White Matter and Internal Capsule

Cerebral white matter is organised into three functional fibre categories, based on what the fibres connect.

Association fibres:

• Connect different cortical regions within the same hemisphere: short association (U) fibres connect adjacent gyri; long association fibres (e.g., the superior longitudinal/arcuate fasciculus, connecting Broca's and Wernicke's areas) connect more distant cortical regions within the same hemisphere.

Commissural fibres:

• Connect corresponding regions between the two hemispheres, crossing the midline, the corpus callosum, anterior commissure, and posterior commissure are the three major named commissures.

Projection fibres:

• Connect the cerebral cortex to subcortical structures (thalamus, brainstem, spinal cord), both ascending (sensory, toward the cortex) and descending (motor, from the cortex), the internal capsule is the critical anatomical bottleneck through which the great majority of these fibres pass.

Corpus callosum:

• The largest commissure by far, a broad band of fibres connecting the two hemispheres' neocortex, arched over the diencephalon, divided into four named parts (anterior to posterior): rostrum, genu, body, splenium: its lesion (e.g., surgical section, historically performed for refractory epilepsy, "split-brain" procedures) produces characteristic disconnection syndromes, where information presented to one hemisphere (e.g., via one visual half-field or one hand) cannot be verbally reported if that hemisphere lacks the relevant capacity (e.g., left visual field information reaching the non-dominant right hemisphere, which typically lacks robust language function), a classic illustration of hemispheric lateralisation and interhemispheric communication.

Anterior commissure:

• A smaller commissure, connecting the two temporal lobes (including olfactory-related structures and parts of the amygdala), located anterior to the columns of the fornix.

Posterior commissure:

• A small commissure at the junction of the third ventricle and cerebral aqueduct, involved in the pupillary light reflex's consensual response (carrying crossing fibres that allow light shone in one eye to produce bilateral pupillary constriction), its involvement is relevant to the relative afferent pupillary defect (RAPD) testing paradigm and to certain midbrain lesion patterns (e.g., Parinaud syndrome, affecting upward gaze and pupillary responses, from pineal region/dorsal midbrain compression involving this structure).

• A compact, C-shaped band of white matter (projection fibres) located between the thalamus/caudate nucleus (medially) and the lentiform nucleus (putamen and globus pallidus) (laterally), representing the single most concentrated bottleneck through which the great majority of ascending and descending fibres connecting the cortex to the brainstem/spinal cord/thalamus must pass, its compact size relative to the enormous volume of fibres it carries is the key anatomical fact explaining why even small lesions here (e.g., small lacunar strokes) can produce extensive, severe neurological deficits disproportionate to the lesion's physical size.

Anterior limb:

• Located between the head of the caudate nucleus (medially) and the lentiform nucleus (laterally); carries frontopontine fibres (frontal lobe to pons, part of the cortico-ponto-cerebellar pathway) and thalamocortical fibres connecting the anterior/medial thalamic nuclei to the frontal lobe (relevant to limbic/emotional circuitry).

Genu:

• The bend connecting the anterior and posterior limbs, medial to the apex of the lentiform nucleus; carries corticobulbar fibres (cortex to cranial nerve motor nuclei).

Posterior limb:

• Located between the thalamus (medially) and the lentiform nucleus (laterally), the most clinically significant part, carrying the corticospinal tract (in a somatotopic arrangement, face fibres most anterior/near the genu, leg fibres most posterior, arm fibres in between), along with thalamocortical sensory fibres relaying to the postcentral gyrus (somatosensory information).

Retrolentiform part:

• Located posterior to the lentiform nucleus, carrying optic radiations (from the lateral geniculate nucleus to the primary visual cortex) and parieto-occipital-pontine fibres.

Sublentiform part:

• Located inferior to the lentiform nucleus, carrying auditory radiations (from the medial geniculate nucleus to the primary auditory cortex) and temporopontine fibres.

• The internal capsule's rich but vulnerable blood supply is dominated by small, deep perforating (lenticulostriate) branches of the middle cerebral artery (supplying most of the posterior limb and genu), with additional contribution from the anterior choroidal artery (a branch of the internal carotid artery, supplying part of the posterior limb) and small perforators from the anterior cerebral artery (anterior limb).
• These small perforating vessels are classic end-arteries, with minimal collateral circulation, and are particularly prone to the lipohyalinosis/microatheroma changes associated with chronic hypertension, making the internal capsule (along with the basal ganglia and thalamus, supplied by similar perforating vessel patterns) the classic anatomical site of lacunar infarcts.

• Given the internal capsule's compact concentration of major ascending and descending pathways, even a small lesion here can produce a dense, complete contralateral hemiparesis/hemiplegia (posterior limb corticospinal fibres), often combined with contralateral hemisensory loss (thalamocortical sensory fibres) and, if the genu is involved, contralateral lower facial weakness and other corticobulbar deficits: this combination of extensive, severe, purely motor-plus-sensory deficit without cortical signs (e.g., without aphasia, neglect, or visual field cut, which require cortical involvement) is a classic clinical pattern distinguishing a pure internal capsule (lacunar) stroke from a larger cortical stroke affecting a similar vascular territory more diffusely.
• The disproportionate severity of deficit relative to lesion size is the single most important teaching point of this structure, a lesion of only a few millimetres can cause a complete, dense hemiplegia, given the extraordinary concentration of fibres passing through this small anatomical bottleneck.