Psychomotor functions in developing rats: ontogenetic approach to structure-function relationships.

The functional development of the central nervous system (CNS) in the rat was studied from the 10th to the 45th postnatal day, through the ontogeny of psychomotor and sensory functions, by a battery of behavioral tests. The ontogenetic development of 10 different functions was described. The results showed that novelty-induced functions matured progressively in an adult-like pattern of functioning in the 3rd postnatal week. Indeed, exploratory activity was low at the 10th day, increased significantly to reach highest values from 20th to 30th postnatal day, then declined at the 45th postnatal day. No habituation was exhibited by 10- and 15-day-old rats; it appeared at the age of 20 days. Emotional reactivity induced by the novelty of surroundings clearly appeared from the 20th postnatal day, when the features of adult animal were reached. It appeared also that reflex and automatic motor functions came to maturity by the age of 3 wk, while voluntary motor functions continued to improve until the 30th day. Thus, the latency of the hind paw lifting reflex occurrence significantly decreased from the 10th to the 20th postnatal day, when the most improved values were reached. The wire-grasping times increased from the 10th to the 25th postnatal day in an exponential fashion. Locomotor activity developed significantly from the 10th to the 15th day, when the mature locomotion pattern was exhibited. The coordination of complex movements and motor initiative appeared only after the 20th postnatal day. The latencies of execution of crawling along the wire and of leap onto the ground decreased significantly from the 20th to the 45th day. These studies reveal the presence of the caudal to rostral sequence of CNS development, predicting a spatio-temporal functional maturation of nuclei and centers in the rat's CNS. The building of the time-sequence of regional maturation of the brain integrated activities was attempted.