Contribution of the Motor Cortex to the Structure and the Timing of Hindlimb Locomotion in the Cat: A Microstimulation Study

While walking in a straight path, changes in speed result mainly from adjustments in the duration of the stance phase while the swing phase remains relatively invariant, a basic feature of the spinal central pattern generator (CPG). To produce a broad range of locomotor behaviors, the CPG has to integrate modulatory inputs from the brain and the periphery and alter these swing/stance characteristics. In the present work we raise the issue as to whether the CPG can adapt or reorganize in response to a chronic change of supraspinal inputs, as is the case after spinal cord injury (SCI). Kinematic data obtained from six adult cats walking at different treadmill speeds were collected to calculate the cycle and subphase duration at different stages after a first spinal hemisection at T(10) and after a subsequent complete SCI at T(13) respectively aimed at disconnecting unilaterally and then totally the spinal cord from its supraspinal inputs. The results show, first, that the neural control of locomotion is flexible and responsive to a partial or total loss of supraspinal inputs. Second, we demonstrate that a hemisection induces durable plastic changes within the spinal locomotor circuitry below the lesion. In addition, this study gives new insights into the organization of the spinal CPG for locomotion such that phases of the step cycle (swing, stance) can be independently regulated for adapting to speed and also that the CPGs controlling the left and right hindlimbs can, up to a point, be regulated independently.

Section: Discussionsupporting

confidence: 81%

Incomplete spinal cord injury promotes durable functional changes within the spinal locomotor circuitry

Martinez

Delivet-Mongrain

Leblond

et al. 2012

“…Indeed, Asante and Martin (2010) recently found in the mouse that spinal projections from shoulder-, elbow-, and wrist-related areas in the motor cortex primarily contact those spinal premotor circuits that connect to shoulder-, elbow-, and wrist-related motoneuron pools, respectively. On the basis of results of experiments with microstimulation in the motor cortex, analogous mechanisms for control of limb joints have been previously suggested by Drew (1991) for the forelimb and by Bretzner and Drew (2005) for the hindlimb of the cat. However, these authors now stress the likelihood that the motor cortex controls locomotion movements based on muscle synergies that appear to form during stepping Krouchev et al 2006).…”

Section: Discussionsupporting

confidence: 57%

Pyramidal tract neurons receptive to different forelimb joints act differently during locomotion

Stout

Beloozerova

2012

During locomotion, motor cortical neurons projecting to the pyramidal tract (PTNs) discharge in close relation to strides. How their discharges vary based on the part of the body they influence is not well understood. We addressed this question with regard to joints of the forelimb in the cat. During simple and ladder locomotion, we compared the activity of four groups of PTNs with somatosensory receptive fields involving different forelimb joints: 1) 45 PTNs receptive to movements of shoulder, 2) 30 PTNs receptive to movements of elbow, 3) 40 PTNs receptive to movements of wrist, and 4) 30 nonresponsive PTNs. In the motor cortex, a relationship exists between the location of the source of afferent input and the target for motor output. On the basis of this relationship, we inferred the forelimb joint that a PTN influences from its somatosensory receptive field. We found that different PTNs tended to discharge differently during locomotion. During simple locomotion shoulder-related PTNs were most active during late stance/early swing, and upon transition from simple to ladder locomotion they often increased activity and stride-related modulation while reducing discharge duration. Elbow-related PTNs were most active during late swing/early stance and typically did not change activity, modulation, or discharge duration on the ladder. Wrist-related PTNs were most active during swing and upon transition to the ladder often decreased activity and increased modulation while reducing discharge duration. These data suggest that during locomotion the motor cortex uses distinct mechanisms to control the shoulder, elbow, and wrist.

“…Although there is an important contribution of the spinal cord, brain stem, and motor cortex in the control of locomotion (Bretzner and Drew 2005;Hayes et al 2009;Le Ray et al 2011), the cerebellum is important for the adaptation of the locomotor pattern. The cerebellum is believed to have a role in monitoring errors and updating the motor outputs (Shadmehr and Krakauer 2008), and, not surprisingly, the cerebellum has been shown to be essential for many forms of motor adaptation (Criscimagna-Hemminger et al 2010;Lewis and Zee 1993;Maschke et al 2004;Tseng et al 2007), including split-belt walking adaptation (Morton and Bastian 2006).…”

Section: Discussionmentioning

confidence: 99%

“…2B). We think these timings might be updated in a feedforward manner during split-belt adaptation since it has been shown that there is a large contribution of higher centers, relative to feedback processes, in the control of heel-strike in cats (Bretzner and Drew 2005). Accordingly, we defined T out as the difference in slow and fast step times (t s and t f , respectively) normalized by the stride time (T stride ):…”

Section: Subjectsmentioning

confidence: 99%

How does the motor system correct for errors in time and space during locomotor adaptation?

Malone

Bastian

Torres-Oviedo

2012

110

158

Malone LA, Bastian AJ, Torres-Oviedo G. How does the motor system correct for errors in time and space during locomotor adaptation? J Neurophysiol 108: 672-683, 2012. First published April 18, 2012 doi:10.1152/jn.00391.2011.-Walking is a complex behavior for which the healthy nervous system favors a smooth, symmetric pattern. However, people often adopt an asymmetric walking pattern after neural or biomechanical damage (i.e., they limp). To better understand this aberrant motor pattern and how to change it, we studied walking adaptation to a split-belt perturbation where one leg is driven to move faster than the other. Initially, healthy adult subjects take asymmetric steps on the split-belt treadmill, but within 10 -15 min people adapt to reestablish walking symmetry. Which of the many walking parameters does the nervous system change to restore symmetry during this complex act (i.e., what motor mappings are adapted to restore symmetric walking in this asymmetric environment)? Here we found two parameters that met our criteria for adaptive learning: a temporal motor output consisting of the duration between heel-strikes of the two legs (i.e., "when" the feet land) and a spatial motor output related to the landing position of each foot relative to one another (i.e., "where" the feet land). We found that when subjects walk in an asymmetric environment they smoothly change their temporal and spatial motor outputs to restore temporal and spatial symmetry in the interlimb coordination of their gait. These changes in motor outputs are stored and have to be actively deadapted. Importantly, the adaptation of temporal and spatial motor outputs is dissociable since subjects were able to adapt their temporal motor output without adapting the spatial output. Taken together, our results suggest that temporal and spatial control for symmetric gait can be adapted separately, and therefore we could potentially develop interventions targeting either temporal or spatial walking deficits. human locomotion; kinematics; motor control; motor learning; walking IN OUR EVERYDAY LIVES we experience a variety of perturbations to movement and quickly learn to correct for predictable errors-we adjust walking for icy surfaces or hand movements for an unfamiliar computer mouse. We refer to this shorttimescale motor learning process as motor adaptation. Motor adaptation is defined here as an error-driven process that allows us to adjust stored movement calibrations used to make predictions of movement outcomes. So rather than constantly correcting our movements upon sustained perturbations, motor adaptation enables us to adjust sensorimotor mappings of well-learned movements when there are changes in the environment or the body (Bastian 2008;Krakauer 2009). Motor adaptation is studied by exposing subjects to novel situations (i.e., adaptation periods), where movements are initially perturbed and errors abruptly increase (Fig. 1A). To minimize these errors, movements (i.e., motor outputs) are actively modified from a preperturbation (i.e., pa...