Subpopulations and functions of long C3-C5 propriospinal neurones.

Long C3-C5 propriospinal neurones (PNs) are classified in 3 types depending on their pyramidal and vestibular input. The first type of PNs received pyramidal excitation but lacked vestibular effects. The second type of PNs was excited from the medial vestibular nucleus but not from the pyramid. The third type of PNs was excited from the lateral vestibular nucleus either from second order neurones or from non-second order neurones. Monosynaptic excitatory postsynaptic potentials from neck afferents and/or oligosynaptic postsynaptic potentials from forelimb afferents were found in some of the PNs of the second and third type but not in those of the first type. Collision experiments revealed that cortico- and rubrospinal fibres to the long C3-C5 PNs terminate in the rostral spinal cord, presumably in the forelimb segments. Vestibular and reticular effects on the PNs are partly from fibres terminating in the rostral spinal cord and partly from fibres projecting to the lumbar cord. It is postulated that the different types of PNs contribute to the adjustment of hindlimb posture which is required during different movements of the forebody. It is suggested that the basic tonus is maintained mainly by the direct projection to the hindlimb segments from lateral vestibulospinal and reticulospinal neurones which excite antigravity muscles via lumbar interneurones and that the long C3-C5 PNs converge onto the same interneurones so that they act by modulation of the basic tonus.