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 ...

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 Agenesis of the corpus callosum (ACC) is a rare birth defect in which there is a complete or partial absence of the corpus callosum. It occurs when the development of the corpus callosum, the band of white matter connecting the two hemispheres i ...

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 Axon guidance (also called axon pathfinding) is a subfield of neural development concerning the process by which neurons send out axons to reach their correct targets. Axons often follow very precise paths in the nervous system, and how they mana ...

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 ...

. These longitudinal callosal fascicles were originally described by

Moriz Probst Moriz Probst (1 October 1867 in Deutschlandsberg - 21 March 1923 in Vienna) was an Austrian psychiatrist and neuroanatomist. He is best known for first description of so-called Probst bundles, the anomalous brain structures found in dysgenesis ...

in 1901 by gross anatomical observation. More recently, these anomalies are detected by magnetic resonance imaging or

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