Spinal Circuits Mediate a Stretch Reflex Between the Upper Limbs in Humans

Essential Tremor accentuates the pattern of tremor-band coherence between upper-limb muscles

Although Essential Tremor is one of the most common movement disorders, current treatment options are relatively limited. Peripheral tremor suppression methods have shown potential, but we do not currently know which muscles are most responsible for patients' tremor, making it difficult to optimize suppression methods. The purpose of this study was to quantify the relationships between the tremorogenic activity in muscles throughout the upper limb. Muscle activity was recorded from the 15 major superficial upper-limb muscles in 24 subjects with Essential Tremor while they held various postures or made upper-limb movements. We calculated the coherence in the tremor band (4-12 Hz) between the activity of all muscle pairs and the time-varying phase difference between sufficiently coherent muscle pairs. Overall, the observed pattern somewhat mirrored functional relationships: agonistic muscle pairs were most coherent and in phase, whereas antagonist and unrelated muscle pairs exhibited less coherence and were either consistently in phase, consistently antiphase, consistently out of phase (unrelated pairs only), or else inconsistent. Patients exhibited significantly more coherence than control subjects (p<0.001) in the vast majority of muscle pairs (95 out of 105). Furthermore, differences between patients and controls were most pronounced among agonists; thus, the coherence pattern existing in control subjects was accentuated in patients with ET. We conclude that tremor-band activity is broadly distributed among the muscles of the upper limb, challenging efforts to determine which muscles are most responsible for a patient's tremor.

A Cadaver Feasibility Study of Extradural Contralateral C7 Ventral Root Transfer Technique for Treating Upper Extremity Paralysis

STUDY DESIGN

A total of 6 formalin-fixed cadavers were included in the cadaver feasibility study.

OBJECTIVE

The aim was to ascertain the anatomical feasibility of extradural contralateral C7 ventral root transfer technique by cervical posterior.

SUMMARY OF BACKGROUND DATA

Upper limb spastic hemiplegia is a common sequela after stroke. In our previous study, the authors established a method by transferring contralateral C7 dorsal and ventral roots to the corresponding C7 dorsal and ventral roots on the affected side in the cervical posterior.

METHODS

In the present study, six formalin-fixed cadavers were dissected to confirm the anatomical feasibility. Experimental anastomosis in cadavers was conducted. The pertinent lengths of the extradural nerve roots were measured. The tissue structures surrounding regions between the extradural CC7 nerve roots and the vertebral artery were observed. The cervical magnetic resonance imaging scans of 60 adults were used to measure the distance between the donor and recipient nerves.

RESULTS

Experimental anastomosis showed that the distance between the donor and recipient nerves was approximately 1 cm; the short segment of the sural nerve needed bridging. The distance between both exit sites of the exit of the extradural dura mater was 33.57±1.55 mm. The length of the extradural CC7 ventral root was 22.00±0.98 mm. The ventral distance (vd) and the dorsal distance (dd) in males were 23.98±1.72 mm and 30.85±2.22 mm ( P <0.05), while those in females were 23.28±1.51 mm and 30.03±2.16 mm, respectively. C7 vertebral transverse process, ligaments, and other soft tissues were observed between the vertebral artery and the extradural C7 nerve root.

CONCLUSION

Under the premise of less trauma, our study shows that the extradural contralateral C7 ventral root transfer technique, in theory, yields better surgical results, including better recovery of motor function and complete preservation of sensory function.

LEVEL OF EVIDENCE

5.

Translations of the Humeral Head Elicit Reflexes in Rotator Cuff Muscles That Are Larger Than Those in the Primary Shoulder Movers

Muscle activation helps stabilize the glenohumeral joint and prevent dislocations, which are more common at the shoulder than at any other human joint. Feedforward control of shoulder muscles is important for protecting the glenohumeral joint from harm caused by anticipated external perturbations. However, dislocations are frequently caused by unexpected perturbations for which feedback control is essential. Stretch-evoked reflexes elicited by translations of the glenohumeral joint may therefore be an important mechanism for maintaining joint integrity, yet little is known about them. Specifically, reflexes elicited by glenohumeral translations have only been studied under passive conditions, and there have been no investigations of how responses are coordinated across the functional groupings of muscles found at the shoulder. Our objective was to characterize stretch-evoked reflexes elicited by translations of the glenohumeral joint while shoulder muscles are active. We aimed to determine how these responses differ between the rotator cuff muscles, which are essential for maintaining glenohumeral stability, and the primary shoulder movers, which are essential for the large mobility of this joint. We evoked reflexes using anterior and posterior translations of the humeral head while participants produced voluntary isometric torque in six directions spanning the three rotational degrees-of-freedom about the shoulder. Electromyograms were used to measure the stretch-evoked reflexes elicited in nine shoulder muscles. We found that reflex amplitudes were larger in the rotator cuff muscles than in the primary shoulder movers, in part due to increased background activation during torque generation but more so due to an increased scaling of reflex responses with background activation. The reflexes we observed likely arose from the diversity of proprioceptors within the muscles and in the passive structures surrounding the shoulder. The large reflexes observed in the rotator cuff muscles suggest that feedback control of the rotator cuff augments the feedforward control that serves to compress the humeral head into the glenoid. This coordination may serve to stabilize the shoulder rapidly when preparing for and responding to unexpected disturbances.

Voluntary modification of rapid tactile-motor responses during reaching differs from its visuomotor counterpart

People commonly hold and manipulate a variety of objects in everyday life, and these objects have different physical properties. To successfully control this wide range of objects, people must associate new patterns of tactile stimuli with appropriate motor outputs. We performed a series of experiments investigating the extent to which people can voluntarily modify tactile-motor associations in the context of a rapid tactile-motor response guiding the hand to a moving target (previously described in Pruszynski JA, Johansson RS, Flanagan JR. Curr Biol 26: 788-792, 2016) by using an anti-reach paradigm in which participants were instructed to move their hands in the opposite direction of a target jump. We compared performance to that observed when people make visually guided reaches to a moving target (cf. Day BL, Lyon IN. Exp Brain Res 130: 159-168, 2000; Pisella L, Grea H, Tilikete C, Vighetto A, Desmurget M, Rode G, Boisson D, Rossetti Y. Nat Neurosci 3: 729-736, 2000). When participants had visual feedback, motor responses during the anti-reach task showed early automatic responses toward the moving target before later modification to move in the instructed direction. When the same participants had only tactile feedback, however, they were able to suppress this early phase of the motor response, which occurs <100 ms after the target jump. Our results indicate that while the tactile motor and visual motor systems both support rapid responses that appear similar under some conditions, the circuits underlying responses show sharp distinctions in terms of their malleability.NEW & NOTEWORTHY When people reach toward a visual target that moves suddenly, they automatically correct their reach to follow the object; even when explicitly instructed not to follow a moving visual target, people exhibit an initial incorrect movement before moving in the correct direction. We show that when people use tactile feedback, they do not show an initial incorrect response, even though early muscle activity still occurs.