MedUni Wien RESEARCHER OF THE MONTH, October 2018
Sensory axons by far outnumber motor axons in the human arm
Any surgical nerve reconstruction must take into account the amount of individual nerve axons at any given level of injury. Despite comprehensive knowledge on the topographical anatomy of nerves innervating the human upper limbs, 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, harvested from heart-beating organ donors, based on highly specific molecular features from spinal cord level to the terminal nerves at wrist level. Three hundred fifty thousand axons emerge from the spinal cord to innervate the human upper limb, of which 10% are motor neurons. In all mixed nerves studied, sensory axons outnumber motor axons by a ratio of at least 9:1. The sensory axon contribution increases when moving distally, whereas only 1,700 motor axons reach the hand to innervate the intrinsic musculature. 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. Since this ratio is relatively constant for motor nerves at different levels of the extremity these results challenge the traditional view of fibre distribution and innervation density in man.
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