Abstract
The anatomy of the cortical motor system and its relationship to motor repertoire in artiodactyls is for the most part unknown. We studied the origin and termination of the corticospinal tract (CST) and cortico-brainstem projections in domestic pigs. Pyramidal neurons were retrogradely labeled by injecting aminostilbamidine in the spinal segment C1. After identifying the dual origin of the porcine CST in the primary motor cortex (M1) and premotor cortex (PM), the axons descending from those regions to the spinal cord and brainstem were anterogradely labeled by unilateral injections of dextran alexa-594 in M1 and dextran alexa-488 in PM. Numerous corticospinal projections from M1 and PM were detected up to T6 spinal segment and showed a similar pattern of decussation and distribution in the white matter funiculi and the gray matter laminae. They terminated mostly on dendrites of the lateral intermediate laminae and the internal basilar nucleus, and some innervated the ventromedial laminae, but were essentially absent in lateral laminae IX. Corticofugal axons terminated predominantly ipsilaterally in the midbrain and bilaterally in the medulla oblongata. Most corticorubral projections arose from M1, whereas the mesencephalic reticular formation, superior colliculus, lateral reticular nucleus, gigantocellular reticular nucleus, and raphe received abundant axonal contacts from both M1 and PM. Our data suggest that the porcine cortical motor system has some common features with that of primates and humans and may control posture and movement through parallel motor descending pathways. However, less cortical regions project to the spinal cord in pigs, and the CST neither seems to reach the lumbar enlargement nor to have a significant direct innervation of cervical, foreleg motoneurons.
Highlights
The artiodactyl forelimb is specialized for standing and locomotion
In both M1 and premotor cortex (PM), more than 90% of corticospinal neurons (CSNs) was concentrated in a length of 10–15 mm in the most caudal part of the gyri (Figure 1C), while the remaining 10% was scattered in a few millimeters rostral to this region
Analyzes of the injection sites at C1 of pig 1 showed that the tracer was mostly confined to the targeted side of the spinal cord, spreading in both the gray matter (GM) and white matter (WM) (Supplementary Figure 1A)
Summary
Pigs are even-toed ungulates with four digits in the forelimbs and hindlimbs. Frontiers in Neuroanatomy | www.frontiersin.org del Cerro et al. The Porcine Cortical Motor System allows for better walking stability but constrains other type of movements. Pigs usually perform skilled motor acts (for instance searching food) with the snout instead than with the forelimb (Goursot et al, 2018). Pigs can still execute targetdirected movements such as lever press maintenance with the forehoof and using the forelimb can be preferred over the snout in a context-dependent manner (Ferguson et al, 2009). The cerebral cortex is involved in the initiation of volitional movements and the selection of motor commands by selectively filtering sensory information and by coordinating posture with the activation of proximal and distal limb muscles (Canedo, 1997). There is a gap of knowledge regarding the forelimb motor repertoire in swine and the precise neural basis for its volitional control
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