The Pyramidal Tract

Transcranial Magnetic Stimulation during Gait: A Review of Methodological and Technological Challenges

Transcranial magnetic stimulation (TMS) is widely used for therapeutic and research purposes such as cognitive studies, treatment of psychiatric disorders, and Parkinson's disease. In research, TMS is perhaps the only technique that can establish a functional connection between brain regions and task performance. In gait research, often TMS is used to identify the extent to which leg motor cortex is involved in different phases on gait cycle. However, using TMS in gait can be challenging for several technical reasons and physiological variations. The objective of this narrative review is to summarize literature in the field of TMS and gait research and present comprehensive challenges. A comprehensive literature search was conducted in PubMed and Google Scholar to identify all relevant literature on TMS and gait. Several critical challenges could potentially impact the findings. For instance, the use of different protocols to obtain motor threshold. This review presents some of the critical challenges in applying TMS during gait. It is important to be aware of these variations and utilize strategies to mitigate some challenges.

Activity-dependent plasticity improves M1 motor representation and corticospinal tract connectivity

Motor cortex (M1) activity between postnatal weeks 5 and 7 is essential for normal development of the corticospinal tract (CST) and visually guided movements. Unilateral reversible inactivation of M1, by intracortical muscimol infusion, during this period permanently impairs development of the normal dorsoventral distribution of CST terminations and visually guided motor skills. These impairments are abrogated if this M1 inactivation is followed by inactivation of the contralateral, initially active M1, from weeks 7 to 11 (termed alternate inactivation). This later period is when the M1 motor representation normally develops. The purpose of this study was to determine the effects of alternate inactivation on the motor representation of the initially inactivated M1. We used intracortical microstimulation to map the left M1 1 to 2 mo after the end of left M1 muscimol infusion. We compared representations in the unilateral inactivation and alternate inactivation groups. Alternate inactivation converted the sparse proximal M1 motor representation produced by unilateral inactivation to a complete and high-resolution proximal-distal representation. The motor map was restored by week 11, the same age that our present and prior studies demonstrated that alternate inactivation restored CST spinal connectivity. Thus M1 motor map developmental plasticity closely parallels plasticity of CST spinal terminations. After alternate inactivation reestablished CST connections and the motor map, an additional 3 wk was required for motor skill recovery. Since motor map recovery preceded behavioral recovery, our findings suggest that the representation is necessary for recovering motor skills, but additional time, or experience, is needed to learn to take advantage of the restored CST connections and motor map.

Integration of motor and visual information in the parietal area 5 during locomotion

The parietal cortex receives both visual- and motor-related information and is believed to be one of the sites of visuo-motor coordination. This study for the first time characterizes integration of visual and motor information in activity of neurons of parietal area 5 during locomotion under conditions that require visuo-motor coordination. The activity of neurons was recorded in cats during walking on a flat surface-a task with no visuo-motor coordination required (flat locomotion), walking along a horizontal ladder or a series of barriers-a task requiring visuo-motor coordination for an accurate foot placement on surface that is heterogeneous along the direction of progression (ladder and barriers locomotion), and walking along a narrow pathway-a task requiring visuo-motor coordination on surface homogeneous along the direction of progression (narrow locomotion). During flat locomotion, activity of 66% of the neurons was modulated in rhythm of stepping, usually with one peak per cycle. During ladder and barrier locomotion, the proportion of rhythmically active neurons significantly increased, their modulation became stronger, and the majority of neurons had two peaks of activity per cycle. During narrow locomotion, however, the activity of neurons was similar to that during flat locomotion. We concluded that, during locomotion, parietal area 5 integrates two types of information: signals about the activity of basic locomotion mechanisms and signals about heterogeneity of the surface along the direction of progression. We describe here the modes of integration of these two types of information during locomotion.