Abstract
Descending systems have a crucial role in the selection of motor output patterns by influencing the activity of interneuronal networks in the spinal cord. Commissural interneurons that project to the contralateral grey matter are key components of such networks as they coordinate left-right motor activity of fore and hind-limbs. The aim of this study was to determine if corticospinal (CST) and reticulospinal (RST) neurons make significant numbers of axonal contacts with cervical commissural interneurons. Two classes of commissural neurons were analysed: 1) local commissural interneurons (LCINs) in segments C4-5; 2) long descending propriospinal neurons (LDPNs) projecting from C4 to the rostral lumbar cord. Commissural interneurons were labelled with Fluorogold and CST and RST axons were labelled by injecting the b subunit of cholera toxin in the forelimb area of the primary somatosensory cortex or the medial longitudinal fasciculus respectively. The results show that LCINs and LDPNs receive few contacts from CST terminals but large numbers of contacts are formed by RST terminals. Use of vesicular glutamate and vesicular GABA transporters revealed that both types of cell received about 80% excitatory and 20% inhibitory RST contacts. Therefore the CST appears to have a minimal influence on LCINs and LDPNs but the RST has a powerful influence. This suggests that left-right activity in the rat spinal cord is not influenced directly via CST systems but is strongly controlled by the RST pathway. Many RST neurons have monosynaptic input from corticobulbar pathways therefore this pathway may provide an indirect route from the cortex to commissural systems. The cortico-reticulospinal-commissural system may also contribute to functional recovery following damage to the CST as it has the capacity to deliver information from the cortex to the spinal cord in the absence of direct CST input.
Highlights
Descending systems have a crucial role in the selection of motor output patterns by influencing the activity of interneuronal networks in the spinal cord
In this study we investigated the connectivity of corticospinal tract (CST) and reticulospinal tract (RST) axons with two classes of commissural interneurons (CINs) located within the cervical spinal cord; those with local intrasegmental (LCINs) projections and long-range descending propriospinal neurons (LDPNs) that project to the contralateral or ipsilateral lumbar cord
In this study we have shown that in the rat, CST axons originating from the forelimb region of the sensorimotor cortex form very small numbers of contacts with cervical LCINs and LDPNs whereas RST axons make approximately ten times the numbers of contacts with these cells
Summary
Descending systems have a crucial role in the selection of motor output patterns by influencing the activity of interneuronal networks in the spinal cord. In this study we investigated the connectivity of CST and RST axons with two classes of CINs located within the cervical spinal cord; those with local intrasegmental (LCINs) projections and long-range descending propriospinal neurons (LDPNs) that project to the contralateral or ipsilateral lumbar cord Both LCINs and LDPNs are known to make monosynaptic connections with contralateral motoneurons [13, 14] and have the capacity to influence directly motor activity in the contralateral side of the cord
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