Axonal components of nerves innervating the human arm.

Area of Science:

• Neuroscience
• Anatomy

Background:

• The brachial plexus facilitates arm and hand movement.
• The precise number of sensory and motor axons in the upper limb remains unknown.

Purpose of the Study:

• To quantitatively analyze the axonal components of human upper limb nerves.
• To differentiate motor and sensory axons from spinal cord to wrist.

Main Methods:

• Collected nerve specimens from 9 organ donors.
• Utilized double immunofluorescence staining to identify motor (choline-acetyltransferase) and sensory (neurofilament) axons.

Main Results:

• 350,000 axons innervate the upper limb; 10% are motor.
• Sensory axons outnumbered motor axons at a ratio of at least 9:1.
• Only 1,700 motor axons reach the hand for intrinsic muscle innervation.

Conclusions:

• Dexterous hand movement relies on a small number of motor neurons.
• Extensive sensory input likely supports feedback control for precise motor execution.