Regionally distinct cutaneous afferent populations contribute to reflex modulation evoked by stimulation of the tibial nerve during walking

During walking, cutaneous reflexes in ankle flexor muscle [tibialis anterior (TA)] evoked by tibial nerve (TIB) stimulation are predominantly facilitatory at early swing phase but reverse to suppression at late swing phase. Although the TIB innervates a large portion of the skin of the foot sole, the extent to which specific foot-sole regions contribute to the reflex reversals during walking remains unclear. Therefore, we investigated regional cutaneous contributions from discrete portions of the foot sole on reflex reversal in TA following TIB stimulation during walking. Summation effects on reflex amplitudes, when applying combined stimulation from foot-sole regions with TIB, were examined. Middle latency responses (MLRs; 70–120 ms) after TIB stimulation were strongly facilitated during the late stance to mid-swing phases and reversed to suppression just before heel (HL) strike. Both forefoot-medial (f-M) and forefoot-lateral stimulation in the foot sole induced facilitation during stance-to-swing transition phases, but HL stimulation evoked suppression during the late stance to the end of swing phases. At the stance-to-swing transition, a summation of MLR amplitude occurred only for combined f-M&TIB stimulation. However, the same was not true for the combined HL&TIB stimulation. At the swing-to-stance transition, there was a suppressive reflex summation only for HL&TIB stimulation. In contrast, this summation was not observed for the f-M&TIB stimulation. Our results suggest that reflex reversals evoked by TIB stimulation arise from distinct reflex pathways to TA produced by separate afferent populations innervating specific regions of the foot sole.

The effect of sensory feedback on crayfish posture and locomotion: II. Neuromechanical simulation of closing the loop

Neuromechanical simulation was used to determine whether proposed thoracic circuit mechanisms for the control of leg elevation and depression in crayfish could account for the responses of an experimental hybrid neuromechanical preparation when the proprioceptive feedback loop was open and closed. The hybrid neuromechanical preparation consisted of a computational model of the fifth crayfish leg driven in real time by the experimentally recorded activity of the levator and depressor (Lev/Dep) nerves of an in vitro preparation of the crayfish thoracic nerve cord. Up and down movements of the model leg evoked by motor nerve activity released and stretched the model coxobasal chordotonal organ (CBCO); variations in the CBCO length were used to drive identical variations in the length of the live CBCO in the in vitro preparation. CBCO afferent responses provided proprioceptive feedback to affect the thoracic motor output. Experiments performed with this hybrid neuromechanical preparation were simulated with a neuromechanical model in which a computational circuit model represented the relevant thoracic circuitry. Model simulations were able to reproduce the hybrid neuromechanical experimental results to show that proposed circuit mechanisms with sensory feedback could account for resistance reflexes displayed in the quiescent state and for reflex reversal and spontaneous Lev/Dep bursting seen in the active state.

Crossed reflex reversal during human locomotion

During human walking, precise coordination between the two legs is required in order to react promptly to any sudden hazard that could threaten stability. The networks involved in this coordination are not yet completely known, but a direct spinal connection between soleus (SOL) muscles has recently been revealed. For this response to be functional, as previously suggested, we hypothesize that it will be accompanied by a reaction in synergistic muscles, such as gastrocnemius lateralis (GL), and that a reversal of the response would occur when an opposite reaction is required. In the present study, surface EMGs of contralateral SOL and GL were analyzed after tibial nerve (TN), sural nerve (SuN), and medial plantar nerve (MpN) stimulation during two tasks in which opposite reactions are functionally expected: normal walking (NW), just before ipsilateral heel strike, and hybrid walking (HW) (legs walking in opposite directions), at ipsilateral push off and contralateral touchdown. Early crossed facilitations were observed in the contralateral GL after TN stimulation during NW, and a reversal of such responses occurred during HW. These results underline the functional significance of short-latency crossed responses and represent the first evidence for short-latency reflex reversal in the contralateral limb for humans. Muscle afferents seem to mediate the response during NW, while during HW cutaneous afferents are likely involved. It is thus possible that different afferents mediate the crossed response during different tasks.

Control of reflex reversal in stick insect walking: effects of intersegmental signals, changes in direction, and optomotor-induced turning

In many animals, the effects of sensory feedback on motor output change during locomotion. These changes can occur as reflex reversals in which sense organs that activate muscles to counter perturbations in posture control instead reinforce movements in walking. The mechanisms underlying these changes are only partially understood. As such, it is unclear whether reflex reversals are modulated when locomotion is adapted, such as during changes in walking direction or in turning movements. We investigated these questions in the stick insect Carausius morosus, where sensory signals from the femoral chordotonal organ are known to produce resistance reflexes at rest but assistive movements during walking. We studied how intersegmental signals from neighboring legs affect the generation of reflex reversals in a semi-intact preparation that allows free leg movement during walking. We found that reflex reversal was enhanced by stepping activity of the ipsilateral neighboring rostral leg, whereas stepping of contralateral legs had no effect. Furthermore, we found that the occurrence of reflex reversals was task-specific: in the front legs of animals with five legs walking, reflex reversal was generated only during forward and not backward walking. Similarly, during optomotor-induced curved walking, reflex reversal occurred only in the middle leg on the inside of the turn and not in the contralateral leg on the outside of the turn. Thus our results show for the first time that the nervous system modulates reflexes in individual legs in the adaptation of walking to specific tasks.

Gait acts as a gate for reflexes from the foot

During human gait, electrical stimulation of the foot elicits facilitatory P2 (medium latency) responses in TA (tibialis anterior) at the onset of the swing phase, while the same stimuli cause suppressive responses at the end of swing phase, along with facilitatory responses in antagonists. This phenomenon is called phase-dependent reflex reversal. The suppressive responses can be evoked from a variety of skin sites in the leg and from stimulation of some muscles such as rectus femoris (RF). This paper reviews the data on reflex reversal and adds new data on this topic, using a split-belt paradigm. So far, the reflex reversal in TA could only be studied for the onset and end phases of the step cycle, simply because suppression can only be demonstrated when there is background activity. Normally there are only 2 TA bursts in the step cycle, whereas TA is normally silent during most of the stance phase. To know what happens in the stance phase, one needs to have a means to evoke some background activity during the stance phase. For this purpose, new experiments were carried out in which subjects were asked to walk on a treadmill with a split-belt. When the subject was walking with unequal leg speeds, the walking pattern was adapted to a gait pattern resembling limping. The TA then remained active throughout most of the stance phase of the slow-moving leg, which was used as the primary support. This activity was a result of coactivation of agonistic and antagonistic leg muscles in the supporting leg, and represented one of the ways to stabilize the body. Electrical stimulation was given to a cutaneous nerve (sural) at the ankle at twice the perception threshold. Nine of the 12 subjects showed increased TA activity during stance phase while walking on split-belts, and 5 of them showed pronounced suppressions during the first part of stance when stimuli were given on the slow side. It was concluded that a TA suppressive pathway remains open throughout most of the stance phase in the majority of subjects. The suggestion was made that the TA suppression increases loading of the ankle plantar flexors during the loading phase of stance.Key words: human gait, cutaneous reflexes, sural nerve, tibialis anterior, split belt, reflex reversal.

Possible contributions of CPG activity to the control of rhythmic human arm movement

There is extensive modulation of cutaneous and H-reflexes during rhythmic leg movement in humans. Mechanisms controlling reflex modulation (e.g., phase- and task-dependent modulation, and reflex reversal) during leg movements have been ascribed to the activity of spinal central pattern generating (CPG) networks and peripheral feedback. Our working hypothesis has been that neural mechanisms (i.e., CPGs) controlling rhythmic movement are conserved between the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that for rhythmic leg movement. This hypothesis has been tested by studying the regulation of reflexes in arm muscles during rhythmic arm cycling and treadmill walking. This paper reviews recent studies that have revealed that reflexes in arm muscles show modulation within the movement cycle (e.g., phase-dependency and reflex reversal) and between static and rhythmic motor tasks (e.g., task-dependency). It is concluded that reflexes are modulated similarly during rhythmic movement of the upper and lower limbs, suggesting similar motor control mechanisms. One notable exception to this pattern is a failure of contralateral arm movement to modulate reflex amplitude, which contrasts directly with observations from the leg. Overall, the data support the hypothesis that CPG activity contributes to the neural control of rhythmic arm movement.Key words: central pattern generator, locomotion, motor control, neural control.