Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C67
Ipsilateral corticospinal contributions to control of the forelimb in monkey
D. Soteropoulos1, S. Edgley2, S. Baker1
1. Institute of Neuroscience, Newcastle University, Newcastle Upon Tyne, United Kingdom. 2. Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge, United Kingdom.
Figure 1: A. Intracellular responses of forearm flexor motoneuron (MN) responding to single shock stimulation of contralateral corticospinal tract (cPT: gray trace) and ipsilateral corticospinal tract (iPT: black trace). Top two traces are intracellular recordings and bottom two traces are cord dorsum recordings. B. Same as A but multiple shocks given to iPT and cPT. C. Histogram showing the types of MNs tested with iPT/cPT, and maximum number of stimuli used. Bars to the right of the dotted line correspond to cPT stimulation to single shock. Gray bars indicate oligosynaptic responses, black bars monosynaptic responses.
Figure 2: Stimulus triggered averages of bilateral rectified muscle activity (Extensor Digitorum Communis: EDC, first Dorsal Interosseus: 1DI), using left corticospinal tract electrical stimulation at intensities of 500 and 1000 μA (black & gray; number of stimuli: 1511 & 919 respectively). The arrows under each trace indicate the onset latency of the response of the right hand side muscle. The stimulation was given while the monkey was performing a bimanual precision grip task - this involved a 1s precision grip with either left hand only, right hand only, or both hands together. The trial type order (left hand, right hand or bimanual) was randomised.
Strong experimental evidence implicates the corticospinal tract in voluntary control of the contralateral forelimb. Its potential role in controlling the ipsilateral forelimb is less well understood, although anatomical projections to ipsilateral spinal circuits are identified. We investigated inputs to motoneurons innervating hand and forearm muscles from the ipsilateral corticospinal tract using multiple methods. Intracellular recordings were made from 62 motoneurons in three anaesthetized macaque monkeys. Monkeys were deeply anesthetised with sevoflurane (3-5% in 100% O2) and alfentanil (7-23 µg kg-1 h-1 by IV infusion) during a laminectomy for exposure of spinal segments C6-T1. The anesthetic regime was then switched to an intravenous infusion of propofol (5-14 mg kg-1 h-1) and alfentanil (doses as above). Neuromuscular blockade was achieved by infusion of atracurium (0.6-1.2 mg kg-1 h-1). Continuously monitored vital signs included heart rate, arterial and venous blood pressure, blood oxygen saturation, end-tidal CO2, and core temperature. Depth of anaesthesia was verified by ensuring that there were no changes in heart rate or arterial blood pressure in response to peripheral nerve stimulation). No monosynaptic post-synaptic potentials were observed following single and multiple shock stimulation of the ipsilateral corticospinal tract (300μA, Fig. 1). Single stimulus intracortical microstimulation of the primary motor cortex (M1) in two awake behaving monkeys (up to 30μA) failed to produce any responses in ipsilateral muscles. Strong stimulation (>500μA, single shock) of the majority of corticospinal axons at the medullary pyramids revealed only weak suppressions in ipsilateral muscles at longer latencies than the robust facilitations seen contralaterally (Fig. 2). Spike triggered averaging of ipsilateral muscle activity from M1 neural discharge (184 cells) did not reveal any post-spike effects consistent with monosynaptic corticomotoneuronal connections. We conclude that, in normal adults, any inputs to forelimb motoneurons from the ipsilateral corticospinal tract are likely to be weak and indirect.
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