Factors that control the efficacy of group Ia synapses in alpha-motoneurons

1) This review considers factors that produce systematic variations in the amplitude of monosynaptic group Ia EPSPs in triceps surae α-motoneurons belonging to different motor unit types. 2) Anatomical studies using horseradish peroxidase to label functionally-identified group Ia afferents and motoneurons postsynaptic to them, and combined anatomical-electrophysiological studies of type-identified α-motoneurons, have constrained some of the factors that produce variations in peak Ia EPSP amplitude in different cells. 3) Computer modeling studies based on these experimental data, together with other evidence in the literature, suggest that the major factor that produces systematic variation in Ia EPSP amplitudes in type FF, FR, and S motoneurons is a corresponding variation in the density of active group Ia synapses. 4) Although EPSP amplitudes are also affected by the relative conductance of the somatic membrane, as reflected in the dendritic-to-somatic conductance ratio, it is possible that at least some of this influence is an artifact produced by microelectrode penetration.