ReviewSpinal interneuronal networks in the cat: Elementary components
Introduction
Elementary spinal interneuronal networks are very simple. In the simplest cases, there are just one or two interneurones in series between input neurones and motoneurones. However, even in the simplest networks, there is a number of interneurones of each kind in parallel and these neurones integrate somewhat different combinations of information, from not only their main sources of input, e.g. muscle and skin afferents, but also from other neuronal networks. They forward it also to somewhat different combinations of their target neurones, including interneurones of other neuronal networks. Because of their links with other networks, all elementary networks may thus be considered to be components of more complex networks.
This arrangement may be illustrated with any of the previously investigated networks of spinal interneurones, from Renshaw cells and interneurones mediating Ia reciprocal inhibition, which were among the first interneurones to be analysed (for references, see Jankowska, 1992), through cervical propriospinal neurones (Lundberg, 1979) and interneurones mediating reflex actions of group II muscle spindle afferents (Jankowska et al., 2002a, Jankowska et al., 2002b), to mention only those known in most detail. In this review it will be illustrated with the recently investigated networks of commissural interneurones. These networks have become of particular interest as being attributed a critical role in locomotor networks (for references, see Buchanan, 1999, Grillner, 2003, Kiehn, 2006, Soffe et al., 1984) because they are needed to adjust rhythmic activity of neurones on both sides of the spinal cord and because they are one of the major targets of reticulospinal neurones that are involved in initiation of locomotion. There is also a growing body of evidence that commissural interneurones may be of critical importance for other centrally or reflexly initiated phasic movements, including voluntary movements and postural adjustments, and that individual commissural interneurones may contribute to several of these movements.
Section snippets
Functional differentiation of the population of commissural interneurones
As other spinal interneuronal populations, the population of commissural interneurones is not homogenous. It includes subpopulations of both excitatory (glutamatergic) and inhibitory (glycinergic) neurones (Bannatyne et al., 2003, Bannatyne et al., 2006, Butt and Kiehn, 2003, Nissen et al., 2005, Roberts et al., 1988, Sugiuchi et al., 1995), at different locations (Bannatyne et al., 2003, Bannatyne et al., 2006, Harrison et al., 1986, Huang et al., 2000, Kiehn and Butt, 2003, Lu et al., 2001,
Comparison of internal organization of elementary interneuronal networks
One of the common features of the so far analysed elementary interneuronal networks is that input to each interneuronal population is drawn from a number of sources, and that input from any of these sources is distributed to several populations, although in different combinations, e.g. both Ia and Ib afferents provide input to interneurones mediating non-reciprocal inhibition of motoneurones (Jankowska et al., 1981), but Ia afferents are the main source of peripheral input to interneurones
Which neurones do and which do not belong to a neuronal network
Boundaries between different neuronal networks may be considered as not being sharp, especially when individual neurones form part of different networks under different circumstances and when neuronal networks change their configuration and elements depending on which movements they subserve. It may thus be a matter of personal preferences whether different kinds of neurones are classified as belonging to the same, or to different, networks. However, independently of how spinal interneuronal
Acknowledgments
The studies carried out in the author's laboratory were supported by grants from the NINDS/NIH (R01 NS040863) and the Swedish Research Council (15393-01A).
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