Analysis of basic motor behaviors in quadrupeds

Abstract

Ability to perform locomotion in different directions and maintain upright body posture is crucial for normal life. At present, mice, which allows employing genetic approaches, are widely used in studying the locomotor system. In these investigations different experimental setups are used to evoke locomotion. First aim of the present study was to compare kinematics of forward (FW) and backward (BW) locomotion performed in different environmental conditions (i.e. in a tunnel, on a treadmill and on an air-ball). On all set-ups, average speed, step amplitude and swing duration during BW locomotion were significantly smaller compared to those observed during FW locomotion. The extent of rostro-caudal paw trajectory in relation to the hip projection to the surface (HP) strongly depended on hip height. With high hip height, the trajectory was symmetrical in relation to HP (middle steps). When hip was low, steps were either displaced rostrally (anterior steps) or caudally (posterior steps) in relation to HP. During FW locomotion, predominantly anterior and posterior steps were observed, respectively, on the treadmill and air-ball, while all three stepping forms were observed in the tunnel. We observed only anterior steps during BW locomotion. Intralimb coordination depended on the form of stepping.

Second aim of the present study was to reveal the role of two populations of commissural interneurons (V0V and V0D CINs) in control of a number of basic motor behaviours (BW locomotion, scratching, righting, and postural corrections). For this purpose two types of knockout mice (Vglut2Cre;Dbx1DTA mice and Hoxb8Cre;Dbx1DTA mice with only V0V and all V0 CINs ablated, respectively) as well as wild-type littermates were used. Our results suggest that the functional effect of excitatory V0V CINs during BW locomotion and scratching is inhibitory, and that execution of scratching involves active inhibition of the contralateral scratching CPG mediated by V0V CINs. By contrast, V0D CINs are elements of spinal postural network, generating postural corrections. Finally, both V0D and V0V CINs contribute to generation of righting behavior. Thus, our study shows the differential contribution of V0 neuron subpopulations in generation of diverse motor acts.

Single steps in different directions are used for control of balance or body configuration. However, our knowledge about neural mechanisms responsible for their generation is limited. The third aim of the present study was to characterize postural response to disturbance of basic body configuration caused by forward, backward or outward displacement of the hindlimb. In intact rabbits, displacement of the hindlimb in any direction caused a postural response consisting of two components. First, a lateral trunk movement towards the supporting (contralateral) hindlimb was performed, and then a corrective step in the direction opposite to the direction of the initial limb displacement was executed. These two components were generated by different mechanisms activated in a strict order by sensory information from the deviated limb signalling distortion of the limb/limb-trunk configuration. We have shown that the integrity of the forebrain was not critical for generation of this postural response. We proposed a hypothesis about operation of mechanisms generating the postural response characterized in the present study.