What Is the Posterior Internal Capsule?

A posterior internal capsule, also called a posterior limb or crus posterius, is an area of white matter in the brain that contains ascending and descending axons. It is one of the three parts of the internal capsule, which include the anterior limb, the posterior limb, and the genu. Many pathways to the body from the cerebral cortex, brainstem, spinal cord, and other associated brain structures are contained within the posterior internal capsule.

Positioned between the lenticular nucleus and the thalamus in the brain, the posterior internal capsule is located just behind the genu, a V-shaped area visible on a horizontally dissected internal capsule. The bend in the V of the genu is used to mark the separation of the anterior and posterior sections of the internal capsule.

Within the posterior internal capsule are corticospinal, corticobulbar, and sensory fibers. Corticospinal fibers run from the genu to the brainstem and cortex. Electrical impulses processed through the pathways in the posterior internal capsule communicate sensory data, such as pain and temperature, to the brain from the peripheral nervous system. It also processes other tactile sensory information from the skin, including vibrations, light touches, and pressure signals.

The primary motor cortex of the brain stores its axons in the posterior limb of the internal capsule. Axons are the part of a nerve cell that conducts electrical impulses away from the cell through long thin projections. They attach to another nerve cell at a point called a synapse and then transmit the stored information.

Blood is supplied to the posterior internal capsule by the lenticulostriate arteries, which also provide blood to the genu and the anterior internal capsule. The lenticulostriate arteries branch off a section of the middle cerebral artery. A person with high blood pressure, or hypertension, is at risk for the narrowing of the lenticulostriate arteries that supply blood to the posterior internal capsule. A lucunar stroke occurs when the complete occlusion, or blockage, of the artery causes a lack of fresh blood circulating in the brain; this type of stroke may result in brain damage from the blocked posterior internal capsule pathway to the medial lemniscus.

Lesions or injury to the posterior capsule can cause weakness on one side of the body. An anterior choroidal artery stroke causes major damage in the posterior limb of the internal capsule, which may result in the paralysis of the arms and legs. Minor strokes may cause a person to be unable to feel the sensations of pain, temperature, and touch.