Internal Capsule and White Matter of Brain
Learning Objectives:
At the end of lecture,students should be able to know:
; Internal capsule’s anatomical location and its parts,fibre tracts.
; Arterial supply of internal capsule.
; White matter of brain.
; Types of axons in cerebrum.
Introduction
The internal capsule is an area of white matter in the brain that separates the caudate nucleus and the thalamus from the lenticular nucleus. The internal capsule contains both ascending and descending axons. It consists of axonal fibres that run between the cerebral cortex and the pyramids of the medulla.
Location:
The capsule arises within the cerebrum but the axons that pass through it continue down to the brainstem & spinal cord.
What does it connect?
It connects the cerebral cortex with the thalamus and brain stem Its appearance on a horizontal and coronal section of the brain The internal capsule is V-shaped when cut both coronally and
horizontally/transversely
When cut horizontally:
the bend in the V is called the "genu".
the part in front of the genu is the "anterior limb". or crus anterius.
the part behind the genu is called the "posterior limb" or crus
posterius.
Parts of Internal Capsule
Anterior limb
Posterior limb
Genu
Retrolentiform
sublentiform
Parts of the Internal capsule:
The fibre tracts found in each part of the Internal Capsule.
The posterior limb of the internal capsule contains corticospinal fibers and sensory fibers from the body.
The genu contains corticobulbar fibers, which run between the cortex and the brainstem.
The anterior limb of the internal capsule contains:
1) frontopontine (corticofugal) fibers project from frontal cortex to
pons.
2) thalamocortico fibers connect the medial and anterior nuclei of the
thalamus to the frontal lobes.
Fibre Tracts
The retrolenticular part contains fibers from the optic system, coming from the lateral geniculate nucleus of the thalamus.
More posteriorly, this becomes the optic radiation.
Some fibers from the medial geniculate nucleus (which carry auditory information) also pass in the retrolenticular internal capsule, but most are in the sublenticular part
Fibre tracts
Arterial supply of the Internal Capsule
The internal capsule is supplied by the anterior and middle cerebral arteries via their medial and lateral straite branches, respectively, and from the branches of the internal carotid, especially the anterior choroidal.
The dorsal part of the posterior limb and genu is supplied by the lateral straite arteries from the middle cerebral.
The ventral part is supplied by the anterior choroidal and small unnamed branches from the internal carotid.
Internal capsule is supplied by the middle cerebral which is a branch of the internal carotid.
Striate arteries come off middle cerebral and enter through the anterior perforated substance in the base of the brain.
Blood supply of the Internal Capsule:
Clinical correlations
The anterior cerebral arteries supply blood to the medial cortex, including the medial aspects of the motor and sensory strips.
Due to the upside-down representation of the homunculus in both of these area, a blockage in an anterior cerebral artery could cause paralysis or sensory deficits which affect the opposite side of the body from the hips on down.
Corpus Callosum:
The corpus callosum connects the two hemispheres.
It is a large bundle of myelinated and nonmyelinated fibers, that crosses the longitudinal cerebral fissure and interconnects the hemispheres. The corpus callosum serves to integrate the activity of the two hemispheres and permits them to communicate with each other.
Most parts of the cerebral cortex are connected with their counterparts in the opposite hemisphere by axons that run in the corpus callosum.
White Matter
The white matter of the adult cerebral hemisphere contains myelinated nerve fibers of many sizes as well as neuroglia.
Transverse (commissural) fibres
interconnect the two cerebral hemispheres (mainly the corpus callosum)
Projection fibres connect the cerebral cortex with lower portions of the brain or the spinal cord.
White matter continued
Association fibres connect the various portions of a cerebral hemisphere and permit the cortex to function as a coordinated whole.
Cerebral Cortex and Central White Matter
Gray surface (cortex) with white tracts internally
Commissures – connect corresponding gyri of the two hemispheres
1) corpus callosum
2) anterior commissure
Projection tracts (fibers) – connect more or less vertically
Association tracts (fibers) – connect one gyrus to another in the
same hemisphere
Types of Axons in the Cerebrum
• Association fibers:
Connections within 1 hemisphere
• Commissural fibers:
Bands of fibers connecting 2 hemispheres • Projection fibers:
Link cerebral cortex with diencephalon, brain stem, cerebellum, and
spinal cord
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