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VI./1.4.: Projection tracts
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These tracts make connections between the cortex and subcortical grey matter areas. As the fibres approach the relatively large cortex in a heavily sprawled fashion, in total they are called corona radiata. Among the projection fibres or tracts short projection fibres between the thalamus and the cortex, and long projection fibres between the cortex and other grey matter areas (spinal cord, brain stem, basal ganglia) can be distinguished. Both include ascending (corticopetal) and descending (corticofugal) fibres.
The short projection tracts are formed by the thalamocortical and corticothalamic fibres, which are collectively called radiatio thalami. Fibres of the radiatio thalami (other than the lower ones) can actually be understood as part of the capsula interna’s medial section, and they reach all of the lobes. Based on their direction the following parts can be distinguished: the radiatio thalami anterior runs to the frontal, the radiatio thalami superior to the parietal, while the radiatio thalami posterior to the occipital lobe. Fibres of the radiatio thalami inferior run to the occipital lobe.
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Of the short projection paths we highlight the radiation of the visual pathway, the radiatio optica or Gratiolet’s radiation. After leaving the corpus geniculatum laterale (CGL), its fibres run through the posterior limb of the capsula interna and spread like a fan in the white matter of the parietal, temporal and occipital lobes, surrounding the inferior and posterior horn of the lateral ventricle. The fibres raise a loop in front of the inferior horn, and only after this do they continue backwards under and next to the cornu inferior seu temporalis. This loop-like shape is called Meyer’s loop. The fibres arrive at the visual cortex on the medial surface of the occipital lobe, the area striata, which is located inside the sulcus calcarinus and along its upper and lower wall (primary, secondary and tertiary visual cortex; area Brodmann 17., 18., 19.).
Radiation of the auditory pathway, the radiatio acustica runs near the former. It originates from the corpus geniculatum mediale and runs in a smaller curve, roughly in the frontal plane towards the auditory cortex, the upper part of the gyrus temporalis, reaching the Heschl’s convolutions.
The fornix is also listed among the short projection tracts.
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The long projection tracts, located a bit laterally from the short ones, form the mass of the capsula interna, running through the crus cerebri. On the horizontal cut surface the capsula interna recalls a letter „V”, open to the lateral direction. Accordingly a front leg (crus anterius), a rear leg (crus posterius) and a knee (genu capsulae internae) connecting the two can be distinguished.
The tracts pass through the capsula interna in a specified order.
The long projection tracts are sometimes distinguished according to whether they reach the caudal motor nuclei directly or indirectly.
Tracts reaching the motor nuclei directly are located in the capsula interna’s knee and the areas neighbouring the knee in the anterior and posterior limb. The corticospinal, corticonuclear and the corticobulbar tracts are listed here.
The tractus corticospinalis is also referred to as the pyramidal tract, as its fibres, running towards the spinal cord, pass along the ventral part of the medulla oblongata corresponding to the double, longitudinal protuberations called pyramids.
Most of these fibres (~85%) decussate in the caudal section of the medulla oblongata and run downwards in the contralateral lateral tract of the spinal cord (tractus corticospinalis cruciatus seu lateralis), while the rest (~15%) run downwards in the ipsilateral anterior tract, decussating only in the spinal cord (tractus corticospinalis directus seu ventralis).
Organization of the fibers inside the capsula interna is worth remembering: fibres of the face and upper limb are located in the front, directly behind the knee, followed by the trunk’s, and slightly to the back, along the tractus corticobulbaris, the lower limb’s fibres.
Fibres of the corticobulbar tract run medially from the ones above, in the posterior limb of the capsula interna. The fibres terminate on the cranial nerves’ nuclei in the medulla oblongata.
The motor fibres of the corticonuclear tract are located in front of the above, in the knee of the capsula interna. Typically they do not originate from the primary motor cortex, but the frontal gaze centres, and some from the occipital lobe. They terminate in the brain stem on the oculomotor nerve’s nuclei.
As opposed to the ones above, fibres of the tractus corticostrialis, corticoreticularis, corticopontinus do not terminate directly, but indirectly on the motor nuclei. The tractus corticopontinus can be further split into tractus frontopontinus (Arnold) in the brain stem medially from the pyramid tract, and the lateral tractus temporooccipitopontinus (Türck). The former is located in the anterior limb of the capsula interna, while the latter in the posterior one. Its fibres terminate on the nuclei pontis.
The capsula interna, as shown in the above, is not only a white matter section, but the collection and distribution centre of the brain’s ascending and descending tracts. It has a close topographic relation to the head of the nucleus caudatus; as parts of the nucleus lentiformis with the putamen and the globus pallidus; and the thalamus. The corpus striatum is named after the stripe-like grey matter bridges, clearly visible on the brain’s cut surface, connecting the putamen and the caput nuclei caudati. Its two main parts are separated by the anterior limb of the capsula interna. A bit to the back the posterior limb and the knee of the capsula separates the nucleus lentiformis from the thalamus. Downwards its fibres become more and more compact, forming the crus cerebri. On its external side the tractus opticus runs, and laterally from that the gyrus parahippocampalis, as a part of the temporal lobe.
As the capsula interna and the crus cerebri contain several brain tracts in a compact space, the injury of which – e.g. due to circulatory disturbances – causes severe paralytic symptoms, their haemoperfusion is worth summarizing.
Blood supply of the anterior limb is practically shared with the neighbouring basal ganglia. Branches come from the arteria cerebri anterior’s so-called precommunicans or A1-segment, and through the substantia perforata anterior
from the Heubner’s recurrent artery which is usually the postcommunicans or A2-segment of the former trunk. The initial, M1-segment of the arteria cerebri media can also provide supplying vessels. The genu capsulae internae gets its blood vessels mainly from the arteria choroidea anterior beside the formers. The arteria basilaris, the arteria cerebri posterior’s precommunicans or P1-segment provides branches to the posterior limb of the capsula interna. These and the branches of the arteria cerebelli superior supply the crus cerebri.
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