Projections of pyramidal tract cells to alpha‐motoneurones innervating hind‐limb muscles in the monkey.

Abstract

SUMMARY1. We have investigated the spatial organization of monosynaptic corticospinal projections to hind-limb motoneurones, using near threshold stimulation of the surface of the precentral gyrus to activate pyramidal tract (PT) cells and intracellular recording from motoneurones to detect the resulting e.p.s.p.s.2. Monosynaptic e.p.s.p.s of cortical origin were seen in all motoneurone species investigated, those of distal as well as of proximal hind-limb muscles. The proportion of motoneurones in which the e.s.p.s were evoked and the amplitudes of the latter indicated a more extensive cortical projection to motor nuclei for distal than for proximal muscles, as previously found for forelimb motoneurones.3. Cortical areas from which monosynaptic e.p.s.p.s were evoked in individual motoneurones were remarkably large, most often between 3 and 7 mm2. Several motoneurones appeared to have two or three separate areas within the hind-limb division of the motor cortex.4. Areas of location of pyramidal tract cells projecting to various motoneurones innervating one muscle were usually not identical. They overlapped often only partially or did not overlap at all.5. Areas of location of pyramidal tract cells projecting to motor nuclei for different muscles often showed an extensive overlap. When it occurred, various motoneurones of a given motor nucleus had common cortical projection areas with motoneurones of other motor nuclei, either to synergistic or to antagonistic muscles. Our results give further evidence for overlapping of areas of cortical projections to motoneurones and speak

Connected Corticospinal Sites Show Enhanced Tuning Similarity at the Onset of Voluntary Action

“…This indicates that in behaving primates, CS interactions are substantially stronger than their presumed strength as might be indirectly estimated based on studies performed in reduced preparations (Jankowska et al, 1976;Baldissera et al, 1981), where often large-scale cortical stimulation (Jankowska et al, 1975(Jankowska et al, , 1976 or stimulation in the pyramidal tract were used. Similar differences in functional efficacy between awake animals and reduced preparation was also reported for CM connectivity (Porter and Lemon, 1993).…”

“…Motor cortical neurons were shown to be spatially organized in a mosaic-like pattern (Jankowska et al, 1975;Amirikian and Georgopoulos, 2003;Ben-Shaul et al, 2003) where neighboring neurons share similar response properties (Lee et al, 1998) and input/output connectivity patterns (Jackson et al, 2003) whereas response properties across patches are unrelated. It is therefore expected that single-pulse stimulation should mostly excite local cells with some degree of homogeneity in their response properties in addition to more remote cells with randomly varying tuning properties.…”

“…This latter, indirect pathway, is more suggestive of a high level of cortical representation of information which is further processed by spinal systems (Tresch et al, 1999;Bizzi et al, 2000). Nevertheless, the indirect pathway is considered to be weak (Jankowska et al, 1975;Baldissera et al, 1981), particularly in primates compared with lower mammals (Maier et al, 1998b;Nakajima et al, 2000) presumably because of increased inhibitory control (Alstermark et al, 1999;Isa et al, 2006).…”

Abductor hallucis for monitoring lower-limb recovery after spinal cord injury in man

“…They found monosynaptic inputs to both populations of motoneurons, but motoneurons to distal muscles showed greater synaptic efficacy, indicating a greater density of innervation from motor cortex to these distal muscles. 9 If there are indeed a larger number of corticospinal axons innervating motoneuron pools of intrinsic foot muscles (including the AbH) compared to lower-limb muscles of the thigh, then simple probability would argue that a larger absolute number of these axons may escape permanent damage after motor-incomplete SCI. This argument is based upon the premise that axons within the corticospinal tract are not arranged in a laminar pattern within the lateral tracts, but are instead randomly dispersed throughout the ventrolateral cord.…”

“…9 In humans, AbH is more easily recruited (ie has a lower threshold) by transcranial magnetic stimulation (TMS) in awake humans than other lower-limb muscles, 10 and the same is true in the anesthetized human when tested with transcranial electrical stimulation during spine surgery. 11 These findings point to a relatively prominent role of the AbH in cortically controlled voluntary movements.…”