CONVERGENCE ON INTERNEURONES IN THE RECIPROCAL Ia INHIBITORY PATHWAY TO MOTONEURONES
Authors: HULTBORN HANS 
Published: December 2008

1. The present studies have aimed at analyzing the excitatory and inhibitory convergence from various neuronal systems onto the inter‐neurones mediating the reciprocal inhibition of motoneurones from large muscle spindle (Ia) afferents. The conclusions regarding the convergence on the Ia inhibitory interneurones rest on studies of how transmission in the Ia inhibitory pathway is influenced from other neuronal systems; an excitatory convergence being indicated by facilitation and an inhibitory by depression of the Ia IPSPs. The depression of Ia IPSPs by volleys in motor axon collaterals has also been an important tool in the analysis.

2. It is known that Ia afferents from several muscles often can evoke inhibition in a given species of motoneurones. It has now been demonstrated that this convergence, at least partly, takes place already at the interneuronal level.

3. In spinal cats under chloralose anesthesia Ia IPSPs in flexor as well as extensor motoneurones were regularly facilitated by volleys in cutaneous afferents and high threshold muscle and joint afferents. In decerebrate cats with a low pontine lesion (in which volleys in these afferents evoke inhibition in both extensor and flexor motoneurones) Ia inhibitory transmission was not facilitated but depressed from high threshold muscle and joint afferents. Volleys in low threshold cutaneous afferents, on the other hand, facilitated Ia IPSPs also in this preparation. It is postulated that the Ia inhibitory interneurones receive excitatory actions from the ipsi‐lateral flexor reflex afferents (transmission depressed in the decerebrate state) and through a separate pathway from low threshold cutaneous afferents.

4. A bulk of evidence has been gathered demonstrating that excitatory as well as inhibitory actions from various segmental and descending pathways, which control Ia inhibitory transmission, in fact do converge onto the same Ia inhibitory interneurones. Thus the interneuronal pool mediating Ia inhibition to a given motor nucleus can so far be regarded as functionally homogeneous.

5. Whenever conditioning volleys in the ventral root depressed transmission of inhibition to motoneurones from segmental or descending pathways, volleys in these pathways were found to facilitate Ia inhibitory transmission. It was postulated that the recurrent depression of IPSPs in motoneurones indicaces to which extent these IPSPs are mediated by the Ia inhibitory interneurones.

6. The investigation of the relative contribution from different efferent nerves to recurrent depression of Ia inhibitory interneurones revealed that the strongest depression is always evoked from motor fibres to muscles whose Ia afferents produce the IPSPs. For example, the Ia IPSP from the knee extensor recorded in motoneurones to a knee flexor is most effectively depressed by antidromic stimulation of motor fibres to the knee extensor. The prevailing pattern of recurrent effects indicates that activity in one motor nucleus will cause recurrent inhibition of aL‐ and γ‐moto‐neurones within that motor nucleus and other nuclei innervating synergists and also of Ia inhibitory interneurones impinging on motoneurones to antagonists.

7. The convergence onto the Ia inhibitory interneurones was discussed in the light of the recently introduced concept of an “α‐γ‐linkage in the reciprocal inhibition”.