Longitudinal callosal fascicles, or Probst bundles, are aberrant bundles of

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...

s that run in a front-back (antero-posterior) direction rather than a left-right direction between the cerebral hemispheres. They are characteristic of patients with agenesis of the corpus callosum and are due to failure of the callosally-projecting neurons (mostly layer 2/3

pyramidal neuron Pyramidal cells, or pyramidal neurons, are a type of multipolar neuron found in areas of the brain including the cerebral cortex, the hippocampus, and the amygdala. Pyramidal neurons are the primary excitation units of the mammalian prefrontal cor ...

s) to extend axons across the midline and therefore form the corpus callosum. The inability of these axons to cross the midline results in anomalous axonal guidance and front-to-back projections within each hemisphere, rather than connecting between the hemispheres in the normal

corpus callosum The corpus callosum (Latin for "tough body"), also callosal commissure, is a wide, thick nerve tract, consisting of a flat bundle of commissural fibers, beneath the cerebral cortex in the brain. The corpus callosum is only found in placental mam ...

. These longitudinal callosal fascicles were originally described by Moriz Probst in 1901 by gross anatomical observation. More recently, these anomalies are detected by

magnetic resonance imaging Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio wave ...

diffusion tensor imaging Diffusion-weighted magnetic resonance imaging (DWI or DW-MRI) is the use of specific MRI sequences as well as software that generates images from the resulting data that uses the diffusion of water molecules to generate contrast in MR images. It ...

References

Central nervous system disorders Corpus callosum {{Neuroanatomy-stub