Preferential motor reinnervation (PMR) refers to the tendency of a regenerating axon in the peripheral nervous system (PNS) to reinnervate a motor pathway as opposed to a somatosensory pathway. PMR affects how nerves regenerate and reinnervate within the PNS after surgical procedures or traumatic injuries. It is important to understand in order to further develop axonal regrowth surgical techniques. Further research of preferential motor reinnervation will lead to a better understanding of peripheral nervous system function in the human body regarding cell roles and abilities.
Summary
Motor vs sensory nerve reinnervation
The peripheral nervous system has the ability to regrow cut nerves. Motor axons preferentially reinnervate motor pathways. The tendency of motor axons to reinnervate motor pathways instead of cutaneous pathways is influenced by a number of factors in the PNS system. Some factors include Schwann cell characteristics, neurotrophic factors, and nerve branch size. These factors influence the pathway preference of the motor neuron. The different nervous systems are illustrated in the image displayed on the right. Preferential motor reinnervation is a tendency that is specifically seen in the peripheral nervous system, which is illustrated in the photos of the bottom of the system shown.
Regeneration vs reinnervation When peripheral axons are severed, the distal part of the cut axon degenerates. The only remaining distal parts from the original nerve are the Schwann cells which myelinate the peripheral axons. The basal lamina components that the Schwann cells secrete help to guide axon regeneration. The more precisely the axon stump is able to regrow along its original path, the better the recovery of function – especially when it comes to experiencing fine touch and movements. The growth of the axon stump to its original target is regeneration. Reinnervation on the other hand, is the recovery of function through reestablishing synaptic connections. Even though the original axon degenerates, the Schwann cells and acetylcholine receptors remain in place, allowing for the junction to reestablish the original synapses once the axon stump regenerates. In medical jargon, regeneration and reinnervation are not commonly distinguished. Regardless of the fact that there is a technical difference, many professionals use the terms interchangeably. This is because without regeneration, there would not be a nerve to innervate, but without reinnervation, the nerve would not function.
PMR relevancy Knowledge of preferential motor reinnervation is necessary because of how it affects the regeneration of nerves. When a patient loses nerve function, PMR can interfere with (or help) the different methods of repair that physicians use. Physicians understanding of natural nerve repair processes will allow for overall improvement in surgery, because they will be able to better interface their repair efforts with natural ones. Axon reinnervation is greatly affected by the pathway the regenerated nerve has chosen to grow along. The nerves' ability to properly function after damage is very dependent on successful reinnervation, which is why the effects of PMR are so relevant. The success of nerve reinnervation after different grafting attempts is a current research area. Grafting aims to solve the problem of incorrect targeting of regenerating axons, resulting in less-than-perfect reinnervation. PMR effects are being investigated to see how they can help grafting, and ultimately patient recovery.
How do nerves regrow?
A cut nerve regenerating A cut axon in the peripheral nervous system has two parts: a distal and a proximal axon stump. The space in between the two stumps is known as the gap, and it is what the nerve must grow through in order to fully regenerate and reinnervate. The distal axon is degenerated through the body's own mechanisms, mostly macrophage consumption and enzymes breaking it down. The proximal part of the cut axon is often able to regenerate. The regeneration and reinnervation of the cut nerve are affected by multiple factors, including how far the nerve must regrow, what kind of environment it is growing in, and the different Schwann cells and pathway options available. PMR indicates that a regenerating motor neuron will choose a motor pathway Schwann cell over a cutaneous pathway Schwann cell when regenerating.
The role of Schwann cells
Schwann cells are the myelination cells that surround nerves. When multiple nerves are cut, they must regrow and enter back through one of the Schwann cells that makes up the distal stump of the gap. These Schwann cells support axonal regrowth through their production of trophic factors as well as surface expression of multiple cell adhesion molecules that help influence axonal growth.
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