Icon module 22

Commissures of the telencephalon

In the white matter of the pallium three basic kinds of neuronal connections can be distinguished:

  • Association pathways that connect various cortex areas within the same hemisphere with each other
  • Commissure pathways that travel between homotopic fields of the two hemispheres
  • Projection pathways that as ascending or descending fibers connect the cerebral cortex with lower lying (subcortical) nucleus regions of the entire CNS

The commissures of the telencephalon thus contain nerve fibers that connect homologous cortex areas in the left and right hemispheres with each other. Their development proceeds from the rostrally lying lamina terminalis; their upper part thickens to become the commissural plate.

Fig. 96 - Commissures of the prosencephalon in the early fetal phase

  1. Corpus callosum (arrow: future course to the occipital poles)
  2. Corpus callosum in the commissural plate
  3. Posterior commissure
  4. Habenular commissure
  5. Commissure of fornix (hippocampi)
  6. Anterior commissure
  7. Optic chiasm in the lamina terminalis

Fig. 96

This schematic median section shows the commissures in the region of the lamina terminalis. Moreover, there are two small commissures in the area of the epithalamus to be discerned: the habenular commissure and the posterior commissure.

NB: The dark blue area corresponds to the lamina terminalis, the upper segment of which forms the commissural plate.

Fig. 97 - Commissures of the prosencephalon in the fetal phase

  1. Corpus callosum
    (advanced development of the connection up to the occipital poles)
  2. Corpus callosum
  3. Posterior commissure
  4. Habenular commissure
  5. Commissure of fornix (hippocampi)
  6. Anterior commissure
  7. Optic chiasm
  8. Expanding septum pellucidum (= translucent partition)

Fig. 97

Median section in the 4th month. Emerging from the lamina terminalis the corpus callosum bends forward and over the roof of the diencephalon. At the same time, the anterior commissure and the commissure of fornix are pressed apart. The septum pellucidum lies between the corpus callosum and the trigonum (not shown).

NB: The dark blue area corresponds to the lamina terminalis, the upper segment of which forms the commissural plate

The anterior commissure, which joins the palaeocortex of the two cerebral halves, arises first (7th week). It runs between the olfactory bulb and the smelling centers of the two hemispheres. The corresponding fibers run in the anterior part of the lamina terminalis and remain where they originate.

The fornix comprises fibers that go from the cortex of the hippocampus (archicortex) to the mammillary body. Due to the arch-shaped growth of the hemispheres the archicortex comes to lie in the temporal lobes making the fornix also take on a horseshoe-shaped form. The corresponding connection between both sides of the archicortex is the commissure of fornix (or hippocampi). They are formed at about the 10th week.

More info
The cortex fields:
In the cortex one distinguishes among primary, secondary and tertiary areas.
The primary sensory cortex processes primarily raw data that it receives from the associated receptors.
In the secondary sensory cortex area a more complex processing and analysis of the data takes place.
Finally, the tertiary or association cortex integrates the information that stems from various sources and brain areas.

The corpus callosum represents the strongest connection between the hemispheres, connecting the neocortex on both sides. The fibers that link the cortex area of the frontal lobes form the forceps minor (or frontal), those that pass between the cortex areas of the parietal lobes and the occipital lead to the forceps major (or occipital). The cortex areas, between which no commissure fibers are formed, are designated as primary cortex areas. As a result, the remaining cortex areas that are connected with their contralateral counterparts, form the secondary and tertiary cortex areas (association cortex). The corpus callosum develops between the 3rd to 6th months. It lies in the depths of the sagittal cerebral fissure, above the lamina terminalis. With its tapered rostrum it emerges out of the lamina terminalis, moves then in a rostral direction and, in the genu of the corpus callosum, bends around in the back. The trunk of the corpus callosum (Corpus corporis callosi) crosses above the roof of the diencephalon in the occipital direction and ends with the splenium. Between the corpus callosum and the fornix lies the translucent septum pellucidum that separates the anterior horns of the lateral ventricles from each other and represents the medial margins of the hemisphere vesicles.

Between the splenium of the corpus callosum and the roof of the IIIrd ventricle there is a transverse gap, the transverse cerebral fissure  (fissure of Bichat). Through this space the important great cerebral vein (vena magna cerebri) runs that collects the blood from the thalamostriate vein and from the choroid plexus.

The commissures of the diencephalon travel between its nucleus regions. The most important connections are the habenular commissure and optic chiasm.