Alterations of synapses on axotomized motoneurones

1. Properties of medial gastrocnemius motoneurones in cats were examined with intracellular electrodes 1-77 days after partial transection of the medial gastrocnemius muscle nerve. 2. Three sequential stages of reaction were defined: (i) firstly, conduction velocity of axotomized motoneurones fell, but e.p.s.p. properties and Ia-motoneurone connectivity were normal; (ii) secondly, e.p.s.p.s became characterized by prolonged rise times and half-widths and by diminished amplitudes; (iii) finally, failures of connectivity were also seen, that is, Ia spindle afferent fibres connected with fewer axotomized than normal homonymous motoneurones. 3. The reason for the change of e.p.s.p. properties cannot be stated with certainty: among the factors may be (i) a change of motoneurone membrane properties and morphology; (ii) partial detachment of synaptic boutons before their total disconnexion, and (iii) detachment of Ia afferent branches terminating on the motoneurone soma before those on the dendrites. 4. The previous conclusion that alteration of e.p.s.p. profiles following motoneurone axotomy was due to total disconnexion of those Ia fibres making synapses on the soma and proximal dendrites ("somatic stripping") must be modified to account for the findings that e.p.s.p. changes are seen before connectivity changes.