Properties of human spinal interneurones: normal and dystonic control

Psychophysiological and Neurophysiological Correlates of Dropping Objects from Hands in Carpal Tunnel Syndrome

BACKGROUND

Dropping objects from hands (DOH) is a common symptom of carpal tunnel syndrome (CTS). We evaluated the clinical, neurophysiological, and psychophysiological features of 120 CTS patients to elucidate the DOH pathophysiology. Forty-nine healthy controls were included.

METHODS

In the patients, the Boston Carpal Tunnel Questionnaire (BCTQ), the Douleur Neuropathique 4 questions (DN4), and a numeric rating scale for pain (NRS) were evaluated. In patients and controls, we evaluated bilateral median and ulnar motor and sensory nerve conduction studies, cutaneous silent period and cutaneomuscular reflexes (CMR) of the abductor pollicis brevis, cold-detection threshold (CDT) and heat-pain detection threshold (HPT) at the index, little finger, and dorsum of the hand, and vibratory detection threshold at the index and little finger by quantitative sensory testing.

RESULTS

CTS with DOH had higher BCTQ, DN4 and NRS, lower median sensory action potential, longer CMR duration, lower CDT and higher HPT at all tested sites than controls and CTS without DOH. Predictive features for DOH were abnormal CDT and HPT at the right index and dorsum (OR: 3.88, p: 0.03) or at the little finger (OR: 3.27, p: 0.04) and a DN4 higher than 4 (OR: 2.16, p < 0.0001).

CONCLUSIONS

Thermal hypoesthesia in median and extra-median innervated territories and neuropathic pain are predictive of DOH in CTS.

Effect of the Triceps Brachii Facilitation Technique on Scapulohumeral Muscle Activation during Reach and Point in a Healthy Population

Purpose: Neurodevelopmental techniques are commonly used in upper limb rehabilitation, but little evidence supports the facilitation techniques associated with this concept. This exploratory study determined whether a facilitation technique at the triceps muscle affected scapulothoracic muscle activity during reach in healthy participants compared with self-selected posture and reach. The secondary aim was to determine whether muscle activation levels differed between the facilitation technique and the optimized posture or guided movement. We also hypothesized that activity in the scapular stabilizers (lower trapezius [LT] and serratus anterior [SA]) would be increased during the facilitated movement than in the other conditions. Methods: The study included 17 healthy participants (aged 20-70 y). Surface electromyography recorded muscle activity in the upper trapezius (UT), middle trapezius (MT), and LT muscles and in the SA, middle deltoid (MD), and triceps during five performance conditions. We used Friedman's test to explore differences in muscle activity across conditions and Bonferroni's post hoc test to explore the differences between conditions. Results: The facilitation technique produced decreased activity in the SA, MD, and triceps muscles (p < 0.01) compared with the self-executed control condition. Compared with optimized posture with independent reach, facilitated movement again produced similar reductions in MD and triceps activity, with decreased LT activity also noted (p < 0.01). Lower activity levels were noted during facilitation than during manual guidance, with or without optimized posture, in the UT, MT, (p < 0.01), SA, and MD muscles (p < 0.05). Conclusions: Triceps facilitation did not increase scapular stability activity, but the activity levels in several other muscle groups (SA, MD, and triceps) were reduced during triceps facilitation compared with optimized posture or guided movement. Detailed analysis of this technique, including co-registered kinematic data and timing of muscle onset, is needed.

Retraining Reflexes: Clinical Translation of Spinal Reflex Operant Conditioning

Neurological disorders, such as spinal cord injury, stroke, traumatic brain injury, cerebral palsy, and multiple sclerosis cause motor impairments that are a huge burden at the individual, family, and societal levels. Spinal reflex abnormalities contribute to these impairments. Spinal reflex measurements play important roles in characterizing and monitoring neurological disorders and their associated motor impairments, such as spasticity, which affects nearly half of those with neurological disorders. Spinal reflexes can also serve as therapeutic targets themselves. Operant conditioning protocols can target beneficial plasticity to key reflex pathways; they can thereby trigger wider plasticity that improves impaired motor skills, such as locomotion. These protocols may complement standard therapies such as locomotor training and enhance functional recovery. This paper reviews the value of spinal reflexes and the therapeutic promise of spinal reflex operant conditioning protocols; it also considers the complex process of translating this promise into clinical reality.

Corticospinal control from M1 and PMv areas on inhibitory cervical propriospinal neurons in humans

Inhibitory propriospinal neurons with diffuse projections onto upper limb motoneurons have been revealed in humans using peripheral nerve stimulation. This system is supposed to mediate descending inhibition to motoneurons, to prevent unwilling muscle activity. However, the corticospinal control onto inhibitory propriospinal neurons has never been investigated so far in humans. We addressed the question whether inhibitory cervical propriospinal neurons receive corticospinal inputs from primary motor (M1) and ventral premotor areas (PMv) using spatial facilitation method. We have stimulated M1 or PMv using transcranial magnetic stimulation (TMS) and/or median nerve whose afferents are known to activate inhibitory propriospinal neurons. Potential input convergence was evaluated by studying the change in monosynaptic reflexes produced in wrist extensor electromyogram (EMG) after isolated and combined stimuli in 17 healthy subjects. Then, to determine whether PMv controlled propriospinal neurons directly or through PMv-M1 interaction, we tested the connectivity between PMv and propriospinal neurons after a functional disruption of M1 produced by paired continuous theta burst stimulation (cTBS). TMS over M1 or PMv produced reflex inhibition significantly stronger on combined stimulations, compared to the algebraic sum of effects induced by isolated stimuli. The extra-inhibition induced by PMv stimulation remained even after cTBS which depressed M1 excitability. The extra-inhibition suggests the existence of input convergence between peripheral afferents and corticospinal inputs onto inhibitory propriospinal neurons. Our results support the existence of direct descending influence from M1 and PMv onto inhibitory propriospinal neurons in humans, possibly though direct corticospinal or via reticulospinal inputs.

Interlimb Reflexes Induced by Electrical Stimulation of Cutaneous Nerves after Spinal Cord Injury

Whether interlimb reflexes emerge only after a severe insult to the human spinal cord is controversial. Here the aim was to examine interlimb reflexes at rest in participants with chronic (>1 year) spinal cord injury (SCI, n = 17) and able-bodied control participants (n = 5). Cutaneous reflexes were evoked by delivering up to 30 trains of stimuli to either the superficial peroneal nerve on the dorsum of the foot or the radial nerve at the wrist (5 pulses, 300 Hz, approximately every 30 s). Participants were instructed to relax the test muscles prior to the delivery of the stimuli. Electromyographic activity was recorded bilaterally in proximal and distal arm and leg muscles. Superficial peroneal nerve stimulation evoked interlimb reflexes in ipsilateral and contralateral arm and contralateral leg muscles of SCI and control participants. Radial nerve stimulation evoked interlimb reflexes in the ipsilateral leg and contralateral arm muscles of control and SCI participants but only contralateral leg muscles of control participants. Interlimb reflexes evoked by superficial peroneal nerve stimulation were longer in latency and duration, and larger in magnitude in SCI participants. Interlimb reflex properties were similar for both SCI and control groups for radial nerve stimulation. Ascending interlimb reflexes tended to occur with a higher incidence in participants with SCI, while descending interlimb reflexes occurred with a higher incidence in able-bodied participants. However, the overall incidence of interlimb reflexes in SCI and neurologically intact participants was similar which suggests that the neural circuitry underlying these reflexes does not necessarily develop after central nervous system injury.

Non-invasive cerebellar stimulation in dystonia

Primary isolated dystonia is a hyperkinetic movement disorder whereby involuntary muscle contractions cause twisted and abnormal postures. Dystonia of the cervical spine and upper limb may present as sustained muscle contractions or task-specific activity when using the hand or upper limb. There is little understanding of the pathophysiology underlying dystonia and this presents a challenge for clinicians and researchers alike. Emerging evidence that the cerebellum is involved in the pathophysiology of dystonia using network models presents the intriguing concept that the cerebellum could provide a novel target for non-invasive brain stimulation. Non-invasive stimulation to increase cerebellar excitability improved aspects of handwriting and circle drawing in a small cohort of people with focal hand and cervical dystonia. Mechanisms underlying the improvement in function are unknown, but putative pathways may involve the red nucleus and/or the cervical propriospinal system. Furthermore, recent understanding that the cerebellum has both motor and cognitive functions suggests that non-invasive cerebellar stimulation may improve both motor and non-motor aspects of dystonia. We propose a combination of motor and non-motor tasks that challenge cerebellar function may be combined with cerebellar non-invasive brain stimulation in the treatment of focal dystonia. Better understanding of how the cerebellum contributes to dystonia may be gained by using network models such as our putative circuits involving red nucleus and/or the cervical propriospinal system. Finally, novel treatment interventions encompassing both motor and non-motor functions of the cerebellum may prove effective for neurological disorders that exhibit cerebellar dysfunction.

Task-Dependent Modulation of Propriospinal Inputs to Human Shoulder

In the human upper limb a proportion of the descending corticospinal command may be relayed through cervical propriospinal premotoneurons. This may serve to coordinate movements involving multiple joints of the arm, such as reaching. The present study was conducted to determine whether a shoulder stabilizing muscle, infraspinatus (INF), is functionally integrated into the putative cervical propriospinal network, and whether there is task-dependent modulation of the network. Fourteen healthy adults participated in this study. Responses in the right INF were evoked by transcranial magnetic stimulation over the motor cortex and compared with responses conditioned by ulnar nerve stimulation. Interstimulus intervals were chosen to summate inputs at the level of the premotoneurons. Participants performed a forearm and shoulder muscle cocontraction task or a grip-lift task that also coactivated forearm and shoulder muscles. During the cocontraction task, INF motor-evoked potentials were significantly facilitated by ulnar nerve stimulation at low intensities and suppressed at higher intensities. Only facilitation reached significance during the grip-lift task. We have shown for the first time that propriospinal pathways may connect the hand to the rotator cuff of the shoulder. The modulation of facilitation/suppression during the grip-lift task suggests that inhibition of propriospinal premotoneurons is down-regulated in a task-dependent manner to increase the gain in the feedback reflex loop from forearm and hand muscles as required.