Internodal Length in Ventral Roots of Bovine Spinal Nerves Varies Independently of Fibre Calibre

The length of internodes in bovine ventral spinal nerve root fibres varies in an irregular pattern along the spinal cord with short internodes for the cervicothoracic and lumbosacral segments, and long internodes for the high cervical, thoracic and sacral segments. This pattern of variation is independent of axon calibre, which is fairly constant between roots. The data show that internode length and fibre calibre vary independently of each other, resulting in internodes of different proportions (length/diameter quotient) for different roots. Maximum internode length was determined at approximately 2400 microns; the longest internode found measured 2847 microns. The segmental variance in ventral root internode length in the cow differs from that in man or rat, which show a craniocaudal increase in internode length. These species-dependent differences relate to differences in the degree of the 'developmental ascensus' of the spinal cord.