Objective: Axons travelling within the brachial plexus are responsible for the dexterous control of human arm and hand movements. Despite comprehensive knowledge on the topographical anatomy of nerves innervating the human upper limb, the definite quantity of sensory and motor axons within this neural network remains elusive. Our aim was to perform a quantitative analysis of the axonal components of human upper limb nerves based on highly specific molecular features from spinal cord level to the terminal nerves at wrist level.
Methods: Nerve specimen harvest at pre-defined harvesting sites (plexus roots and cords as well as major nerves originating from the brachial plexus innervating the arm and hand) was performed in nine human heart-beating organ donors. Double immunofluorescence staining using antibodies against choline-acetyltransferase and neurofilament was performed to differentiate motor and sensory axons on nerve cross sections.
Results: 350.000 axons emerge from the spinal cord to innervate the human upper limb of which 10% are motor neurons. In all nerves studied sensory axons outnumber motor axons by a ratio of at least 9:1. The sensory axon contribution increases when moving distally while only 1.700 motor axons reach the hand to innervate the intrinsic musculature.
Interpretation: Our results suggest that upper limb motor execution, and particularly dexterous coordination of hand movement require an unexpectedly low number of motor neurons, with a large convergence of afferent input for feedback control. This article is protected by copyright. All rights reserved.