Retrograde determination of motoneuron properties and their synaptic input.

Both motoneurons and the muscle fibers that they innervate can vary widely in their properties. The finding that these properties are highly correlated indicates that these cells are not specified independently, but rather interact in some manner to achieve the observed coherence. The direction of the interaction, i.e., whether orthograde or retrograde, requires analysis beyond the simple observation of these correlated properties. Evidence, largely from experiments involving reinnervation of muscle by the original or a foreign motor nerve, suggests the action of retrogradely transported factors from the muscle as well as orthograde ones. Various possible factors, specifically neurotrophins, are advanced as possible candidates for retrograde specification. In addition, synaptic input to motoneurons also varies in a coordinated fashion in a manner that suggests a retrograde determination from the muscle. However, motoneuron properties and their synaptic input change after peripheral nerve manipulations such that they are no longer in register. This indicates some independence in retrograde specification of motoneurons and their synaptic input.