External capsule

The external capsule is a series of white matter tracts in the brain. They are situated between the putamen and claustrum.  Ventrally, the claustrum is deficient, and thus the boundary between the extreme and external capsule is poorly defined. The external capsule is composed of the claustrocortical fibres dorsally and the combined mass of the uncinate fasciculus (UF) and inferior frontal occipital fasciculus (IFOF) ventrally. The boundary between the UF, IFOF and extreme capsule (comprised of subcortical "U" fibres), as previously described, is indistinct. 

The claustrocortical fibres of the dorsal extreme capsule appear to emanate radially from the periphery of the dorsal claustrum. These fibres pass through the corona radiata to reach to the frontal and parietal lobes (including the supplementary motor area). On their way through the corona radiata, they run lateral to the dorsal part of the frontal horn, all the body and the ventral atrium of the lateral ventricle. It should be noted that the claustrocortical fibres are bi-directional, that is, it is comprised of fibres both emanating and converging on the claustrum.

The IFOF and UF together form the the ventral external capsule. They pass through the temporal stem to the frontal lobe from the temporal lobe. The UF lies just posterior to the limen (insulae) in a hook-like shape (hence "uncinate") and connects the temporopolar area to the orbital frontal lobe. It is sometimes characterised as having two parts, a dorsolateral and a ventromedial parts, the latter also connecting to the septal area. On its way to the limen it envelopes the inferior and anterior aspect of the accumbens nucleus. 

The IFOF lies dorsal to the UF and is a long association fibre tract, coursing from the ventral parts of the middle and inferior frontal gyri, through the limen and under the superior and middle temporal gyri to reach the middle and inferior occipital gyri. In the temproal lobe it runs superior to the inferior longitudinal fasciculus (ILF).

Neuroanatomy
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rID: 5826
Section: Anatomy
Synonyms or Alternate Spellings:

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