Neurobiological basis of state-dependent control of motor behaviors

Deep brain stimulation (DBS) as a therapeutic approach in gait disorders: What does it bring to the table?

Gait deficits are found in various degenerative central nervous system conditions, and are particularly a hallmark of Parkinson's disease (PD). While there is no cure for such neurodegenerative disorders, Levodopa is considered as the standard medication in PD patients. Often times, the therapy of severe PD consists of deep brain stimulation (DBS) of the subthalamic nucleus. Earlier research exploring the effect of gait have reported contradictory results or insufficient efficacy. A change in gait includes various parameters, such as step length, cadence, Double-stance phase duration which may be positively affected by DBS. DBS could also be effective in correcting the levodopa-induced postural sway abnormalities. Moreover, during normal walking, interaction among the subthalamic nucleus and cortex -essential regions which exert a role in locomotion- are coupled. However, during the freezing of gait, the activity is desynchronized. The mechanisms underlying DBS-induced neurobehavioral improvements in such scenarios requires further study. The present review discusses DBS in the context of gait, the benefits associated with DBS compared to standard pharmacotherapy options, and provides insights into future research.

Neurophysiology of gait: from the spinal cord to the frontal lobe

Locomotion is a purposeful, goal-directed behavior initiated by signals arising from either volitional processing in the cerebral cortex or emotional processing in the limbic system. Regardless of whether the locomotion initiation is volitional or emotional, locomotion is accompanied by automatic controlled movement processes, such as the adjustment of postural muscle tone and rhythmic limb movements. Sensori-motor integration in the brainstem and the spinal cord plays crucial roles in this process. The basic locomotor motor pattern is generated by spinal interneuronal networks, termed central pattern generators (CPGs). Responding to signals in proprioceptive and skin afferents, the spinal interneuronal networks modify the locomotor pattern in cooperation with descending signals from the brainstem structures and the cerebral cortex. Information processing between the basal ganglia, the cerebellum, and the brainstem may enable automatic regulation of muscle tone and rhythmic limb movements in the absence of conscious awareness. However, when a locomoting subject encounters obstacles, the subject has to intentionally adjust bodily alignment to guide limb movements. Such an intentional gait modification requires motor programming in the premotor cortices. The motor programs utilize one's bodily information, such as the body schema, which is preserved and updated in the temporoparietal cortex. The motor programs are transmitted to the brainstem by the corticoreticulospinal system, so that one's posture is anticipatorily controlled. These processes enable the corticospinal system to generate limb trajectory and achieve accurate foot placement. Loops from the motor cortical areas to the basal ganglia and the cerebellum can serve this purpose.

Staying safe in the jungles of Borneo: five studies of fatigue and cultural issues in remote mining projects

The global mining industry keeps expanding, and projects are often started in areas previously considered too remote. Due to worker beliefs about safety, and the diversity of cultures in remote projects, the measurement and management of human fatigue is complex. This paper reports on five studies from companies in Kalimantan, the Indonesian part of the island of Borneo, where workers had been killed in likely fatigue-related accidents. Mixed-method approaches, involving qualitative, semi-quantitative, and quantitative measures were used. Participants were 20-45 yr old, had homes of 4.7 people (SD +/- 1.8), shared income outside of the house in 80% of cases, travelled < or = 3 d each-way between blocks of shifts. A major output was a set of camp standards to help ensure recovery sleep. Another requirement identified was access to leave when family members died, since not attending death ceremonies caused a lot of stress and made recovery and safe work less possible. Demanding work conditions and long work hours were also problematic in some operations. Safety systems should better consider fatigue in accidents/incidents and link its data with hours of work information. The interaction of cultures, stress, sleep, fatigue, safety and individual differences must be more effectively addressed in remote mining camps.

[Motor control by the basal ganglia]

The cerebral cortex controls cognitive and voluntary process of movements. The brainstem and spinal cord are involved in the execution of innately acquired motor patterns such as postural reflexes, muscle tone regulation and locomotion. Cortico-reticular projections arising from the motor cortical areas contribute to the postural control that precedes the voluntary movement process. The basal ganglia cooperatively regulates the activities of the cerebral cortex and the brainstem-spinal cord by its strong inhibitory and dis-inhibitory effects upon these target structures so that goal-directed movements could be appropriately performed. We propose that basal ganglia disfunction, including the abnormality in the dopaminergic projection system, may disturb the cooperative regulation, resulting in motor deficiencies expressed in basal diseases.

Substrates for normal gait and pathophysiology of gait disturbances with respect to the basal ganglia dysfunction

In this review, we have tried to elucidate substrates for the execution of normal gait and to understand pathophysiological mechanisms of gait failure in basal ganglia dysfunctions. In Parkinson's disease, volitional and emotional expressions of movement processes are seriously affected in addition to the disturbance of automatic movement processes, such as adjustment of postural muscle tone before gait initiation and rhythmic limb movements during walking. These patients also suffer from muscle tone rigidity and postural instability, which may also cause reduced walking capabilities in adapting to various environments. Neurophysiological and clinical studies have suggested the importance of basal ganglia connections with the cerebral cortex and limbic system in the expression of volitional and emotional behaviors. Here we hypothesize a crucial role played by the basal ganglia-brainstem system in the integrative control of muscle tone and locomotion. The hypothetical model may provide a rational explanation for the role of the basal ganglia in the control of volitional and automatic aspects of movements. Moreover, it might also be beneficial for understanding pathophysiological mechanisms of basal ganglia movement disorders. A part of this hypothesis has been supported by studies utilizing a constructive simulation engineering technique that clearly shows that an appropriate level of postural muscle tone and proper acquisition and utilization of sensory information are essential to maintain adaptable bodily functions for the full execution of bipedal gait. In conclusion, we suggest that the major substrates for supporting bipedal posture and executing bipedal gait are 1) fine neural networks such as the cortico-basal ganglia loop and basal ganglia-brainstem system, 2) fine musculoskeletal structures with adequately developed (postural) muscle tone, and 3) proper sensory processing. It follows that any dysfunction of the above sensorimotor integration processes would result in gait disturbance.

Forebrain control of locomotor behaviors

Activation of different areas in the forebrain evokes different types of goal directed adaptive behaviors. An important component of these different patterns of behavior is the locomotion that brings the animal to or away from a particular location. Here I review the role of projections from forebrain structures to the mesopontine tegmentum of the brainstem where neural mechanisms for initiation of locomotion and regulation of postural muscle tone are located that are activated during locomotor behavior. It is interesting is to understand how signals that converge from the forebrain structures to the mesopontine tegmentum control locomotor behavior, because the mesopontine tegmentum receives inhibitory efferents from the basal ganglia and excitatory efferents from the limbic-hypothalamic system and the neocortex. Here I hypothesize that the mesopontine tegmentum has functional gating mechanisms that determine whether the subject will initiate and select volitionally guided or emotionally triggered locomotor behaviors, depending on the behavioral context.