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The Journal of Neurophysiology Vol. 84 No. 2 August 2000, pp. 698-709
Copyright ©2000 by the American Physiological Society
Sobell Department of Neurophysiology, Institute of Neurology, University College London, London WC1N 3BG, United Kingdom
Nakajima, K.,
M. A. Maier,
P. A. Kirkwood, and
R. N. Lemon.
Striking Differences in Transmission of Corticospinal Excitation
to Upper Limb Motoneurons in Two Primate Species. J. Neurophysiol. 84: 698-709, 2000. 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 C3-C4
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 C5, 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 C5 lesion, 19% after it). Transmission of the late EPSPs via C3-C4 PNs
in the squirrel monkey was indicated by their absence after an
additional C2 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 C3-C4 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.
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