J. Armand

Topography of projections from the motor cortex to rubrospinal units in the cat.

SummaryA topographical study of the cortico-rubrospinal pathway was conducted in cats anesthetized with chloralose. Extracellular unit recordings were made from cells in the red nucleus projecting to the spinal cord. They were identified by antidromic invasion following stimulation of their axones at the 2nd cervical and 9th thoracic levels of the spinal cord.I.The pericruciate cortical regions from which spikes could be induced in rubrospinal neurons were limited to the lateral part of the anterior sigmoid gyrus, the lateral sigmoid gyrus and the anterior part of the posterior sigmoid gyrus. No responses were obtained from stimulation of the medial part of the anterior sigmoid gyrus or the gyrus proreus. Compared to the somatotopic organization of the motor cortex for the cat described by Woolsey (1958), our results show that the rubrospinal cells receive projections from the motor cortex controlling proximal and distal muscles but not axial muscles.II.Neurons projecting to the cervico-thoracic cord receive afferents from the lateral part of the anterior sigmoid gyrus and the lateral sigmoid gyrus whereas those projecting to the lumbo-sacral cord receive projections from the entire surface of the sigmoid gyrus except the medial part of the anterior sigmoid gyrus and the gyrus proreus.III.A latero-medial organization of cells within the red nucleus was found according to the origin of their cortical afferents. Rubrospinal neurons with fibers terminating in the cervical or thoracic cord receive projections from the motor cortex controlling the proximal musculature of the forelimb when they are located in the dorso-lateral region of the nucleus and the entire forelimb motor cortex when they are located in the medial part of the nucleus. It is suggested that this organization may indicate a control of proximal forelimb musculature by dorsolateral rubrospinal cells and distal musculature by medial cells.IV.Rubrospinal cells placed medially in the nucleus receive more convergent projections (i.e. from a greater cortical surface) than cells placed more laterally. It was shown that for certain cells the convergence occurs in the direct pathway. These results are discussed in terms of a functional organization allowing coordinated movements of different segments of a single limb or of different limbs.

Protracted postnatal development of corticospinal projections from the primary motor cortex to hand motoneurones in the macaque monkey.

We have studied the development of corticospinal projections from the hand area of the primary motor cortex to the spinal cord using anterograde transport of WGA-HRP. In the neonate, as in the adult, corticospinal projections to the intermediate zone at the C8/T1 spinal level were clearly present. However, in contrast to the adult, there was only very faint and barely visible labelling in the dorso-lateral motor nuclei which supply the hand muscles. No aberrant projections to other motor nuclei were seen. By 2.5 months, a ring of dense labelling was present around the dorso-lateral motor nuclei, but labelling was still sparse in the central region. This labelling was more pronounced at 11 months, but was still not as heavy as in the adult. There was no labelling among the ventral motoneurones at any age. The conduction velocity (c.v.) of the fastest corticospinal fibres was determined in each of the monkeys. There was an age-related increase in c.v. within the spinal cord. At birth, the fastest axons had a c.v. of only 8 m·s-1. At 11 months c.v. was still substantially slower (55 m·s-1) than the adult value of 73 m·s-1. In contrast, by 11 months, the axonal c.v. within the brain was close to the adult value, suggesting a rostro-caudal maturation of the corticospinal system. Our results demonstrate that corticospinal projections in the macaque monkey mature gradually over a period of at least 11 months, much longer than previously thought.

Differential corticospinal projections in the cat. An autoradiographic tracing study.

An autoradiographic study of the corticospinal projections from different parts of the cat sensorimotor cortex produced the following findings. The lateral part of area 4 projects contralaterally to the lateral intermediate zone of the cervical enlargement only. The intermediate part of area 4 projects throughout the spinal cord, contralaterally to the lateral part of the intermediate zone and bilaterally to its ventromedial part. The lateral and medial part of area 3 project contralaterally to the cervical and lumbosacral dorsal horn (including laminae I and II), respectively.

Topography of rubrospinal units in the cat

SummaryMapping of cells at the origin of the rubrospinal tract was conducted in the cat.1.Rubrospinal neurons sending efferents to cervico-thoracic segments of the spinal cord are located in the dorso-medial part of the nucleus. These neurons are especially medial in the caudal planes and especially dorsal in the rostral planes. Neurons with efferents terminating at the level of lumbo-sacral segments of the cord occupy the ventro-lateral part of the nucleus. These neurons are especially lateral in the caudal planes and especially ventral in the rostral planes. The limit between these two cell populations is clear in the caudal and middle thirds of the nucleus but considerable overlap is seen in the rostral third. These results agree with the anatomical findings of Pompeiano and Brodal (1957).2.For the population of lumbar neurons the conduction velocities ranged from 31 m/sec to more than 120 m/sec with a mean of 85 m/sec.3.Rubrospinal cells are found throughout the nucleus. The most caudal planes are essentially composed of cells with rapidly conducting fibers whereas in the middle and rostral planes a cell population with increasingly slower conducting fibers appears. The results of the present study are discussed in relation to classical data on the magnocellular and parvocellular divisions of the red nucleus.2.The third author acknowledge the personal support of the Medical Research Council of Canada.

Functional differences in corticospinal projections from macaque primary motor cortex and supplementary motor area.

We made a quantitative comparison of the density of macaque corticospinal projections from primary motor cortex (M1) and supplementary motor area (SMA) to spinal motor nuclei supplying hand and finger muscles. We also compared the action of corticospinal outputs from these two areas on 84 upper limb (mostly hand) motoneurones in chloralose-anaesthetised macaques. The hand representations of M1 and SMA were first identified using MRI and intracortical microstimulation. We made focal injections of WGA-HRP into these representations. Densitometric analysis showed that corticospinal projections from M1 were far denser and occupied a much greater proportion of the hand muscle motor nuclei than did SMA projections. Stimulation of M1 and SMA with bipolar intracortical pulses evoked monosynaptic EPSPs. These were significantly larger and more common from M1 (88% of motoneurons) than from SMA (48%). The results demonstrate corticomotoneuronal connections from both M1 and SMA, some converging upon single motoneurons. Both areas give rise to CM projections but that those from M1 are far more numerous and exert stronger excitatory effects than those from the SMA.

The structure and function of the developing corticospinal tract : somme key issues.

Publisher Summary This chapter explains some of the key issues regarding the structure and function of the developing corticospinal tract. Abnormal development of descending motor systems is associated with a variety of movement disorders. In primates, some corticospinal neurones establish a monosynaptic linkage between the primary motor cortex and spinal motoneurones, particularly those innervating hand and finger muscles. The use of electrophysiological approaches to prove the existence of a monosynaptic connection that requires intracellular recording from the target motoneurones. The monosynaptic origin of the excitatory postsynaptic potential (EPSP) is indicated when the segmental delay between the arrival of the tract volley and EPSP onset is too short to involve more than one synapse. The chapter also examines the change in axon diameter of the corticospinal neurons change during development and its relation to conduction velocity. The myelination of corticospinal axons is a postnatal process clearly protracted with respect to that of the other descending pathways. This period of myelination far outlasts that in which the spinal gray matter receives corticospinal innervation. In primates, corticospinal axons seem to be myelinated over their cranial before their spinal course. In the spinal cord, myelination of the corticospinal tract follows a rostral-to-caudal gradient. The chapter also elaborates the use of transcranial magnetic stimulation for studying corticospinal development.

Somatotopic organization of the corticospinal tract in cat motor cortex.

Abstract The somatotopic organization of cat motor cortex (covering areas 4γ and 6aβ according to Hassler and Muhs-Clement) was examined by antidromic stimulation of the corticospinal tract from various spinal levels (high cervical, low cervical, thoracic and lumbar) in 15 cats anesthetized with chloralose. The stimulation parameters were adjusted to recruit the entire corticospinal tract and multiple recording electrode sites were used to localize as precisely as possible the cortical zones activated antidromically for each level of spinal stimulation. Both transcortical bipolar and monopolar depth electrodes were used for recording antidromic responses. In this study, only the population of fast conducting fibers is considered. (1) High cervical (C2) stimulation rarely produced activity in the medial anterior sigmoid gyrus and the gyrus proreus (area 6aβ) whereas a large number of activated sites were found in the middle and lateral anterior sigmoid gyrus, the lateral sigmoid gyrus and the lateral posterior sigmoid gyrus (area 4γ). Area 6aβ seems to contribute few, if any, fibers to the corticospinal tract. This observation is compared with the existing anatomical and electrophysiological data. (2) Different portions of area 4γ projected to different spinal levels; the middle and lateral parts of area 4 were associated with the cervical segments whereas the middle part of area 4 appeared to project to thoracic levels. An exploration of the depths of the cruciate sulcus indicated a focus for lumbar motor outflow in the caudal wall of the sulcus. (3) The overlap of the zones influencing the cervical and thoracic levels is interpreted as evidence for a zone of corticospinal tract origin common to several spinal levels. This zone is situated in the middle part of area 4. The functional role of this particular cortical zone is suggested with respect to a proximodistal distribution of the musculature.

Critical timing of sensorimotor cortex lesions for the recovery of motor skills in the developing cat

Forelimb movements and motor skills were studied in adult cats in order to determine the effect of brain damage inflicted at different postnatal ages. The unilateral lesion included the cortical areas from which the pyramidal tract originates in cat: areas 4 and 6 corresponding to the motor cortex; areas 3, 2 and 1 corresponding to the primary somatosensory cortex; and part of area 2 prae-insularis corresponding to the secondary somatosensory cortex. Forelimb performance of a food-retrieving task requiring proximal as well as distal muscles was assessed by comparing the limb contralateral to the damaged hemisphere (affected limb) with the limb contralateral to the intact hemisphere (non-affected limb) that appeared to perform the task as well as both limbs of control animals. In simple task-related movements, all operated animals were rapidly able to achieve the goal with the affected limb, whatever the age at lesion. In complex tasks, the ability to achieve the goal with the affected limb decreased with increasing age at lesion. Recovery of distal skills, i.e. grasping and wrist rotation, did not occur in animals operated on after the 23rd postnatal day (PND), and recovery of proximal skills, i.e. amplitude and precision of the reaching movement, did not occur in animals operated on after the 45th PND. The critical time for the recovery of distal skills lies somewhere between the 23rd and 30th PND, whereas for the recovery of proximal skills it lies somewhere between the 45th and 60th PND. These critical dates for the recovery of motor skills support the Kennard doctrine. Different critical times for proximal and distal skills may be explained in terms of different stages of sensorimotor development in kitten. It is hypothesised that recovery only occurs if brain damage is inflicted before maturation of the nervous system underlying a given motor skill.