Internal Capsule Physiology Notes

Internal Capsule Definition

• The internal capsule is the broad compact band of afferent and efferent fibers connecting cerebral cortex with the brainstem and spinal cord.
• The cerebral cortex is connected with the brainstem and spinal cord by both afferent and efferent fibers. The fibers arising from different parts of the cerebral cortex descend down into white matter of cerebral hemispheres in the form of radiating mass of fibers called corona radiata.
• The corona radiata, while passing down towards the brainstem, converges in the form of an internal capsule.
• The fibers from spinal cord and brainstem reach cerebral cortex in the same route. A large portion of internal capsule is formed by thalamic radiation.

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Internal Capsule Situation

Internal capsule is situated in between the thalamus and caudate nucleus on the medial side and lenticular nucleus on the lateral side.

Internal Capsule Divisions

Internal capsule has two limbs, the anterior and posterior limbs. In between the two limbs, lies the genu of internal capsule. The distal end of the posterior limb is continued as the caudal portion of the internal capsule.

1. Anterior Limb: Anterior limb of the internal capsule is short and lies between lenticular and caudate nuclei.
   • The anterior limb includes three groups of fibers:
     1. Anterior thalamic radiation
     2. Prefrontal corticopontine (frontopontine) tract
     3. Fibers from orbital cortex to hypothalamus.
2. Posterior Limb: It is long and situated between thalamus and lenticular
   • Posterior limb includes three groups of fibers:
     1. Corticospinal tracts
     2. Superior thalamic radiation
     3. Frontal corticopontine tract.
3. Genu: Genu is situated between the anterior and posterior limbs and it contains the corticobulbar tract.
4. Caudal Portion: Caudal portion is otherwise known as the retrolenticular portion of the internal capsule. It includes posterior thalamic radiation.

Applied Physiology Effect Of Lesions Of Internal Capsule

Lesion of the internal capsule is caused by thrombosis or hemorrhage in branches of the middle cerebral arteries. The effects of lesions depend upon the part of the internal capsule involved.

1. Effect Of Lesion In Anterior Limb: The anterior limb contains thalamocortical and frontopontine fibers. The lesion in this limb causes widespread disability in the body. Both motor and sensory functions are lost.
2. Effect Of Lesion In Posterior Limb: Lesion in the posterior limb affects the sensory fibers – thalamocortical fibers. So, it causes:
   • Contralateral hemianesthesia (loss of sensation in the opposite side of the body)
   • Contralateral hemihyperesthesia (abnormal sensation in the opposite side of the body)
   • 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.
3. Effect Of Lesion In Genu: The lesion in genu causes alteration in motor activities in the opposite side due to damage of corticobulbar fibers.
4. Effect Of Lesion In Caudal Portion: Lesion in this portion of the internal capsule causes contralateral hemianesthesia. It also produces hemianopia and deafness, because of the involvement of the auditory and visual fibers.