Submitted by John C. Kincaid, MD, News Science Editorial Board
Edited by Lisa M. Williams, MD, News Science Editorial Board
Glasslbauer B, Hruby LA, Roche et al. Axonal components of nerves innervating the human arm
. Annals of Neurology
. Sep 2017; 82(3): 396-408.
Control of the human arm and hand are eloquently orchestrated by sensory and motor nerves arising from the cervical spinal cord. To date, the exact quantity of sensory and motor axons responsible for upper limb control are not known. The researchers of this study used 9 human heart-beating organ donors to perform a quantitative analysis of the nerve segments from the trunks and cords of the brachial plexus and segments of the major nerves arising from the cervical spinal cord. Motor and sensory axons were identified by the presence in histologic specimens of choline acetyltransferase (ChAT) and neurofilament. Using double immunofluorescence staining, the researchers were able to distinguish large diameter axons, arising from alpha motor neurons, and small diameter axons, arising from gamma motor neurons and preganglionic sympathetic neurons. The results of this study determined that only 10% of the 350,000 axons emerging from the spinal cord to innervate the upper limb were large diameter. Sensory axons outnumbered motor axons by a ratio of 9:1. Additionally, sensory axons were found to increase as in proportion distally towards the hand.
This study is the first to quantify the motor and sensory axonal components of the upper limb elucidating predominant afferent sensory control needed for fine activities such as finger motions of the upper limb, rather than abundance of motor axons.