What are the pathways in the anterior and posterior limb of the internal capsule and their significance?

From the Guidelines

The internal capsule is a critical white matter structure containing major neural pathways that connect the cerebral cortex with the brainstem and spinal cord, with the anterior limb primarily containing frontopontine, thalamocortical, and anterior thalamic radiations, and the posterior limb containing corticospinal, corticobulbar, sensory, and optic radiations. The anterior limb of the internal capsule contains pathways such as the frontopontine tract, which connects the frontal lobe to the pons, thalamocortical fibers connecting the thalamus to the frontal cortex, and the anterior thalamic radiations. These pathways are essential for higher cognitive functions, executive planning, and personality. The posterior limb of the internal capsule contains critical pathways including the corticospinal tract, which is responsible for motor fibers controlling voluntary movement, the corticobulbar tract, which controls facial and pharyngeal muscles, sensory fibers from the thalamus to the parietal lobe carrying touch, proprioception, and vibration information, and the optic radiations in its retrolenticular portion, which is part of the visual pathway. The significance of these pathways is highlighted by the fact that damage to the internal capsule, such as from intracerebral hemorrhage (ICH) involving the posterior limb of the internal capsule or thalamus, tends to have poorer outcomes compared to other locations 1. Key points about the pathways and their significance include:

• The compact arrangement of these pathways explains why small lesions in the internal capsule can produce widespread neurological deficits affecting motor, sensory, and cognitive functions simultaneously.
• Lesions in the anterior limb may cause cognitive and behavioral changes.
• Posterior limb damage typically results in contralateral hemiparesis, sensory deficits, or visual field defects depending on the specific fibers affected.
• The location of ICH, such as infratentorial or supratentorial, including the internal capsule, has a clear impact on survival and functional outcome 1.

From the Research

Pathways in the Anterior Limb of the Internal Capsule

• The anterior limb of the internal capsule contains pathways from the supplementary motor area (mesial area 6) 2
• It also contains subcortical projection pathways from the subthalamic nucleus (STN), substantia nigra (SNR), red nucleus (RN), ventral tegmental area (VTA), ventrolateral thalamus (VLT), and mediodorsal thalamus (MDT) 3
• These pathways are involved in various networks, including the affect, reward, cognitive control, and default networks, which are relevant to psychiatric disorders such as major depression and obsessive-compulsive disorder 3

Pathways in the Posterior Limb of the Internal Capsule

• The posterior limb of the internal capsule contains the pyramidal tract, which enters the rostral capsule in the anterior half of the posterior limb and shifts into the posterior half in more caudal sections 4
• It also contains axons from the primary motor cortex, which pass through the middle third of the posterior limb 2
• The posterior limb is also involved in sensorimotor projections, and damage to this area can result in reduced integrity of these projections, leading to motor and sensory deficits 5
• Fiber tracts connecting the posterior part of the posterior limb of the internal capsule, supplementary motor area, and posterior aspect of the thalamus are involved in motor neglect syndrome 6

Significance of the Pathways

• The pathways in the internal capsule are crucial for motor function, and damage to these areas can result in significant motor impairments 2, 5, 6
• The anterior and posterior limbs of the internal capsule contain distinct pathways that are involved in different networks and functions, and damage to these areas can have varying effects on motor and cognitive function 2, 3
• Understanding the pathways and networks involved in the internal capsule can provide insights into the neural basis of various neurological and psychiatric disorders, and can inform the development of treatments for these conditions 3