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First published online May 18, 2006
Journal of Experimental Biology 209, 2007-2014 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02213

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Deciphering the organization and modulation of spinal locomotor central pattern generators

Ian T. Gordon and Patrick J. Whelan^*

HSC 2119, Hotchkiss Brain Institute, University of Calgary, Calgary, AB T2N 4N1, Canada

View larger version (25K): [in a new window]   Fig. 1. (A) Pattern of flexor and extensor muscle activity recorded from a conscious adult mouse walking on a treadmill. Adapted from Pearson et al. (Pearson et al, 2005) with permission. (B) A hindlimbs-attached preparation can be used with electromyographs (EMGs) or electroneurograms (ENGs) to record activity from spinal networks (Pearson et al., 2003; Whelan et al., 2000). The thoracosacral spinal cord is preserved in this preparation. ENGs can also be recorded from the lateral gastrocnemius (LGS) and common peroneal (CP) nerve, which allow flexor and extensor activity to be recorded as shown in the example (Whelan et al., 2000). Note that the flexor and extensor pattern produced by the in vitro neonatal mouse preparation is qualitatively similar to that produced by adult mice walking on a treadmill. (C) Isolated spinal cord preparation commonly used to record locomotor-like patterns in vitro. Alternation between ipsilateral L2 and L5 ventral root recordings corresponds to flexor-extensor activity, while segmental alternation between the left (l) and right (r) L2 roots reflects left-right alternation (Whelan et al., 2000). Rhythm evoked using a combination of serotonin (5-HT), N-methyl-D/L-aspartate (NMA) and dopamine (see Jiang et al., 1999). TA, Tibialis anterior; GS, gastrocnemius

View larger version (26K): [in a new window]   Fig. 2. (Ai) In the neonatal mouse, electrical stimulation (4 Hz, 40 pulses) of the cauda equina is capable of evoking coordinated lumbar locomotor-like activity. (Aii) Rectified and filtered neurogram traces recorded from the second and fifth lumbar ventral roots (L2 and L5, respectively). These traces display both left-right (see L2-L2 and L5-L5 traces) and flexor-extensor (see ipsilateral L2-L5 traces) alternation. This pattern is considered to be a signature of locomotor-like activity. (Bi) A schematic of the recording procedure used to examine coordination between cervical and lumbar segments. (Bii) These digitally rectified and filtered traces show left-right alternation in the cervical roots (C8) that is coupled with the pattern recorded from the L2 neurograms. l, left; r, right.

View larger version (41K): [in a new window]   Fig. 3. Genetic techniques can be used to dissect circuits within the spinal cord. Populations of interneurons derived from progenitor cells can be identified in neonatal mice. Schematic shows different populations of cells that can be identified currently. V0 cells are commissural interneurons that consist of both glutamatergic and glycinergic/GABAergic cells. Animals lacking these commissural cells have poor left-right coordination. V1 interneurons are considered to be inhibitory and are a heterogeneous population. At least a portion of them is involved in regulating the speed of the rhythm (Gosgnach et al., 2006). Renshaw cells form part of the V1 population and have been well characterized (Sapir et al., 2004), but the contribution of individual classes of V1 neurons to rhythm generation has not been well established. V2 cells are also likely to be heterogeneous and consist of ipsilateral propriospinal interneurons projecting across several segments. One possibility is that these cells are involved in flexor-extensor coordination. V3 cells are glutamatergic commissural interneurons that may play a role in coordination of patterns. Cells expressing a transcription factor termed Hb9 are present in lamina VIII and exhibit several intrinsic properties thought to be important for rhythm generation (Wilson et al., 2005). Figure adapted from a previously published embryonic schematic (Goulding and Pfaff, 2005).

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