Perinatal development of lumbar motoneurons and their inputs in the rat

The rat is quite immature at birth and a rapid maturation of motor behavior takes place during the first 2 postnatal weeks. Lumbar motoneurons undergo a rapid development during this period. The last week before birth represe nts the initial stages of motoneuron differentiation, including regulation of the number of cells and the arrival of segmental and first supraspinal a fferents. At birth, motoneurons are electrically coupled and receive both a ppropriate and inappropriate connections from the periphery; the control fr om supraspinal structures is weak and exerted mainly through polysynaptic c onnections. During the Ist postnatal week, inappropriate sensori-motor cont acts and electrical coupling disappear, the supraspinal control increases g radually and myelin formation is responsible for an increased conduction ve locity in both descending and motor axons. Both N-methyl-D-aspartate (NMDA) and non-NMDA receptors are transiently overexpressed in the neonatal spina l cord. The contribution of non-NMDA receptors to excitatory amino acid tra nsmission increases with age. Activation of gamma -aminobutyric acid(A) and glycine receptors leads to membrane depolarization in embryonic motoneuron s but to hyperpolarization in older motoneurons. The firing properties of m otoneurons change with development: they are capable of more repetitive fir ing at the end of the Ist postnatal week than before birth, However, matura tion does not proceed simultaneously in the motor pools innervating antagon istic muscles; for instance, the development of repetitive firing of ankle extensor motoneurons lags behind that of flexor motoneurons, The spontaneou s embryonic and neonatal network-driven activity, detected at the levels of motoneurons and primary afferent terminals, may play a role in neuronal ma turation and in the formation and refinement of sensorimotor connections. ( C) 2001 Elsevier Science Inc.