THE BASIS OF REFLEX COORDINATION

Topography and topology in functional recovery of regenreated sensory and motor systems

Motor and sensory nerves can re-establish coordinated movement and accurate sensation when they regrow into denervated tissues of some lower vertebrates. Motor nerves achieve their end by a competitive process in which each motoneuron innervates many muscle fibres but, in the presence of many applicants, only those synapses from motoneurons most appropriate to a muscle cell, with respect to the original pattern of development, are retained in a functional state. The discharge pattern of a motoneuron, determined by its connections with the network of central interneurons, is not sensitive to the location of the muscles in which the axon terminates, but the efficacy of transmission from the terminals is. Sensory nerves re-establish their functional specificity as to receptor type by an inductive process occurring at the terminals along with the cessation of growth. However, in the case of cutaneous nerves they can terminate anywhere over the skin surface. The return of correctly localized reflex behaviour therefore demands a restructuring of the central nervous system in response to local position-specific signals, presumably of developmental origin, that are supplied to the sensory nerves by the skin. The re-arrangement of the central nervous connections made by the central processes of the sensory neurons probably uses the same competitive mechanism of enabling and disabling formed synaptic connections as is used in sorting out the correct site of functional termination of the peripheral processes of motoneurons.

Re-innervation of axolotl limbs. I. Motor nerves

The motor innervation of the hindlimb by the segmental nerves of the lumbar plexus was determined by observation of muscle contraction and leg movement following nerve stimulation and by mapping the location of junction potentials recorded by an intracellular microelectrode. There is variation between animals in the composition of the plexus and in the muscles they supply in the limb but in any one animal the motor innervation of the two limbs is symmetrical. After severing the two main nerves to one limb and re-directing each nerve towards the muscles normally supplied by the other, the limbs became re-innervated and coordinated movement returned. Mapping the segmental innervation of re-innervated limbs showed that the normal symmetrical pattern, as judged by the unoperated normal limb was restored. Therefore motor nerve axons from each segmental nerve had regained control of their original muscle territories in spite of being directed at operation to foreign muscle. The above is probably sufficient to explain the return of coordinated movement to limbs, on the basis of specific functional connexions made between motor nerve fibres and muscles, rather than on any reorganization of the firing pattern of motoneurones with misdirected axons.