Spinal interneuronal networks in the cat: elementary components

This review summarises features of networks of commissural interneurones co-ordinating muscle activity on both sides of the body as an example of feline elementary spinal interneuronal networks. The main feature of these elementary networks is that they are interconnected and incorporated into more complex networks as their building blocks. Links between networks of commissural interneurones and other networks are quite direct, with mono- and disynaptic input from the reticulospinal and vestibulospinal neurones, disynaptic from the contralateral and ipsilateral corticospinal neurones and fastigial neurones, di- or oligosynaptic from the mesencephalic locomotor region and mono-, di- or oligosynaptic from muscle afferents. The most direct links between commissural interneurones and motoneurones are likewise simple: monosynaptic and disynaptic via premotor interneurones with input from muscle afferents. By such connections, a particular elementary interneuronal network may subserve a wide range of movements, from simple reflex and postural adjustments to complex centrally initiated phasic and rhythmic movements, including voluntary movements and locomotion. Other common features of the commissural and other interneuronal networks investigated so far is that input from several sources is distributed to their constituent neurones in a semi-random fashion and that there are several possibilities of interactions between neurones both within and between various populations. Neurones of a particular elementary network are located at well-defined sites but intermixed with neurones of other networks and distributed over considerable lengths of the spinal cord, which precludes the topography to be used as their distinguishing feature.