2 Neuroscience2.6 Neuroanatomy
2.6.5 Major white matter pathways
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Written by Resident Doctor Committee · Published
Classes of cerebral white matter
Class | What it joins | Example |
|---|---|---|
Commissural | Matching areas of the two hemispheres, across the midline | Corpus callosum |
Association | Cortex to cortex within one hemisphere | Arcuate fasciculus |
Projection | Cortex with subcortical, brainstem or spinal targets, in either direction | Internal capsule |
Limbic bundles such as the fornix and cingulum overlap these classes: the cingulum is both an association and a limbic tract.
Commissures
Commissure | Course and connections |
|---|---|
Corpus callosum | The largest; rostrum and genu join frontal cortex, the body joins central and parietal cortex, the splenium joins posterior cortex |
Anterior commissure | Crosses in front of the fornix columns below the ventricles, mainly joining the middle and inferior temporal gyri |
Posterior commissure | Crosses above the cerebral aqueduct and carries pretectal light reflex fibres |
The intercommissural line between the anterior and posterior commissures gives the coordinates for stereotactic surgery and deep brain stimulation.
Alexia without agraphia follows a left occipital and splenium lesion, often a posterior cerebral artery infarct: the person writes but cannot read, usually with a right homonymous hemianopia.
Association tracts
Tract | Course | Function or effect of damage |
|---|---|---|
Arcuate fasciculus | Dorsal; posterior superior temporal gyrus (Wernicke's area) to inferior frontal gyrus (Broca's area) in one hemisphere | Disconnection causes conduction aphasia |
Uncinate fasciculus | Hook-shaped; anterior temporal lobe and amygdala to orbitofrontal and inferior frontal cortex | Limbic link that attaches emotional value to social cues; studied in psychopathy |
Inferior longitudinal fasciculus | Ventral; occipital to anterior temporal cortex in the ventral visual stream | Object and face recognition; damage causes agnosia |
Superior longitudinal fasciculus | Lateral frontoparietal bundle | Right-sided disruption causes neglect |
Cingulum | Cingulate gyrus white matter, curving into the parahippocampal gyrus | Limbic association tract |
Conduction aphasia gives fluent speech and fairly good comprehension but failed repetition, with phonemic errors the person tries to correct.
Internal capsule
The internal capsule is white matter: a compact band of myelinated projection fibres running between cortex and the thalamus, brainstem and spinal cord.
It lies between the thalamus and caudate medially and the lentiform nucleus laterally.
Part | Main fibres |
|---|---|
Anterior limb | Frontopontine and prefrontal thalamic fibres |
Genu | Corticobulbar |
Posterior limb | Corticospinal and somatosensory |
Retrolentiform part | Optic radiation |
Sublentiform part | Auditory radiation |
Corticospinal axons crowd the posterior limb, so a small lacune there weakens the opposite face, arm and leg together.
The lower posterior limb is supplied by the anterior choroidal artery, a direct internal carotid branch; other parts take blood from other circle of Willis vessels.
Visual and auditory projections
Most optic tract fibres synapse in the lateral geniculate body, which projects through the optic radiation (geniculocalcarine tract) to calcarine primary visual cortex.
The lower optic radiation fibres sweep into the temporal lobe as Meyer's loop.
The auditory pathway climbs from the cochlear nuclei through the lateral lemniscus to the inferior colliculus on the dorsal midbrain, then to the medial geniculate body.
Fornix and the Papez circuit
The fornix carries hippocampal output to the mammillary bodies and septal nuclei; bilateral injury impairs episodic learning.
Papez's circuit (1937) was proposed for emotion and now anchors memory anatomy. It begins in the hippocampus.
The hippocampus sends its output through the fornix.
The fornix carries it to the mammillary bodies, which are the first relay.
The mammillothalamic tract joins the mammillary bodies to the anterior thalamic nucleus.
The anterior thalamic nucleus projects to the cingulate gyrus.
The cingulum carries fibres from the cingulate gyrus to the parahippocampal gyrus and entorhinal cortex.
The entorhinal cortex closes the loop by projecting back to the hippocampus.
The amygdala is not part of the 1937 loop; it joined the limbic system only when MacLean widened the concept.
Bilateral damage to the hippocampus, fornix or diencephalic parts causes amnesia; Wernicke-Korsakoff syndrome damages the mammillary bodies and thalamus.