Methods for chronic recording of EMG activity from large numbers of hindlimb muscles in awake rhesus macaques

Abstract

Studies of the neural control of movement often rely on the ability to record EMG activity during natural behavioral tasks over long periods of time. Increasing the number of recorded muscles and the time over which recordings are made allows more rigorous answers to many questions related to the descending control of motor output. Chronic recording of EMG activity from multiple hindlimb muscles has been reported in the cat but few studies have been done in non-human primates. This paper describes two chronic EMG implant methods that are minimally invasive, relatively non-traumatic and capable of recording from large numbers of hindlimb muscles simultaneously for periods of many months to years.

Figure 1

Cross-section of muscle anatomy in the lower leg (A) and upper leg (B). A cadaver hindlimb was frozen and sectioned using a 19 tooth/inch, bi-metal band saw blade (L.S. Starret Co.). Photographs were taken of each cross-section and the boundaries between muscles were traced directly from the sections. Where separation of muscles was difficult to discern, macroscopic ultraviolet epifluorescence was used to identify fascial boundaries. Identification of muscles was confirmed by cross-referencing to a dissected cadaver limb with muscles intact. Medial (M) and lateral (L) aspects of the hindlimb are marked on the photographs in A and B.

Figure 2

T1- and T2-weighted MRIs of the upper and lower hindlimb in a cadaver rhesus monkey leg. Bone is easily identifiable with both types of imaging. Fascial boundaries separating muscles are more pronounced in the T1-weighted image. Bright spots on the periphery of the T1-weighted image are due to placement of the Helmholtz RF coil loop relative to the leg.

Figure 3

(A) Arm-mounted subcutaneous implant. Orange rectangles on forelimb: exteriorized connectors that allow connection of EMG wires to amplifiers. Short black lines: exterior wires leading to connectors. Red lines: subcutaneous paths of EMG wires to individual target muscles, wires continuing onto medial side of hindlimb go to target muscles. Light blue circles: sites of small incisions during tunneling. Long light blue line: back pocket where wire loop is seated. Black arrows: 1 - exterior arm connectors and entry point of EMG wires, 2 - back incision, 3 - puncture incisions on the hip, 4 –puncture incision on lateral hindlimb for wires tunneled from hip incision 1. Hip incisions (arrow 3) are numbered 1–4 from most lateral to most medial. (B) Cranial-mounted subcutaneous implant. Red lines, light blue circles and light blue line as described above. Green circle: EMG connector, affixed to skull with dental acrylic, connects to amplifiers. Black arrows: 1 - cranial connector and entry point of EMG wires, 2 - back incision, 3 - puncture incisions on the hip, 4 – puncture incision on lateral hindlimb for wires tunneled from hip incision 1. Hip incisions (arrow 3) are numbered 1–4 from most lateral to most medial. (C) EMG records and joint angle during two cycles of the hindlimb push-pull task. Angle measurement at each joint is described in graph legend and also depicted in (A) and (B).

Figure 4

EMG signals (left) and response averages (right) from 20 pairs of EMG wires implanted in hindlimb muscles at 1 month (A) and 18 months (B) following the arm-mounted subcutaneous implant in Monkey F. The records shown are from two consecutive trials of the push-pull task. Different channels of EMG activity were amplified from 5–200 K. Filtering was generally 30 Hz to 3 kHz. All channels were digitized at 4 kHz. Muscle abbreviations are given in Materials and Methods. PP: push-pull task. Shaded area indicates extension position zone. Calibration bar to the left of the task signal indicates linear distance of foot movement during the push-pull task.

Figure 5

EMG signals (left) and response averages (right) from 20 pairs of EMG wires implanted in hindlimb muscles at 4 months following the cranial-mounted subcutaneous implant in Monkey C. The records shown are from two consecutive trials of the push-pull task. Different channels of EMG activity were amplified from 5–200 K. Filtering was generally 30 Hz to 3 kHz. All channels were digitized at 4 kHz. Muscle abbreviations are given in Materials and Methods. PP: push-pull task. Shaded area indicates extension position zone. Calibration bar to the left of the task signal indicates linear distance of foot movement during the push-pull task.