Inhibition Of Soleus Research Articles

Spinal mechanisms in man contributing to reciprocal inhibition during voluntary dorsiflexion of the foot.

1. The inhibition of the soleus Hoffmann reflex (H reflex) during voluntary dorsiflexion of the foot--henceforth referred to as 'natural' reciprocal inhibition--was found to be initiated 50 ms before the onset of the EMG activity in the tibialis anterior muscle and to increase gradually during a ramp-and-hold dorsiflexion. There was a positive correlation between strength of tonic dorsiflexion and amount of 'natural' reciprocal inhibition. 2. The change of activity in the disynaptic and a long-latency group Ia inhibitory pathway and the change in presynaptic inhibition of the Ia fibres mediating the soleus H reflex were tested separately during ramp-and-hold dorsiflexion as well as during tonic dorsiflexion of the foot, and the results were compared with the development of the 'natural' reciprocal inhibition of the unconditioned soleus H reflex. 3. The disynaptic group I inhibition of soleus motoneurones was increased, as compared to rest, during the dynamic phase of a ramp-and-hold dorsiflexion movement, but the inhibition generally did not increase during tonic dorsiflexion of the foot. 4. The long-latency group I inhibition was seen only during dorsiflexion of the foot. It appeared around 50 ms before tibial anterior EMG activity and there was a positive correlation between strength of tonic dorsiflexion and amount of this long-latency inhibition. 5. Presynaptic inhibition of Ia afferents terminating on soleus motoneurones was estimated by an indirect method. The increase of presynaptic inhibition started soon after the onset of the ramp-and-hold dorsiflexion, and gradually became more pronounced during the ramp phase. The amount of presynaptic inhibition was positively correlated with strength of tonic dorsiflexion. 6. It is concluded that all investigated mechanisms may contribute to the 'natural' reciprocal inhibition and it seems that the different pathways are used differentially during different types of movement.

Increased inhibitory effects on close synergists during muscle fatigue in the decerebrate cat

We compared the magnitude of reflex inhibition induced in the soleus muscle by contraction or stretch of the medial gastroenemius (MG), before, during, and after electrically induced fatigue of the MG. Our findings are that MG fatigue is accompanied by a substantial increase in soleus inhibition, which then recovers with MG rest. This increased inhibition may explain, at least in part, the decline in motoneuron discharge rate that has been described in fatiguing human muscle.

Assessing changes in presynaptic inhibition of I a fibres: a study in man and the cat.

1. A method to assess changes in presynaptic inhibition of I a afferent terminals in man is proposed. The soleus H reflex was facilitated by a heteronymous I a volley from quadriceps and the amount of reflex facilitation was used to estimate the size of the conditioning I a excitatory post-synaptic potential (e.p.s.p.). It is argued that the size of this e.p.s.p. as measured by the resulting amount of reflex facilitation reflects the amount of presynaptic inhibition on the corresponding I a fibres. A decrease in the reflex facilitation may then be ascribed to an increase in presynaptic inhibition of the I a fibres mediating the conditioning volley. 2. That the heteronymous I a facilitation from quadriceps to soleus is caused by a purely monosynaptic e.p.s.p. is a prerequisite for the validity of the method. Experimental evidence is therefore given in an Appendix that in man the earliest part (first 0.5 ms) of this heteronymous I a facilitation is mediated through a monosynaptic pathway. Evidence is also given that this earliest facilitation is not yet contaminated by any polysynaptic effects from I a or I b afferents. 3. The validity of the method was established in animal experiments in which presynaptic inhibition of I a fibres and post-synaptic events in motoneurones could be assessed by direct tests. It was found that the amount of test reflex facilitation produced by a conditioning I a volley was decreased when I a fibres were subjected to presynaptic inhibition but remained unchanged when the motoneurone pool in which the test reflex was elicited received pure post-synaptic inhibition. 4. In man, presynaptic inhibition of I a fibres was evoked by a short-lasting (three shocks at 200 Hz) vibration applied to the tibialis anterior tendon. Such a vibratory burst reduced the efficiency of the heteronymous I a volley in facilitating the soleus H reflex. By contrast, during a pure post-synaptic inhibition of soleus motoneurones the efficiency of the conditioning volley in facilitating the test reflex remained unchanged. It is therefore argued that the amount of heteronymous I a facilitation can indeed be used to assess the amount of ongoing presynaptic inhibition exerted onto heteronymous I a fibres from the quadriceps muscle to soleus motoneurones. 5. The short-lasting tibialis anterior vibration used here evoked a long-lasting (300-500 ms) depression of soleus and quadriceps H reflexes.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for presynaptic inhibition of muscle spindle Ia afferents in man

A new method for estimating presynaptic inhibition of soleus (Sol) Ia fibres in man is introduced based on the assessment of the amount of facilitation produced by a preceding homonymous Ia volley onto a Sol monosynaptic reflex. It was found in animal experiments that a reduction of such Ia facilitation reflects the amount of presynaptic inhibition exerted on these Ia fibres. Since, in man, a similar reduction of homonymous Ia facilitation was found after tibialis anterior vibration, this provides evidence for presynaptic inhibition of Sol Ia fibres, elicited here by the afferent discharge due to this flexor vibration.