Extensive spinal decussation and bilateral termination of cervical corticospinal projections in rhesus monkeys

J Comp Neurol. 2009 Mar 10;513(2):151-63. doi: 10.1002/cne.21940.


To examine neuroanatomical mechanisms underlying fine motor control of the primate hand, adult rhesus monkeys underwent injections of biotinylated dextran amine (BDA) into the right motor cortex. Spinal axonal anatomy was examined using detailed serial-section reconstruction and modified stereological quantification. Eighty-seven percent of corticospinal tract (CST) axons decussated in the medullary pyramids and descended through the contralateral dorsolateral tract of the spinal cord. Eleven percent of CST axons projected through the dorsolateral CST ipsilateral to the hemisphere of origin, and 2% of axons projected through the ipsilateral ventromedial CST. Notably, corticospinal axons decussated extensively across the spinal cord midline. Remarkably, nearly 2-fold more CST axons decussated across the cervical spinal cord midline (approximately 12,000 axons) than were labeled in all descending components of the CST (approximately 6,700 axons). These findings suggest that CST axons extend multiple segmental collaterals. Furthermore, serial-section reconstructions revealed that individual axons descending in either the ipsilateral or contralateral dorsolateral CST can: 1) terminate in the gray matter ipsilateral to the hemisphere of origin; 2) terminate in the gray matter contralateral to the hemisphere of origin; or 3) branch in the spinal cord and terminate on both sides of the spinal cord. These results reveal a previously unappreciated degree of bilaterality and complexity of corticospinal projections in the primate spinal cord. This bilaterality is more extensive than that of the rat CST, and may resemble human CST organization. Thus, augmentation of sprouting of these extensive bilateral CST projections may provide a novel target for enhancing recovery after spinal cord injury.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Axons
  • Biotin / administration & dosage
  • Biotin / analogs & derivatives
  • Biotin / metabolism
  • Dextrans / administration & dosage
  • Dextrans / metabolism
  • Female
  • Fluorescent Dyes / administration & dosage
  • Fluorescent Dyes / metabolism
  • Macaca mulatta / anatomy & histology*
  • Male
  • Microinjections
  • Motor Cortex / anatomy & histology*
  • Motor Cortex / cytology
  • Nerve Endings*
  • Nerve Fibers*
  • Neural Pathways / anatomy & histology*
  • Neural Pathways / cytology
  • Pyramidal Tracts / anatomy & histology*
  • Pyramidal Tracts / cytology
  • Spinal Cord / anatomy & histology*
  • Spinal Cord / cytology


  • Dextrans
  • Fluorescent Dyes
  • biotinylated dextran amine
  • Biotin