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Mandibular movements and masseter muscle activity were measured in humans during hopping, walking and running to determine whether reflexes contribute to the maintenance of jaw position during locomotion. In initial experiments, subjects hopped so that they landed either on their toes or on their heel. Landing on the toes provoked only small mandibular movements and no reflex responses in the masseter electromyogram (EMG). Landing on the heels with the jaw muscles relaxed caused the mandible to move vertically downwards relative to the maxilla, and evoked a brisk reflex response in the masseter at monosynaptic latency. Neither this relative movement of the mandible nor the reflex was seen when the teeth were clenched: hence the reflex is not the result of vestibular activation during head movement. The same variables were measured in a second series of experiments while subjects stood, walked and ran at various speeds and at various inclinations on a treadmill. During walking, the vertical movements of the head and therefore the mandible were slow and small, and there was no tonic masseter EMG or gait-related activity in the jaw-closing muscles. When subjects ran, the vertical head and jaw movement depended on the running speed and the inclination of the treadmill. Landing on the heels induced larger movements than landing on the toes. About 10 ms after each foot-strike, the mandible moved downwards relative to the maxilla, thereby stretching the jaw-closing muscles and activating them at segmental reflex latency. This caused the mandible to move back upwards. The strength of the reflex response was related to the speed and amplitude of the vertical jaw movement following landing. It is concluded that, during walking, the small, slow movements of the mandible relative to the maxilla are subthreshold for stretch reflexes in the jaw muscles: i.e. the mandible is supported by visco-elasticity of the soft tissues in the masticatory system. However, the brisker downward movements of the mandible after heel-landing during hopping and running evoke segmental reflex responses which contribute to the active maintenance of the posture of the mandible. This is a unique demonstration of how a stretch reflex operates to maintain posture under entirely natural conditions.

Illicit use of stimulant drugs such as methamphetamine, ecstasy, and cocaine is a current and growing problem throughout the world. The aim of the current study was to investigate the long-term effect of illicit stimulant use on human motor cortical and corticospinal circuitry. We hypothesized that individuals with a history of primarily methamphetamine and ecstasy use would exhibit altered corticospinal excitability and intracortical inhibition within motor cortex. The study involved 52 healthy adults (aged 26 ± 7 yr) comprising 26 abstinent stimulant users, 9 cannabis users, and 17 nondrug users. The experiment involved a routine urine drug screen, drug history questionnaire, neuropsychological assessment, and single- and paired-pulse transcranial magnetic stimulation (TMS) over motor cortex. EMG responses to stimulation [motor evoked potentials (MEPs)] were recorded from the contralateral first dorsal interosseus. At a given stimulus intensity, MEP area was significantly larger in abstinent stimulant users than in nondrug users during both relaxation ( P = 0.045) and muscle contraction ( P < 0.001). MEP latency was also significantly longer in abstinent stimulant users ( P < 0.009), and they exhibited significantly greater muscle activity during performance of a given task ( P = 0.004). However, resting motor threshold and the response to paired-pulse TMS were unaffected. The results suggest that abstinent stimulant users exhibit long-term changes in the excitability of motor cortical and corticospinal circuitry and muscle activity during movement. These changes may partly underlie anecdotal and objective reports of movement dysfunction in chronic stimulant users.

Use of illicit stimulant drugs such as methamphetamine, cocaine, and ecstasy are a significant worldwide problem. However, little is known about the effect of these drugs on movement. The aim of the current study was to investigate hand function in adults with a history of illicit stimulant use. We hypothesized that prior use of illicit stimulant drugs is associated with abnormal manipulation of objects. The study involved 22 subjects with a history of illicit stimulant use (aged 29±8 yrs; time since last use: 1.8±4.0 yrs) and two control groups comprising 27 non-drug users (aged 25±8 yrs) and 17 cannabis users with no history of stimulant use (aged 22±5 yrs). Each subject completed screening tests (neuropsychological assessment, medical history questionnaire, lifetime drug history questionnaire, and urine drug screen) prior to gripping and lifting a light-weight object with the dominant right hand. Horizontal grip force, vertical lift force, acceleration, and first dorsal interosseus electromyographic (EMG) activity were recorded during three trials. In trial one, peak grip force was significantly greater in the stimulant group (12.8±3.9 N) than in the control groups (non-drug: 10.3±4.6 N; cannabis: 9.4±2.9 N, P

Use of illicit stimulants such as methamphetamine, cocaine, and ecstasy is a significant health problem. The United Nations Office on Drugs and Crime estimates that 14-57 million people use stimulants each year. Chronic use of illicit stimulants can cause neurotoxicity in animals and humans but the long-term functional consequences are not well understood. Stimulant users self-report problems with tremor whilst abstinent. Thus, the aim of the current study was to investigate the long-term effect of stimulant use on human tremor during rest and movement. We hypothesized that individuals with a history of stimulant use would exhibit abnormally large tremor during rest and movement. Tremor was assessed in abstinent ecstasy users (n = 9; 22±3 yrs) and abstinent users of amphetamine-like drugs (n = 7; 33±9 yrs) and in two control groups: non-drug users (n = 23; 27±8 yrs) and cannabis users (n = 12; 24±7 yrs). Tremor was measured with an accelerometer attached to the index finger at rest (30 s) and during flexion and extension of the index finger (30 s). Acceleration traces were analyzed with fast-Fourier transform. During movement, tremor amplitude was significantly greater in ecstasy users than in non-drug users (frequency range 3.9-13.3 Hz; P

Repetitive transcranial magnetic stimulation (rTMS) can induce short-term reorganization of human motor cortex. Here, we investigated the effect of rTMS during relaxation and weak voluntary muscle contraction on motor cortex excitability and hand function. Subjects ( n = 60) participated in one of four studies. Single transcranial magnetic stimuli were delivered over the motor area of the first dorsal interosseus for measurement of motor evoked potential (MEP) size before and after real or sham rTMS delivered at an intensity of 80% of active motor threshold. rTMS involved trains of stimuli applied at 6 Hz for 5 s and repeated every 30 s for 10 min. Resting MEP size was suppressed for 15 min after rTMS during relaxation. However, MEP suppression was abolished when additional brief voluntary contractions were performed before and after rTMS ( study 1). Resting MEP size was suppressed for 30 min after rTMS during weak voluntary contraction. MEP suppression was present even though voluntary contractions were performed before and after rTMS ( study 2). The MEP suppression most likely reflects a decrease in motor cortical excitability. Surprisingly, rTMS during voluntary contraction did not alter maximal finger tapping speed or performance on a grooved pegboard test, object grip and lift task ( study 3), and visuomotor tracking task ( study 4). These studies document the complex relationship between voluntary movement and rTMS-induced plasticity in motor cortex. This work has implications for the optimization of rTMS parameters for improved efficacy and potential therapeutic applications.