Internal capsule is the broad and compact band of afferent and efferent fibers connecting cerebral cortex with brainstem and spinal cord. Cerebral cortex is connected with brainstem and spinal cord by both afferent and efferent fibers. Fibers arising from different parts of cerebral cortex descend down into white matter of cerebral hemispheres in the form of radiating mass of fibers called corona radiata. While passing down towards the brainstem, corona radiate converges in the form of internal capsule. Fibers from spinal cord and brainstem reach cere bral cortex in the same route. A large portion of internal capsule is formed by thalamic radiation.
SITUATION
Internal capsule is situated
in between thalamus and caudate nucleus on the medial side and lenticular
nucleus on the lateral
side.
DIVISIONS
Internal
capsule has two limbs, the anterior and posterior limbs. In between these two
limbs, lies the genu of internal capsule. Distal end of posterior limb is continued as the caudal portion of internal capsule.
ANTERIOR LIMB
Anterior
limb of internal capsule is short and lies between lenticular and caudate
nuclei.
POSTERIOR LIMB
Posterior
limb is long and situated between thalamus and lenticular nucleus.
GENU
Genu is situated between
the anterior and the posterior limbs.
CAUDAL PORTION
Caudal portion is
otherwise known as retrolenticular portion of internal capsule.
EFFECT OF LESIONS OF INTERNAL CAPSULE
Lesion of internal capsule
is caused by thrombosis or hemorrhage in branches of
middle cerebral arteries. IN ANTERIOR LIMB
Anterior
limb contains thalamocortical and frontopontine fibers. Lesion in this limb
causes widespread disability in the body. Both motor and
sensory functions are lost.
IN POSTERIOR LIMB
Lesion in
posterior limb affects the sensory fibers (thalamocortical fibers). So, it
causes:
1.
Contralateral hemianesthesia (loss of
sensation in opposite side of the body)
2. Contralateral hemihyperesthesia
(abnormal sensation in opposite side of the
body)
3. Hemiplegia
(paralysis of upper and lower limbs in one side
of the body). Hemianesthesia and hemiparesthesia occur because of lesion of
superior thalamic radiation. Hemiplegia is due to injury
of corticospinal tracts.
IN GENU
Lesion in
genu causes alteration in motor activities in opposite side due to damage of corticobulbar fibers.
IN CAUDAL PORTION
Lesion in this portion of
internal capsule causes contralateral hemianesthesia. It also
produces
hemianopia
and deafness, because of the involvement of
the auditory and visual fibers.
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