Modulation of Transmission in the Corticospinal and Group Ia Afferent Pathways to Soleus Motoneurons During Bicycling

Abstract

Pyndt, H. S. and J. B. Nielsen. Modulation of transmission in the corticospinal and group Ia afferent pathways to soleus motoneurons during bicycling. J Neurophysiol 89: 304 -314, 2003. 10.1152/jn.00386.2002. Transmission in the corticospinal and Ia pathways to soleus motoneurons was investigated in healthy human subjects during bicycling. Soleus H reflexes and motor evoked potentials (MEPs) after transcranial magnetic stimulation (TMS) were modulated similarly during the crank cycle being large during downstroke [concomitant with soleus background electromyographic (EMG) activity] and small during upstroke. Tibialis anterior MEPs were in contrast large during upstroke and small during downstroke. The soleus H reflexes and MEPs were also recorded during tonic plantarflexion at a similar ankle joint position, corresponding ankle angle, and matched background EMG activity as during the different phases of bicycling. Relative to their size during tonic plantarflexion, the MEPs were found to be facilitated in the early part of downstroke during bicycling, whereas the H reflexes were depressed in the late part of downstroke. The intensity of TMS was decreased below MEP threshold and used to condition the soleus H reflex. At short intervals (conditioning-test intervals of Ϫ3 to Ϫ1 ms), TMS produced a facilitation of the H reflex that is in all likelihood caused by activation of the fast monosynaptic corticospinal pathway. This facilitation was significantly larger in the early part of downstroke during bicycling than during tonic plantarflexion. This suggests that the increased MEP during downstroke was caused by changes in transmission in the fast monosynaptic corticospinal pathway. To investigate whether the depression of H reflexes in the late part of downstroke was caused by increased presynaptic inhibition of Ia afferents, the soleus H reflex was conditioned by stimulation of the femoral nerve. At a short interval (conditioning-test interval: Ϫ7 to Ϫ5 ms), the femoral nerve stimulation produced a facilitation of the H reflex that is mediated by the heteronymous monosynaptic Ia pathway from the femoral nerve to soleus motoneurons. Within the initial 0.5 ms after its onset, the size of this facilitation depends on the level of presynaptic inhibition of the Ia afferents, which mediate the facilitation. The size of the facilitation was strongly depressed in the late part of downstroke, compared with the early part of downstroke, suggesting that increased presynaptic inhibition was indeed responsible for the depression of the H reflex. These findings suggest that there is a selectively increased transmission in the fast monosynaptic corticospinal pathway to soleus motoneurons in early downstroke during bicycling. It would seem likely that one cause of this is increased excitability of the involved cortical neurons. The increased presynaptic inhibition of Ia afferents in late downstroke may be of importance for depression of stretch reflex activity before and during upstroke.

Corticospinal Excitability Is Lower During Rhythmic Arm Movement Than During Tonic Contraction

“…Our data corroborate findings from leg (Brooke et al 1997;McIlroy et al 1992;Pyndt and Nielsen 2003) and arm ) cycling studies and strengthen the case that the gain of H-reflex (i.e., Group Ia) circuits are reduced during rhythmic locomotor-like movements of both the upper and lower limbs, especially during limb flexion. The most dramatic depression of H-reflexes coincided with the reduction in TMS responses, which indicates that the CNS suppresses excitatory drive from peripheral and descending sources to the wrist flexors during the flexion phase of arm cycling.…”

“…Asterisks denote significant differences between cycling tasks and tonic contractions. rhythmic movement (Pyndt and Nielsen 2003;Zehr et al 2003), whereas corticospinal-motoneuronal synapses are not subject to classical presynaptic inhibition (Nielsen and Petersen 1994).…”

“…The importance of spinal circuitry for the control of rhythmic, human leg movements is well documented (for review see Brooke et al 1997;Zehr and Stein 1999), although there is also evidence from transcranial magnetic stimulation (TMS) studies that the corticospinal pathway plays at least some direct role in the production of locomotor drive in humans during walking and leg cycling (Capaday et al 1999;Petersen et al 1998Petersen et al , 2001Pyndt and Nielsen 2003;Schubert et al 1997Schubert et al , 1999). Here we assessed the excitability of the motor cortex during the execution of rhythmic, locomotor-like movements of the human upper limbs.…”

Increase of Muscle Activities in Hemiplegic Lower Extremity During Driving a Cycling Wheelchair

“…There are some studies on neurophysiological changes during pedaling movement in healthy subjects (Collins et al 1993;Pyndt and Nielsen 2003). These studies indicate presynaptic inhibition reduces H reflex during pedaling via activation of an inhibiting interneuron in the spinal cord.…”

“…These studies indicate presynaptic inhibition reduces H reflex during pedaling via activation of an inhibiting interneuron in the spinal cord. Transcranial magnetic stimulation during pedaling, on the other hand, facilitates motor evoked potential (MEP) according to the rotation cycle (Pyndt and Nielsen 2003). It suggests that transmission in the fast monosynaptic corticospinal pathway selectively increases during pedaling.…”