relationships between supraspinal neurons projecting to either the brachial or lumbosacral enlargement, (2) the distribution and relative frequency of single supraspinal neurons which project to both enlargements by means of axonal branching. In one group of cats large injections of horseradish peroxidase (HRP) were made throughout either the brachial or lumbosacral enlargement. The results from these experiments support recent observations on the multiplicity of brainstem centers giving origin to descending spinal pathways and provide evidence for a population of corticospinal neurons in area 6. In a second set of experiments, HRP was injected in one enlargement, and 3H-apo-HRP (enzymatically inactive) was injected in the other enlargement. Relatively large numbers of neurons with collateral projections to both enlargements (double-labeled) were observed in the medullary and pontine reticular formation, the medial and inferior vestibular nuclei bilaterally, the ipsilateral lateral vestibular nucleus, Edinger-Westphal nucleus, caudal midline raphe nuclei and nuclear regions surrounding the brachium conjunctivum. By contrast, double-labeled neurons were infrequently observed in the red nucleus and sensorimotor cortex, contralateral to the injections. In the red nucleus, lateral vestibular nucleus and sensorimotor cortex, neurons projecting to the brachial enlargement were largely segregated topographically from neurons projecting to the lumbosacral enlargement. However, there was some overlap, and double-labeled neurons were consistently observed within the region of overlap. In the sensorimotor cortex, the overlap between brachial- and lumbar-projecting neurons was most prominent in areas 4 and 3a, along the cruciate sulcus, but also involved other cytoarchitectonic regions in the medial aspect of the hemisphere.

The distribution of motoneurones supplying chick hind limb muscles

1. The motor nuclei supplying many of the hind limb muscles were localized in late chick embryos (stage 36-37; 10-11 days) by utilizing the technique of retrograde transport of horseradish peroxidase. 2. Each nucleus was found to be localized in a characteristic position in both the rostro-caudal and transverse plane of the spinal cord with only slight individual variation. 3. Each motor nucleus consisted of an elongate, coherent cluster of labelled cells, with few cells occurring outside the cluster. Thus, there did not appear to be extensive overlap of nuclei nor extensive intermingling of motoneurones projecting to different muscles. 4. The position of a motor nucleus in the transverse plane was not correlated with whether its muscle was used as an extensor or flexor; nor were adjacent nuclei necessarily co-activated during normal unrestrained walking movements as deduced from e.m.g. recordings. The position of a motor nucleus also was not correlated in a topographical manner with the adult position in the limb of the muscle to which it projected. 5. Further, while no correlation was found between the rostrocaudal position of a motor nucleus and the embryonic muscle mass from which its muscle was derived, such a relationship existed for the medio-lateral position; all muscles arising from the dorsal muscle mass, regardless of their function or adult position, were innervated by laterally situated motoneurones, all muscles arising from the ventral muscle mass by medially situated motoneurones. 6. It is concluded that motoneurone position is most closely correlated with ontogenetic events presumaeriphery. It can also be inferred that the central connexions onto motoneurones, responsible for their proper activation, cannot be achieved by a simple mechanism based largely on the position of the motoneurone soma.

Motor responses to sudden limb displacements in primates with specific CNS lesions and in human patients with motor system disorders

Central feedback pathways for motor control were studied by recording EMG responses to sudden upper limb displacements in humans and monkeys using a precision torque motor to generate step load changes. Normal human subjects showed three short-latency EMG responses (M1, M2 and M3) which appear to correspond to those recorded from trained monkeys. The M2 and M3 components, thought to represent feedback in supraspinal pathways, were significantly increased when the subjects were instructed to actively compensate for the load changes. Parkinsonian patients with rigidity showed evidence of markedly increased feedback over the interval for the M2 and M3 responses and appeared to have lost the ability to modulate feedback according to the motor task being performed. The results are discussed with reference to recent research on motor control mechanisms in primates and a tentative model for the basis of Parkinsonian rigidity is proposed.

Laying the ghost of 'muscles versus movements'

It is certainly a mark of great wisdom to the organizer of this meeting to have chosen Charles Phillips as our Guest Lecturer for this meeting.

Whilst I am grateful for this opportunity of saying a few words of introduction, I am sure it can be said of Charles Phillips that very little introduction is necessary, since we meet him at the most important meetings in Neurophysiology around the world. Any meeting on the motor system, of course, is not complete without the participation of Dr. Phillips. However, some of the less old-timers than myself, and the students, might be interested in the beginnings of Dr. Phillips’ career in neurophysiology and to know that he was first a clinical neurologist beginning his medical studies in Oxford just before the war, where he came under the influence of Sir Charles Sherrington with Jack Eccles as his tutor.

Spinal interneurones involved in presynaptic controls of supraspinal origin

1. Interneurones presenting heterotopic and heterosensory convergence have been identified in laminae VI-VII of the lumbar dorsal horn in the cat. Stimulation of the hind limbs sometimes induced a bimodal response, but we considered only the late convergent discharge.2. The fact that response latencies are longer to hind limb than to forelimb stimulation at this level suggests the intervention of a supraspinal loop in the activation of spinal convergent units. This hypothesis is supported by the relationship between the excitability of supraspinal structures and the discharge intensity of convergent cells as well as by the absence of long latency responses in the spinal preparation.3. Electrophysiological and pharmacological evidence discloses a strong relationship between convergent unit discharges and the occurrence of dorsal root potentials to cortical, heterosegmental and heterosensory stimulation.4. It is suggested that convergent units receive information of supraspinal origin and exert control over sensory input to the cord via primary afferent depolarization.