The organization of spinal cord motor columns innervating 18 selected macaque forelimb muscles was studied with the technique of retrograde transport of horseradish peroxidase. The reliability of the method was evaluated in the cat hindlimb.Motor columns innervating forearm muscles with similar actions on the hand appear to overlap in the anterior horn. Extensor motoneurons are generally positioned ventral and/or lateral to flexor motoneurons. Motoneurons controlling hand movement are located primarily in segments C8 and Tl.Motoneurons that innervate individual muscles are arranged in rostrocaudal columns in the anterior horn of the spinal cord. The segmental localization of motor columns innervating primate forelimb muscles was originally described by Sherrington (1898) in studies involving stimulation of individual ventral nerve roots. Although the somatotopic organization of motor columns in the anterior horn has been reported for the cat lumbosacral cord (Romanes, 1951;Burke et al., 1977;Sato et al., 1978), the only topographical data available for the monkey cervical cord relate to the distribution of motoneurons that contribute to individual ventral roots (Sprague, 1948) and major peripheral nerves (Reed, 1940).In the present study we sought to determine both the segmental and topographical organization of individual motor columns involved in controlling hand movement in the monkey. The retrograde transport of horseradish peroxidase (HRP) from selected muscles to motoneurons was used (Burke et al., 1977;Sato et al., 1978;Richmond et al., 1978; Matusda et al., 1978;Landmesser, 1978).
Materials and MethodsThe studies were carried out in 10 Macaca mulatta and 4 M. fuscicularis monkeys of both sexes (2.9 to 4.2 kg). The reliability of the method was first evaluated in the cat.Injection barbiturate anesthesia the desired muscle was exposed and, with the use of an operating microscope, was carefully dissected from surrounding muscles. Care was taken to preserve the neurovascular pedicle. Muscles were identified by anatomic position, tendinous insertion, and when necessary, by electrical stimulation of the muscle (Hartman and Straus, 1933). The muscles were injected from a 27 gauge needle over 15 min at multiple sites with a saline solution of HRP (25,000 units/ml, Sigma HRP type VI, St. Louis, MO). Sufficient HRP was injected to render each muscle a dark brown color. To prevent the spread of HRP to adjacent muscles, Silastic and cotton barriers were positioned between the muscle to be injected and the surrounding muscle groups. The tissues surrounding the injected muscle were irrigated frequently with normal saline after the injection. By 3 hr, the injected muscle had returned to a more normal appearance and visible HRP leakage had stopped. Less frequent irrigation was continued for approximately 12 hr postinjection and the Silastic and cotton barriers remained in place for the duration of the experiment. Study of the spinal cord. After 48 hr under anesthesia, each animal was heparinized and perfused th...