Internal capsule structure and function

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.