Modulation of spinal motor output by initial arm postures in anesthetized monkeys.
Proper execution of voluntary movement requires a sensorimotor transformation based on the initial limb state. For example, successfully reaching to a stable target requires the recruitment of different muscle groups depending on limb position at movement initiation. To test whether this transformation could occur at the spinal level, we stimulated the cervical spinal cord of anesthetized monkeys while systematically changing initial posture and examined the modulation of the twitch response induced in the upper limb muscles. In three monkeys, a multichannel microelectrode array was implanted into the C6 segment of the spinal cord and electromyographic electrodes were implanted in 12 limb muscles (five hand, four elbow, and three shoulder muscles). The magnitude and onset latency of the evoked response in each electrode-muscle pair were examined by systematically changing the hand position through nine positions in a horizontal plane with the monkey prone. Among 330 electrode-muscle pairs examined, 61% of pairs exhibited significant modulation of either magnitude or latency of twitch responses across different hand/arm configurations (posture dependency). We found that posture dependency occurred preferentially in the distal rather than proximal muscles and was not affected by the location of the electrode within the stimulated spinal segment. Importantly, this posture dependency was not affected by spinalization at the C2 level. These results suggest that excitability in the cervical spinal cord is affected by initial arm posture through spinal reflex pathways. This posture dependency of spinal motor output could affect voluntary arm movement by adjusting descending motor commands relative to the initial arm posture.
Department of Neurophysiology, National Institute of Neuroscience, Tokyo 187-8502, Japan, Graduate School of Information Science and Technology, The University of Tokyo, Tokyo 113-8656, Japan.
Hiroaki
H
Yaguchi
Department of Neurophysiology, National Institute of Neuroscience, Tokyo 187-8502, Japan.
Tomohiko
T
Takei
School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, Pennsylvania 19104.
David
D
Kowalski
Center for Information and Neural Networks, National Institute of Information and Communications Technology, Osaka 565-0871, Japan, and.
Takafumi
T
Suzuki
Graduate School of Information Science and Technology, The University of Tokyo, Tokyo 113-8656, Japan.
Kunihiko
K
Mabuchi
Department of Neurophysiology, National Institute of Neuroscience, Tokyo 187-8502, Japan, Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, Saitama 332-0012, Japan seki@ncnp.go.jp.
Kazuhiko
K
Seki
D000704:Analysis of Variance; D000758:Anesthesia; D000818:Animals; D001132:Arm / Q000502:physiology*; D004558:Electric Stimulation; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology; D008253:Macaca mulatta; D008297:Male; D008839:Microelectrodes; D009046:Motor Neurons / Q000187:drug effects / Q000502:physiology*; D009068:Movement / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D011930:Reaction Time; D013116:Spinal Cord / Q000166:cytology*; D013119:Spinal Cord Injuries / Q000473:pathology / Q000503:physiopathology
EMG; arm posture; microstimulation; monkey; spinal cord; twitch