Primary afferent terminal sprouting after a cervical dorsal rootlet section in the macaque monkey

Area of Science:

• Neuroscience
• Somatosensory System
• Neural Plasticity

Background:

• Localized cervical dorsal root lesions can cause temporary silencing of cortical representations.
• Subsequent re-emergence of these representations suggests underlying neural plasticity mechanisms.

Purpose of the Study:

• To investigate if central axonal sprouting of primary afferent neurons mediates somatosensory cortical reactivation after a dorsal root lesion.
• To provide direct evidence for sprouting in specific spinal cord regions.

Main Methods:

• Electrophysiological mapping of hand representation in the macaque somatosensory cortex.
• Injection of cholera toxin subunit B-horseradish peroxidase into digit pads to label primary afferent central terminals.
• Assessment of primary afferent terminal proliferation in the spinal dorsal horn and cuneate nucleus at different postlesion time points.

Main Results:

• Cortical representations of digits C6-C8 initially silenced after dorsal rootlet section, then re-emerged over 2-4 months.
• Physiologically detectable input from lesioned digits emerged in bordering dorsal rootlets.
• Significantly larger labeled terminal bouton distributions were observed in the dorsal horn and cuneate nucleus on the lesion side at 15-25 weeks compared to 7 days postlesion.

Conclusions:

• Sprouting of spared primary afferent terminals in the dorsal horn and cuneate nucleus occurs after restricted dorsal root injury.
• This axonal sprouting is a key mechanism underlying the observed somatosensory cortical reactivation.