The corticotegmental connectivity as an integral component of the descending extrapyramidal pathway: novel and direct structural evidence stemming from focused fiber dissections.

This study opts to investigate the thus far ill-defined intra-hemispheric topography, morphology, and connectivity of the extrapyramidal fibers that originate from the frontoparietal cortex and project to the tegmental area and to explore structural correlations to the pyramidal pathway. To this end, twenty normal adult, formalin-fixed cerebral hemispheres were studied through the fiber micro-dissection technique. Stepwise and in-tandem medial to lateral and lateral to medial dissections were carried out in all specimens. The cortical termination of the fibers under investigation was carefully defined, and their entry zone at the tegmental area was meticulously recorded. We consistently identified the corticotegmental tract (CTT) as a distinct fiber pathway lying in the white matter of the genu and posterior limb of the internal capsule and travelling medial to the corticospinal tract (CST) and lateral to the thalamic radiations. The CTT exhibits a fan-shaped configuration and can be classified into three discrete segments: a rostral one receiving fibers from BA8 (pre-SMA, frontal eye fields, dorsal prefrontal cortex), a middle one arising from areas BA4 and BA6 (primary motor cortex and premotor cortex), and a caudal one stemming from areas BA1/2/3 (somatosensory cortex). The anatomical location, configuration, trajectory, and axonal connectivity of this tract are attuned to the descending component of the extrapyramidal system, and therefore, it is believed to be implicated in locomotion, postural control, motor inhibition, and motor modification. Our results provide further support on the emerging concept of a dynamic, parallel, and delocalized theory for complex human motor behavior.