Permanent reorganization of Ia afferent synapses on motoneurons after peripheral nerve injuries

Ann N Y Acad Sci, 2010 · DOI: 10.1111/j.1749-6632.2010.05459.x · Published: June 1, 2010

Simple Explanation

After peripheral nerve injuries, the connections between sensory afferents and motoneurons in the spinal cord are altered. Specifically, the synapses retract from their normal location, causing a disconnection from motoneurons. Even after the nerves regenerate and reconnect in the periphery, these changes in the spinal cord remain. This can lead to deficits in the stretch reflex, where muscles don't respond properly to being stretched. A hypothetical model suggests that this failure is due to the combined effect of imperfect reconnection in the periphery and decreased connectivity in the spinal cord.

Study Duration

Not specified

Participants

Rats

Evidence Level

Review

Key Findings

1
Sensory afferent synapses retract from lamina IX, causing a permanent relocation of inputs and disconnection from motoneurons.
2
Peripheral reconnection between proprioceptive afferents and muscle spindles is imperfect, leading to sensory afferents not reconnecting appropriately.
3
The combination of peripheral and central reconnection deficits might explain the failure of muscle stretch to initiate or modulate firing of many homonymous motoneurons.

Research Summary

After peripheral nerve injuries, monosynaptic connections from group I afferents and motoneurons become diminished in the spinal cord. Lingering motor dysfunction might be related to decreased connectivity of Ia afferents centrally, including retraction of sensory afferent synapses and imperfect peripheral reconnection. A model suggests that combined peripheral and central reconnection deficits might explain the failure of muscle stretch to initiate or modulate firing of many homonymous motoneurons.

Practical Implications

Restorative Actions

Facilitate Ia afferent regrowth centrally to ensure motoneurons retain innervation from afferents correctly innervating muscle spindles.

Therapeutic Manipulations

Explore manipulations to facilitate the proliferation of central afferent arborizations to recover the lost stretch reflex.

Further Research

Investigate whether the failure of central axons to repopulate lamina IX is due to lack of peripheral signals or negative factors in the adult spinal cord.

Study Limitations

1
The exact amount of VGLUT1 detachment from motoneuron dendrites might be underestimated.
2
The specific signals that promote or maintain central connections of sensory afferents are presently unknown.
3
The presence of negative factors in the adult spinal cord that impair regrowth of central sensory afferent synaptic arborizations is not fully understood.

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