Activity-Dependent Plasticity of Spinal Circuits in the Developing and Mature Spinal Cord

Table 1

Different aspects of activity-dependent spinal plasticity in the developing and a mature spinal cord, discussed in this paper.

	Developing	Mature
Cat-301	Increase in response to movement and sensory input to SC during critical period. Large nerve crush inhibits the expression of the antibody.	Not substantial. After the critical period, nerve crush does not affect the expression of the antibody.
NMDA receptors	Have role in the induction of synaptic plasticity. Probably have role in the induction of morphological changes. Have role in dendritic growth and retraction.	Blocking the receptors does not affect motoneuron morphology. These receptors are being eliminated from almost all parts of the spinal cord except for substantia gelatinosa. Likely do not have substantial role in reflex transmission.
Elimination	Substantial elimination during maturation. Cortical connections to the ipsilateral side of the spinal cord will be eliminated during maturation. Dendrites grow and retract. This is a model of non-Hebbian activity dependent process. At the neuromuscular junction, many synaptic connections are lost which results in muscle fibers from polyneural innervation to mononeural innervation [23, 24].	Synaptic connections mostly follow Hebbian process. Activity-dependent plasticity does not seem to eliminate synapses.
Sensory input	Sensory input is essential for developing spinal cord. Sensory information generated my movement seems to have role in the development of spinal synapses and circuits.	Have role in both transitional as well as permanent changes in the spinal circuits. Pattern of sensory input has been shown to have role in the induction of plastic changes.
Presynaptic modulation	Likely presynaptic inhibition exists in infants and is being modulated in response to movement. However, the role of presynaptic inhibition in the acquisition of new skills in newborn infants and children has not been extensively studied. Recent studies on mouse models have shown that undernourishment substantially decreases the amount of presynaptic inhibition [25]	Has important role in the modulation of reflex gain during different movements, at the initiation of movement, and for postural control. Skill acquisition (such as dance) can permanently change the amount of presynaptic inhibition. Presynaptic inhibition can also be increased or decreased through operant conditioning (absence of any functional task) and task-related feedback conditioning (presence of a functional task)
Descending influence	Has important role in the expression of Cat-301 and in the elimination of synapses through development.	Has important role in the induction of plastic changes in spinal cord during skill acquisition, operant conditioning and movement control and modulation of presynaptic inhibition, and other spinal mechanisms.