Musculotopic innervation of the primary flight muscles the pectoralis pars thoracicus and supracoracoideus of the pigeon Columba liviaA WGA-HRP study.код для вставкиСкачать
THE ANATOMICAL RECORD 225:35-40 (1989) Musculotopic Innervation of the Primary Flight Muscles, the Pectoralis (Pars Thoracicus) and Supracoracoideus, of the Pigeon (Columba M a ) : A WGA-HRP Study A. SOKOLOFF, T. DEACON, AND G.E. GOSLOW, JR. Biological Anthropology, Harvard University, Cambridge, Massachusetts 02138 (A.S., T.D.);Department of Biological Sciences, Northern Arizona University, Flagstaff,Arizona 8601 1 (G.E.G.) ABSTRACT The distribution of motoneurons innervating the primary depressor and elevator muscles of the wing of the domestic pigeon (Columba liuia) was studied by using the retrograde axonal tracer lectin-conjugated horseradish peroxidase (WGA-HRP). Injection of WGA-HRP into the pectoralis (pars thoracicus) labeled neurons in the ventromedial corner of the lateral motor column of the spinal cord. These neurons were arranged in a column extending from spinal segment X or XI to spinal segment XI1 or XIII. The pectoralis, the primary depressor muscle of the wing, consists of two parts which are anatomically and functionally distinct, the sternobrachialis (SB) and thoracobrachialis (TB). Injection into the SB labeled neurons in the rostra1 and middle regions of the pectoralis motoneuron column. In contrast, injection into the TB labeled neurons in the middle and caudal regions of the pectoralis motoneuron column. Injection into the primary elevator muscle of the wing, the supracoracoideus, labeled neurons in the lateral motor column in spinal segments X and XI. These motoneurons were located dorsolateral to motoneurons labeled following pectoralis injection. These data demonstrate musculotopic segregation of the motoneurons innervating the primary flight muscles in the pigeon and, further, illustrate that the SB and TB subregions of the pectoralis are innervated by discrete aggregations of motoneurons. This report and the preceding one (Kaplan and Goslow, 1989) describe aspects of the neuromuscular organization of the major muscles used for wing elevation and depression during flight in the pigeon (Columba liuia). The major depressor muscle of the wing, the pectoralis (pars thoracicus), consists of two parts, the sternobrachialis (SB) and thoracobrachialis (TB). The two parts are separated by an intramuscular aponeurosis, possess independent origins and fascicle architecture, and exhibit different proportions of fast oxidative and fast oxidative glycolytic fibers (Kaplan and Goslow, 1989). The supracoracoideus (SC), the major elevator muscle of the wing, lies deep to the pectoralis. Its fascicles arise from the dorsal half of the carina, the adjacent body of the sternum, and a small area on the base of the coracoclavicular membrane. The tendon of insertion passes dorsally through the triosseal canal (formed by the clavicle, coracoid, and scapula) to insert onto the dorsal surface of the posterior edge of the proximal humerus. This specialized “pulleylike” arrangement is an avian feature which appears fundamental for wing elevation. Flight in birds represents a highly derived and successful form of locomotion among tetrapods. Neither the evolution of bird flight (Padian, 1986) nor its neural control is understood. Requisite to such understand0 1989 ALAN R. LISS, INC ing is a description of the anatomical organization of the neurons which innervate the muscles of flight. In many vertebrate species, peripheral muscles are innervated by spatially discrete pools of motoneurons (e.g., Hollyday, 1980; McHanwell and Biscoe, 1981; Fetcho, 1987). Recent studies have shown that populations of motor units in some muscles are organized into functional subunits. These functional subunits often receive innervation from a discrete aggregation of motor pool neurons whose axons course through a single firstorder nerve branch (Iliya and Dum, 1984; English and Weeks, 1987; Weeks and English, 1987). Such anatomical segregation suggests that the central nervous system may recognize distinct functional compartments within individual peripheral muscles (English and Weeks, 1987). The musculotopic segregation of motoneurons innervating flight muscles has been demonstrated in the chick and chicken (Martin and Hrycyshyn, 1981; Ohmori et al., 1982; Straznicky and Tay, 1983)) but the topographic organization of motoneurons in- Received March 15, 1988; accepted September 27, 1988 Please address correspondence to A. Sokoloff, Biological Anthropology, Harvard University, 11 Divinity Ave., Cambridge, MA 02138. 36 A. SOKOLOFF ET AL and viewed under brightfield, darkfield, and polarized light microscopy. TABLE 1. Location of muscle injections Case 1 2 3 43 54 64 74 SB/TB2 X X X X Muscles iniectedl SB TB sc X X X X X X X 84 9 10 ‘TB-thoracobrachialis; SB-sternobrachialis; SC-supracoracoideus. ’Entire pectoralis injected. 31njections into the left SB/TB and the right SC. 41njections into the right SB and the left TB. nervating the anatomically and functionally distinct SB and TB parts of the pectoralis has not been studied. The central organization of the motoneurons which innervate the SC and the SB and TB parts of the pectoralis has not been described in the pigeon. This study illustrates the organization of the spinal cord motoneurons which innervate the SC and the SB and TB parts of the pectoralis in the adult pigeon. MATERIALS AND METHODS Ten adult pigeons, C . Zivia (250-450 g), were anesthetized (25 mg/kg ketamine and 2 mg/kg xylazine, I.M.), and in various combinations, the SB, TB, and SC were surgically exposed and injected with a 5% solution of lectin-conjugated horseradish peroxidase (WGAHRP, Sigma) (Table 1).To sample a wide range of motor units while minimizing the possibility of tracer leakage to adjacent muscles, WGA-HRP was delivered via multiple, small intramuscular injections with a 10pl Hamilton syringe. A total of 2 p1 was delivered to the SC (three or four injections of 0.5-0.7 p1 each) and a total of 10-15 p1 was delivered to the SB and to the TB (three or four injections of 3-4 pl each). During surgery, care was taken to preserve the integrity of the fascial membranes separating the SB and TB and the SB and SC. It has been reported that intact fascial membranes form a barrier to the leakage of tracer from an injected muscle (Haase and Hrycyshyn, 1986). After postinjection survival times of 44-50 hr the animals were euthanatized and immediately perfused through the left ventricle with 0.9% saline followed by a fixative containing 0.5%paraformaldehyde, 1.5%glutaraldehyde, and 1.5% sucrose in 0.1 M phosphate buffer. The spinal cord was exposed, spinal nerves I-XX were identified, and segments VIII-XVI were removed and stored for 1-2 days in a 20%sucrose solution in 0.1 M phosphate buffer. Spinal cord tissue was embedded in gelatin and cut in serial 75-pm sections in either transverse or horizontal planes on a freezing microtome. Tissue sections were collected in a solution containing 30% sucrose and 30% ethylene glycol in 0.1 M phosphate buffer and stored for up to 1 week at 0°C. Spinal cord tissue was processed with a modified version of Mesulam’s (1978)tetramethylbenzidine method (Deacon et al., 19831, counterstained in neutral red, OBSERVATIONS The brachial enlargement extended from the middle or caudal part of spinal segment X t o caudal XIII. The ventral horn a t these levels is divisible into separate medial and lateral motor columns (Huber, 1936) which together constitute lamina 9 of the pigeon spinal cord (Leonard and Cohen, 1975). In the present study, unilateral intramuscular injections into the SB, TB, and SC only labeled cells in the lateral motor column (LMC) of the ipsilateral ventral horn. Injection into the ipsilateral SB and TB resulted in ipsilateral labeling in the LMC from the caudal part of spinal segment X to middle XI11 (Fig. 1A). In four birds, the extent of the column of labeled neurons differed by as much as one segment in both rostral and caudal directions. Labeled neurons were observed from caudal X to middle XI11 (case l),from rostral XI to middle XI1 (case 2), and from middle XI t o caudal XI1 (cases 3 and 4; Fig. 2A) In each case, the column of neurons labeled with SB and TB injections was restricted in its rostral and caudal extremes to the ventromedial corner of the LMC whereas a t middle levels (caudal XI and rostral XII) it expanded dorsolaterally to fill much of the LMC (Figs. l A , 3). Neurons labeled with SB and TB injections possessed round or polygonal cell bodies ranging from 22 to 50 pm in diameter. Individual differences were also noted in the rostrocaudal extent of neuron labeling following injections into the right SB and left TB. Neurons labeled after SB injections extended from middle X to rostral XI1 or caudal XI1 (cases 5 and 6, respectively), from middle XI to rostral XI11 (case 7), and from caudal XI to middle XI1 (case 8). Neurons labeled after TB injections extended from caudal X to caudal XI1 (case 51, from rostral XI1 to caudal XI1 (case 71, and from rostral XI1 t o caudal XI11 (cases 6 and 8). In general, neurons innervating the SB extended from one-half to one-and-one-half spinal segments rostral to neurons innervating the TB and neurons innervating the TB extended from one-half to one-and-one-half spinal segments caudal to neurons innervating the SB (Fig. 2B). Rostrocaudal overlap in the position of neurons innervating the two parts was observed in the middle levels of the pectoralis column (segment XI1 in cases 6-8 and segment XI in case 5). Within this region of rostrocaudal overlap, neurons labeled with TB injections were located dorsally or dorsolaterally to neurons labeled with SB injections (Fig. 3). Intramuscular injection of WGA-HRP into the SC resulted in the labeling of neurons in a column extending from rostral X to caudal XI (case 9) and from middle X to caudal XI or rostral XI1 (cases 4 and 10, respectively; Fig. 2A). These neurons were situated in the ventrolateral corner of the LMC in spinal segment X, but in segments XI and XI1 they were located in a tight cluster in the center of the LMC directly dorsolateral to the pectoralis column (Figs. lB, 3). Neurons labeled with SC injections formed a morphologically heterogeneous population with perikarya generally ranging from 25 to 55 pm in diameter. A few larger polygonal cells (55-70 pm) were also observed. MUSCULOTOPIC ORGANIZATION; FLIGHT MUSCLES 37 Fig. 1. Motoneurons labeled following intramuscular injection of WGA-HRP. A: Photomicrograph of a section from caudal segment XI following injections into the sternobrachialis and thoracobrachialis parts of the pectoralis muscle (case 3). The diagram on the right indi- cates the location of the photo. B: Photomicrograph of a section from middle segment X following injection into the supracoracoideus muscle (case 9). Calibration bar = 100 pm. DISCUSSION ally to cells innervating the pectoralis muscle and were located laterally to pectoralis neurons in regions where the SC and pectoralis columns overlapped. Because of the experimental paradigm used in the present study it was not possible to definitively determine the relative rostrocaudal position of the motoneurons innervating the pectoralis and SC muscles. However, our data suggest that motoneurons innervating the SC extend more rostrally than motoneurons innervating the pectoralis and that SC motoneurons are located dorsolateral to pectoralis motoneurons in regions where the pectoralis The localization of motoneurons innervating the pectoralis and SC described in the present study is in general agreement with previous studies of the innervation of the wing muscles of the chicken (Ohmori et al., 1982) and chick (Martin and Hrycyshyn, 1981; Straznicky and Tay, 1983). Ohmori et al. (1982) severed nerves to the pectoralis and SC muscles in the adult chicken and reported cell degeneration in the ventromedial corner of the ipsilateral LMC. They noted that cells innervating the SC were offset crani- A. SOKOLOFF ET AL. 38 A Caudal Fig. 2. Rostrocaudal distribution of supracoracoideus and pectoralis motoneurons. A Labeled motoneurons following injections into the right supracoracoideus (SC) and left pectoralis (case 4). B: Labeled motoneurons following injections into the right sternobrachialis (SB) and left thoracobrachialis (TB) (case 5). Spinal segments X-XI1 are B Caudal indicated. Note: the black dots indicate the rostrocaudal location of labeled neurons only; the number of dots does not correspond to the actual number of motoneurons innervating the SC, SB, or TB muscles. and SC columns overlap (Figs. 2A, 3). Ohmori et al. the chick by Martin and Hrycyshyn (1981) may reflect (1982) also investigated the innervation of the rostral differences in the techniques employed, species differand caudal nerve branches to the pectoralis and re- ences in the innervation of the pectoralis and the SC, or ported that the neurons composing the rostral branch differences in the ages of the animals studied. The abwere located rostral to neurons composing the caudal sence of contralateral degeneration in the experiments branch. This offset pattern in chickens is reminiscent of Ohmori et al. (1982) suggests that the contralateral of the rostrocaudal offset in labeled neurons following innervation of the chick pectoralis muscle observed by right SB and left TB injections in pigeons and suggests Martin and Hrycyshyn (1981) may be a transient feaa similar topographic organization of the wing muscle ture in the development of wing muscle innervation. neurons in the two species. The absence of large labeled neurons following injecStraznicky and Tay (1983) injected HRP into the pec- tions into the pigeon TB and SB muscles in the present toralis muscle in the chick and observed labeled neu- study is consistent with previous cytoarchitectonic rons in the ventromedial corner of the LMC in the mid- studies of the pigeon spinal cord (Huber, 1936; Leonard dle segments of the brachial enlargement. Martin and and Cohen, 1975). The results from the present study demonstrate an Hrycyshyn (1981) also noted labeled neurons in the ventromedial corner of the LMC following HRP injec- anatomical segregation of the motoneurons innervattions into the pectoralis muscle of chicks, but further ing the SB and TB parts of the pectoralis. The SB, reported 1) labeled neurons in dorsolateral regions of which differs from the TB in fiber composition, fascicle the LMC, 2) a few labeled neurons in the contralateral architecture, force distribution, and patterns of electriLMC, 3) the presence of very large labeled neurons cal activity during flight (Dial et al., 1987, 1988; Ka(70-pm diameter) in the pectoralis motor pool, and 4) plan and Goslow, 19891, is innervated by the rostral overlap in the transverse position of neurons labeled branch of the ventral cord of the brachial plexus by with SC and pectoralis injections. We were unable to neurons located in the rostral and middle portions of confirm any of these observations for the pigeon. Dif- the pectoralis motor column. The TB, in contrast, is ferences in the present results from those reported for innervated by the caudal branch of the ventral cord of MUSCULOTOPIC ORGANIZATION; FLIGHT MUSCLES 39 Fig. 3. Rostrocaudal extent and relative location of motoneurons innervating the supracoracoideus (SC), sternobrachialis (SB), and thoracobrachialis (TB) muscles. Diagram produced by superimposing data from all cases. Symbol: 0 , SC; W, SB; A, TB. Note: the symbols indicate the spatial distribution of neurons only; the number of symbols does not correspond to the actual number of motoneurons innervating the SC, SB, or TB muscles. the brachial plexus by neurons located in the middle and caudal regions of the pectoralis motor column. The segregation of motoneurons innervating the SB and TB, in association with histological and electromyographic differences between the two parts, suggests an organization of the pigeon pectoralis which is similar to the compartmental organization described for the lateral gastrocnemius and tibialis anterior muscles of the cat (Iliya and Dum, 1984; English and Weeks, 1987). By virtue of this anatomical segregation, the TB and SB motoneuron populations may be capable of independent activation. Comparative kinematic and electromyographic studies of selected tetrapods have led to speculation that the transition from a primitive musculoskeletal design to a more specialized state may have occurred with little change in the basic motor pattern that controls the shoulder during locomotion (Goslow et al., 1989). Any test of such a hypothesis requires the identification of the homologous musculoskeletal elements from one group to the next. At present we have only a superficial understanding of the homologies of shoulder muscles (see for review Jenkins and Goslow, 1983).The rostrocaudal segregation of motoneurons innervating the different heads of the pectoralis in the dog (Krogh and Towns, 1986) and rat (Baulac and Meininger, 1981) is reminiscent of the separate innervation of the SB and TB in the pigeon, but comparable data on the innervation of the reptile pectoralis are lacking. And, although the pigeon and chicken show similar organization in the rostra1 and lateral location of the SC motoneuron pool relative t o the pectoralis motoneuron pool, we are unaware of information on the innervation of the presumed homologue of the SC in either reptiles or mammals. The apparent conservatism of spinal motoneuron organization (McHanwell and Biscoe, 1981; Fetcho, 1987) may help to determine the homologies of the shoulder muscles in this system in which musculoskeletal patterns are highly derived to serve different locomotor specializations. ACKNOWLEDGMENTS We thank Mary Kramer for comments on the manuscript. One of us (G.E.G.) was funded by NSF grant DCB87-18727. 40 A. SOKOLOFF ET AL. LITERATURE CITED Baulac, M., and V. Meininger 1981 Organisation des motoneurones des muscles pectoraux chez la rat. Acta Anat. (Basel), 109: 209-217. Deacon, T.W., H. Eichenbaum, P. Rosenberg, and K. Eckmann 1983 Afferent connections of the Derirhinal cortex in the rat. J . Comu. ~~~~.~~~ Neurol., 220: 168-190. Dial, K.P., S.R. Kaplan, G.E. Goslow, Jr., and F.A. Jenkins, J r . 1987 Structure and neural control of the pectoralis in pigeons: Implications for flight mechanics. Anat. Rec., 218:284-287. Dial, K.P., S.R. Kaplan, G.E. Goslow, Jr., and F.A. 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