Sequential output-input maturation of kitten motor cortex.

Average response histograms of the responses of single cortical neurons to imposed forelimb displacements in the kitten were utilized to quantify: (1) Background activity, (2) excitatory response latency, (3) percentage responding neurons, and (4) synaptic effectiveness. Motor cortical neuronal responses to the input did not attain adult values until 55--65 days of age, while adult-like responses were found in somatosensory cortical neurons as early as 9 days postnatally. Motor cortical output to alpha-motoneurons innervating triceps brachii, as tested by intracortical and subcortical microstimulation, first appeared over an interval ranging from 37--45 days postnatally. Thus, motor cortical responses to forelimb mechanoreceptors develop after completion of the output linkages from motor cortex to forelimb motoneurons and, though the motor cortex can output at 45 days, motor cortical functions requiring somatosensory feedback cannot be operative until about 60 days. This system provides a discrete and "late" maturing model in which to study the postnatal development of neuronal networks underlying normal and abnormal motor behavior.