The Journal of Neuroscience, December 1993, 73(12): 5105-5119 Corticospinal Terminations in Two New-World Primates: Further Evidence That Corticomotoneuronal Connections Provide Part of the Neural Substrate for Manual Dexterity Gregory A. Bortoff and Peter L. Strick Research Service, V.A. Medical Center and Departments of Neurosurgery and Physiology, SUNY Health Science Center at Syracuse, Syracuse, New York 13210 Anterograde transport of 2-10% WGA-HRP was used to dexterity of some primates. In fact, the capacity of selected examine the pattern of termination of efferents from the pri- primates to manufacture and usetools may derive, in part, from mary motor cortex to cervical segments of the spinal cord their ability to perform relatively independent movements of in cebus (Cebus ape/la) and squirrel (Saimiri sciureus) mon- the fingers. keys. We have compared the pattern of termination in these Yet, the appropriate peripheral apparatus is, in itself, not monkeys because of marked differences in their manipu- sufficient to explain the unique motor capacities of some pri- lative abilities. Both primates have pseudo-opposable mates. There are multiple instances of animals that possess thumbs; however, only cebus monkeys use independent fin- similar hands, but differ in their ability to perform dexterous ger movements to pick up small objects. movements of the fingers (e.g., Torigoe, 1985; seeNapier and We found that corticospinal terminations in cervical seg- Napier, 1985). For example, two speciesof new-world primates, ments of the cebus monkey are located in three main zones: cebus monkeys and squirrel monkeys, have similar hands with a dorsolateral region of the intermediate zone, a dorsomedial pseudo-opposablethumbs. However, recent behavioral studies region of the intermediate zone, and the ventral horn. The have demonstratedthat the cebusmonkey and the squirrel mon- projection to the ventral horn in these monkeys is particularly key differ markedly in their manual dexterity (Fragaszy, 1983; dense at C8-Tl segments, where terminations form a “ring” Costello and Fragaszy, 1988). Becausethe thumbs of thesetwo that encircles the lateral motoneuronal cell group. In con- monkeys cannot rotate about the carpo-metacarpaljoint, it was trast, there are only two main zones of terminations in the thought that neither primate was capable of performing a pre- squirrel monkey: a dorsolateral region of the intermediate cision grip (e.g., Napier and Napier, 1967, 1985). In spite of zone and a dorsomedial region of the intermediate zone. As this anatomical constraint, the cebusmonkey is capableof per- others have noted, efferents from the primary motor cortex forming relatively independent finger movements and usesa of squirrel monkeys have, at best, only sparse terminations precision grip to pick up small objects and manipulate tools within the ventral horn. Thus, there are marked differences (Antinucci and Visalberghi, 1986; Westergaard and Fragaszy, between cebus and squirrel monkeys in the extent of cor- 1987; Costello and Fragaszy, 1988). In contrast, the squirrel ticospinal terminations within the ventral horn. These ob- monkey is incapable of performing relatively independent servations provide further support for the concept that movements of the fingers and does not usea precision grip for monosynaptic projections from the primary motor cortex to any activity (Fragaszy, 1983; Costello and Fragaszy, 1988). In- motoneurons in the ventral horn provide part of the neural stead, the squirrel monkey uses a “power grip” in which an substrate for dexterous movements of the fingers. object is grasped by using a sweepingmotion that involves all [Key words: cervical spinal cord, hand movement, motor the fingers of the hand working in concert. The differences in control, primary motor cortex] grip usage between the two speciesof monkey cannot be ex- plained by differences in the mobility of joints in the wrist or The hand endowsmost primates with a number of specialmotor hand (Fragaszy et al., 1989). skills. For example, selectedprimates can perform a “precision In the present study, we explored the possibility that the cebus grip” in which an object is graspedbetween two fingers without and squirrel monkey have different neural substratesfor gen- the useofthe palm (e.g., Napier, 1956, 1961). Hand movements, erating and controlling finger movements. Specifically, we hy- such as a precision grip, are clearly the basis for the enhanced pothesized that efferents from the primary motor cortex ter- minate directly on hand motoneurons in the cebusmonkey, but Received Feb. 19, 1993; revised June 7, 1993; accepted June 15, 1993. not in the squirrel monkey. This hypothesis is basedon results This work was supported by funds from the Veterans Administration Medical from prior anatomical, physiological, and behavioral studies Research Service and Rehabilitation Research and Development Service, and U.S. that suggestedthat the ability to perform highly fractionated Public Health Service Grants 02957 and 24328. We thank Mr. Mike Page for movements of the fingers dependson a monosynaptic connec- development ofcomputer programs, Ms. Mary Lou Chemey and Sarah Fitzpatrick for expert technical assistance, and Dr. Richard P. Dum for advice. This material tion between the primary motor cortex and hand motoneurons has been presented by G.A.B. in partial fulfillment ofthe requirements for a Doctor (for references,see Kuypers, 1981). of Philosophy degree in Physiology from SUNY-HSC at Syracuse. Correspondence should be addressed to Dr. Peter L. Strick, Research Service There already is evidence from prior studies that the squirrel (lSI), V.A. Medical Center, Syracuse, NY 13210. monkey lacks substantial projections from its primary motor Copyright 0 1993 Society for Neuroscience 0270-6474/93/135105-14$05.00/O cortex to the ventral horn (Harting and Noback, 1970; Tigges 5106 Bortoff and Strick - Corticospinal Terminations et al., 1979). However, there is little information available on and the wound was closed in anatomical layers. The animal was returned corticospinal connections in the cebus monkey (Petras, 1968). to its home cage, where recovery was carefully monitored. After the survival period, animals were reanesthetized and perfused Therefore, we used anterograde transport techniques to define transcardially using a four-step procedure (Rosene and Mesulam, 1978; the pattern of projections from the primary motor cortex of the Mesulam, 1982). The perfusates included (1) 0.1 M phosphate buffer cebus monkey to the cervical spinal cord. We also examined solution, pH 7.4 (PBS); (2) 2.5% paraformaldehyde in PBS; (3) 2.5% this projection in the squirrel monkey for comparison. paraformaldehyde in 10% glycerin and PBS; and (4) cold 10% glycerin We found that there is a major difference between the cebus in PBS. The spinal cord and the brain were removed and stored in PBS with lO-20% glycerin at 4°C for 5-7 d. The cervical spinal cord was and squirrel monkey in the pattern of corticospinal terminations divided into two blocks (C2-C5 and C6-T2). The brain was blocked to in the cervical spinal cord. The cebus monkey has abundant include the injection site and adjacent cortical areas. corticospinal terminations in the regions of the ventral horn where the cell bodies of hand motoneurons are located. In con- Histological procedures trast, comparable terminations at this si?e are absent in the The brain and spinal cord blocks were quick-frozen in cold isopentane squirrel monkey. Thus, our results suggest that the differences (Rosene et al., 1986). Transverse sections of spinal cord and coronal in corticomotoneuronal connections between cebus and squirrel sections of brain were cut at 50 Frn intervals on a microtome. Every 10th section was postfixed and reacted with cresyl violet for cytoarchi- monkeys may be part of the basis for the differences in hand tecture, according to Gower (in Mesulam, 1982). The remaining sections function between these two primates. Furthermore, our results were processed for HRP using a tetramethylbenzidine (TMB) method, provide additional support for the concept that the direct cor- which incorporates the procedures of Mesulam (1982) and Gibson et ticomotoneuronal connection is part of the neural substrate for al. (1984). relatively independent movements of the fingers. A brief report of some of the results has been presented pre- Analytical procedures viously (Bortoff and Strick, 1990). The outlines of sections and the locations of injections sites, labeled processes, and tissue landmarks (e.g., blood vessels, spinal lamina) were plotted and stored using a computerized charting system (Minnesota Materials and Methods Datametrics). This system uses optical encoders to sense x-y movements We used anterograde transport of wheat germ agglutinin conjugated to of the microscope stage and stores the coordinates of charted structures horseradish peroxidase (WGA-HRP) to label the terminations of effer- on an IBM-compatible computer. ents from the primary motor cortex within cervical segments of the Injection site analysis. At least every fourth section of the frontal spinal cord in two new-world primates: the squirrel monkey (Suimiri cortex was examined under bright-field and dark-field illumination with sciureus, n = 2) and the tufted capuchin (Cebus apella, n = 2). polarized light and plotted to define the extent of the injection site. The injection site was considered to include the densely stained regions adjacent to the needle track, but not the adjacent region with light Surgical procedures background staining where individual neurons could be distinguished One day prior to surgery, each animal was pretreated with dexameth- (Mesulam, 1982). The sections stained with cresyl violet were examined asone (0.5 mg/kg, i.m.). The animal was restricted from food and water under bright-field illumination to determine the area 3a/4 border using 6-12 hr prior to surgery. On the day of surgery, the animal was anes- the appearance of a granular layer IV as the defining criterion (Lucier thetized with ketamine HCI (25 mg/kg, i.m.) and sodium pentobarbital et al., 1975; Jones and Porter, 1980; Strick and Preston, 1982). (20 mg/kg, i.p.).