Internal Capsule
Published: Jul 17, 2023
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Updated: Jul 28, 2023
Written by Oseh Mathias
Founder, SpeechFit
The internal capsule is a bundle of nerve fibres that connects the cerebral cortex with the brainstem and other lower brain centres[1]. It acts as a "highway" carrying important neural connections, allowing for the transmission of signals involved in functions, such as movement, sensation, and cognition[2].
The internal capsule is located deep within the brain, sandwiched between the thalamus and the basal ganglia, specifically the caudate nucleus and the lentiform nucleus (which is composed of the putamen and the globus pallidus)[4].
In the image below depicting the corticospinal tract, the internal capsule is located where the fibers of the tract are shown to be drawing together in the brain.
The internal capsule is divided into several sections, namely the anterior limb, the genu (or bend), and the posterior limb[2]. There is also a retrolenticular part and a sublenticular part extending posteriorly from the posterior limb, but these areas are sometimes considered part of the posterior limb itself[8].
Here's a quick overview of the functions and projections associated with the different parts of the internal capsule:
Anterior limb: This section primarily contains fibres running between the thalamus and the prefrontal cortex, such as thalamocortical and corticothalamic fibers, which are implicated in cognitive functions[10]. It also contains fibres from the anterior cingulate gyrus and from the orbital surface of the frontal lobe[11].
Genu: This is the bend between the anterior and posterior limbs. It mainly carries corticobulbar fibres that run from the cortex to the brainstem's 'bulbs' or nuclei[12]. These are important for the control of facial muscles and other head and neck movements[13].
Posterior limb: The bulk of the fibres in this section are corticospinal tracts, which are vital for voluntary motor control[14]. These fibres run from the cerebral cortex to the spinal cord[15]. There are also sensory fibres here, specifically somatosensory fibres running from the body to the thalamus[16].
Retrolenticular part: This contains fibres from the optic radiation, which carry visual information from the thalamus to the primary visual cortex[17].
Sublenticular part: This contains fibres of the auditory radiation, which carry auditory information from the medial geniculate body of the thalamus to the primary auditory cortex[18].
The internal capsule serves as a critical pathway for bidirectional communication between different brain areas[19]. The fibres within the internal capsule are organised in a way that separates motor-related fibres from sensory-related fibres, with motor fibres located more anteriorly and sensory fibres positioned more posteriorly[20].
Given the extensive tracts running through it, damage to the internal capsule can result in severe sensory and motor deficits, among other issues[2]. The specific deficits will depend on the precise location of the damage within the internal capsule[2]. For example, stroke or injury affecting the motor fibres in the internal capsule can lead to weakness or paralysis on one side of the body, while damage to the sensory fibres can result in sensory loss or abnormalities[21].
Oseh is a software engineer, entrepreneur and founder of SpeechFit. Oseh is passionate about improving health and wellbeing outcomes for neurodiverse people and healthcare providers alike.
References
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DataBase Center for Life Science (DBCLS). (2021). Corticospinal tract [Image]. https://doi.org/10.7875/togopic.2021.063
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