Voluntary suppression of neck reflexes during passive head-on-trunk rotations - reflex gain control vs. proprioceptive feedback

Normal subjects can completely eliminate resistance upon imposed head-on-trunk rotations when they are asked to relax. It is not, however, clear how neck reflexes to stretch can be voluntarily suppressed. Reflexive responses might be modified by adjusting the gain of the reflex loop through descending control. Theoretically, necessary corrections upon interfering disturbances during coordinated motor performace requiring the interplay of relaxation/activation may be missing if muscle relaxation is taking place exclusively by this mechanism. It has been alternatively proposed, that sensory information from the periphery may be allowed to "neutralize" neck reflexes if it is fed back with opposite sign to the structures driving the reflexes. Six healthy subjects were asked to relax while subjected to head-on-trunk rotations generated by a head motor. After any initial resistance had completely subsided, the head was unexpectedly exposed to "ramp-and-hold" perturbations of up to 2○ amplitude and 0.7 s duration. Resistance to stretch consistently reappeared thereupon suggesting that stretch reflex gain had not been set to zero during the previously achieved complete relaxation. Resistance to perturbations under these circumstaces was compared to the forces generated when the same "ramp-and-hold" displacements were delivered unpredictably to the head held stationary. A quantitative model of neck proprioceptive reflexes suppression has been thus constructed. Gain scheduling or "motor set" cannot sufficiently account for the voluntary reflex suppression during slow passive head rotations. Instead, we propose as underlying mechanism the "neutralization" of the controlling servo by means of continuous feedback tracking displacement and force signals from the periphery.

Recording of Proprioceptive Muscle Reflexes in the Lower Extremity

Electromyography (EMG) is routinely used in diagnostics of root syndromes in the lower extremity. By studying signs of axonal damage of different root levels in the corresponding myotomes of the lower extremity and back muscles with needle EMG reveals, which of the motor roots are injured in patients with suspected root compression. But by EMG study only injuries of the anterior motor roots are diagnosed. Routine electroneuromyography does not disclose specific injury of the afferent sensory posterior roots. However, the integrity of some the posterior roots is readily studied with myotatic reflexes. We have routinely measured a proprioceptive reflex, the H-reflex of the soleus muscle with stimulation of the posterior tibial nerve, and found it to be useful in the diagnostics of the S1 root syndrome. It seems to be possible to record H-reflex of the peroneus longus muscle at the L5 level. We discuss the serious problems with volume conduction, when trials to measure proprioceptive reflexes of the L4 and L5 levels are performed. It may also be useful to record the medium latency reflexes in the area of the posterior tibial nerve, which seems to have a different reflex arch (II-afferents – β-efferents) from H-reflex (Ia afferents – α efferents). These measurements are non-invasive and not time consuming, and we hope to be able to add them for the routine ENMG diagnostics, when appropriate.

Canine cauda equina neuritis secondary to neospora

A five-year-old, female spayed, crossbreed dog was presented for lethargy, fever, hyporexia and right hindlimb lameness of two weeks’ duration. Examination revealed an intermittent weightbearing right hindlimb lameness with delayed proprioceptive reflexes. When the right hindlimb was placed, weightbearing was through the dorsal pes. The other limbs were neurologically normal. The neurological deficits localised to L6–S2. CT and myelogram revealed focal nerve root enlargement with contrast enhancement of L7–S1 nerve roots. Neospora caninum neuritis was diagnosed by rising serum titres, along with a neutrophilic pleocytosis on cerebrospinal fluid. The dog showed clinical improvement with prednisone, clindamycin and trimethoprim/sulfadiazine but relapsed six months after cessation of treatment with similar presentation. The same treatment was initiated, and a positive response was seen again. There is only one other case of focal cauda equina neuritis secondary to Neospora reported in the literature and the response to treatment is not well documented.

Design of a Magnetic Resonance-Safe Haptic Wrist Manipulator for Movement Disorder Diagnostics

Tremor, characterized by involuntary and rhythmical movements, is the most common movement disorder. Tremor can have peripheral and central oscillatory components which properly assessed may improve diagnostics. A magnetic resonance (MR)-safe haptic wrist manipulator enables simultaneous measurement of proprioceptive reflexes (peripheral components) and brain activations (central components) through functional magnetic resonance imaging (fMRI). The presented design for an MR-safe haptic wrist manipulator has electrohydraulic closed-circuit actuation, optical position and force sensing, and consists of exclusively nonconductive and magnetically compatible materials inside the MR-environment (Zone IV). The MR-safe hydraulic actuator, a custom-made plastic vane motor, is connected to the magnetic parts and electronics located in the shielded control room (Zone III) via hydraulic hoses and optical fibers. Deliberate internal leakage provides backdriveability, damping, and circumvents friction. The manipulator is completely MR-safe and therefore operates safely in any MR-environment while ensuring fMRI imaging quality. Undesired external leakage in the actuator prevented the use of prepressure, limiting the control bandwidth. The compact end effector design fits in the MR-scanner, is easily setup, and can be clamped to the MR-scanner bed. This enables use of the manipulator with the subject at the optimal fMRI location and allows it to be setup quickly, saving costly MR-scanner time. The actuation and sensor solutions performed well inside the MR-environment and did not deteriorate image quality, which allows for various motor control experiments. Enabling prepressure by carrying out the recommendations on fabrication and sealing should improve the bandwidth and fulfill the requirements for proprioceptive reflex identification.