White matter

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The white matter of the cerebral hemisphere is composed of axonal fibres which facilitate processing of information and tasks between cortical areas, and between cortex and subcortical areas (e.g., from basal ganglia downwards).

Fibre Categories

Association fibres

Short association fibres

  • Also known as U-fibres because of their shape
  • run between neurons of adjacent gyri within the same lobe
  • e.g., from the primary sensory (postcentral gyrus) to the adjacent sensory association area in the parietal lobe

Long association fibres

  • run between cortical areas of one lobe to another lobe within the same hemisphere
  • Specific, named bundles of long association fibres are:
    1. Superior longitudinal or superior occipito-frontal fasciculus: linking occipital to frontal lobe
    2. Inferior longitudinal fasciculus: linking occipital and temporal lobes
    3. Cingulum: linking the cingulate gyrus to parahippocampal gyrus
    4. Uncinate fasciculus: linking the paralimbic areas of the frontal and temporal poles
    5. Arcuate fasciculus: linking Wernicke's area to Broca's area

Commissural fibres

  • relay information between corresponding cortical areas of the left and right hemispheres
  • The Corpus callosum is the major bundle of commissural fibres
  • Other commissural fibres include the anterior commissure, posterior commissure, and the commissure between the fornices (axonal bundles of the limbic system)

Projection fibres

  • These fibres ascend and descend and synapse along the neuroaxis of the central nervous system
  • they connect the cortical areas with subcortical regions
  • Axonal fibres from different functional areas converge towards the region of the basal ganglia, forming the #corona radiata

Corona Radiata

  • The corona radiata of the hemispheres form two compact bundles in the shape > <
  • Each bundle (internal capsule) is situated between subcortical nuclei of the basal ganglia and diencephalon
  • Internal capsules contain both ascending and descending fibres
  • The three anatomical regions of each internal capsule are
    1. anterior limb
    2. genu
    3. posterior limb
  • major descending tracts: corticospinal and corticobulbar tracts
    • can be mapped somatotopically to the genu and the posterior limb within the internal capsule
    • Cell bodies (upper motor neurons) of the corticospinal and corticobulbar tracts are located in the motor cortex (pre-central gyrus)
  • nerves of the corticobulbar tract cross at the after the pons
  • nerves of the corticospinal tract cross at the spinal level
  • Since the histological cell type of the upper motor neuron cell in the motor cortex is pyramidal, these tracts are also called pyramidal tract

Funiculi and columns


  • There are three major funiculi/columns: lateral, ventral, and dorsal (fasciculus gracile medially and fasciculus cuneate laterally)
  • Each spinal tract in the spinal cord is represented by a somatotopic map arranged in a segmental fashion
  • This knowledge is important in assessing the location and extent of the lesion in the tracts

Figure 1. Somatotropic mapping of the spinal cord Somatotropic mapping of spinal cord.gif

Corticospinal pathways

Lateral corticospinal tract

  • Lesion of the upper motor neuron (UMN) below the decussation of the pyramid results in impairment of ipsilateral muscles below the level of the lesion
  • Lesion above the decussation of the pyramid results in impairment of contralateral muscles
  • In UMN lesion, Babinski's sign is seen
    • Babinski's sign is the pathological reflex is extension (dorsiflexion) of the big toe, and fanning of the other toes
  • LMN neuropathy results in areflexia and hyporeflexia of spinal reflexes
  • Also see Physiology of the motor cortex: upper motor lesions
  • Pathway:
    1. Cell bodies of the upper motor neuron originating from various somatotopic regions of the precentral gyrus (motor cortex)
    2. corona radiata
    3. internal capsule
    4. crus cerebri (midbrain)
    5. basal pons (deep to pontine nuclei)
    6. pyramid of the medulla
    7. decussate to the contralateral side at the decussation of the pyramid lateral funiculus
    8. at the spinal level of exit, fibers of the UML enter the ventral horn synapses with the cell body of the lower (alpha) motor neuron
    9. axons leave via the ventral root to the skeletal muscle

Anterior corticospinal tract

  • 10% of upper motor neuron fibers from the motor cortex do not cross to contralateral spinal cord at the decussation of the pyramid
  • They descend ipsilaterally in the anterior funiculus
  • At the level of exit, they decussate via the anterior commissure (white matter anterior to the central canal) to the contralateral ventral horn
  • Then, they synapse with the lower motor neurons
  • These neurons exit via the ventral root to skeletal muscles
  • The anterior corticospinal tract is destined for proximal trunk muscles only
  • Its fibers gradually disappear as it descends past the thoracic segments of the spinal cord

Corticobulbar tract

  • This tract is not in the spinal cord, but is discussed here as part of the pyramidal tract
  • from the primary motor cortex
  • responsible for cognitive motor functions of the muscles innervated by cranial nerves
  • descends alongside the corticospinal tract
  • Fibres decussate to the contralateral side and synapse in the somatic or branchial motor nuclei of CN III, IV, V, VI, VII, IX, X, XI, and XII
  • Post-synaptic fibers travel to target muscles via the designated cranial nerves

Dorsal column / lemniscal pathway

  • dorsal column pathway: spinal cord portion of this pathway
  • lemniscal pathway: the brain stem portion
  • Conveys the following sensations:
    • two-point discrimination
    • vibration
    • proprioception (position sense) from skin and joint
  • Ascending fibres from the sacral and lumbar levels travel in the medial portion of the dorsal column (fasciculus gracilis)
  • Fibres from the thoracic and cervical levels travel in the lateral portion (fasciculus cuneatus)

Fasciculus gracilis

  1. Axons of mechanoreceptors from skin, muscle spindles, and joints returning from the sacral and lumbar levels
  2. cell bodies of first order neurons in DRG
  3. dorsal root
  4. enters dorsal horn
  5. travels immediately to the ipsilateral fasciculus gracilis of the dorsal column
  6. ascends in fasciculus gracilis
  7. synapses with 2nd order neuron in nucleus gracilis in caudal medulla
  8. 2nd order neuron immediately decussates to the medial lemniscus on the contralateral side
  9. ascends in medial lemniscus until it reaches the ventral posterior group of lateral nucleus in the thalamus
  10. 3rd order neuron in the thalamus projects to the lower limb and trunk areas of the primary sensory cortex

Fasciculus cuneatus

  • Involves fibres of mechanoreceptors from skin, muscle spindles, and joints from the thoracic and cervical levels
  • ascend in the laterally placed fasciculus cuneatus of the dorsal column and synapse in the nucleus cuneatus
  • From there, the pathway is identical to the lemniscal pathway from nucleus gracilis
  • Projections from the thalamus travel to the upper limb and face areas of the primary sensory cortex.
  • Lesion of the medial lemniscus results in loss of contralateral sensation
  • Lesion of the ascending 1st order neuron in the dorsal funiculus affects ipsilateral sensation. Both effects will be observed below the level of the lesion

Spinothalamic tract pathway (afferent)

  • conveys somatic sensation of pain and temperature
  1. Pain and mechanoreceptors in the skin travel in nerve fibers of the 1st order neuron (pseudounipolar cell body is located in the dorsal root ganglion)
  2. enters the dorsal horn via the dorsal root
  3. synapses with a short interneuron (2nd order neuron)
  4. 2nd order neuron immediately synapses with a 3rd order interneuron
  5. still at the level of spinal cord entry, the 3rd order neuron decussates via the anterior commissure to the lateral funiculus on the contralateral side
  6. axonal fibre of the 3rd order neuron ascends in the spinothalamic tract in the lateral funiculus
  7. it then travels in the lateral aspects of the brain stem and terminates in the ventral posterior group of lateral nucleus in the thalamus
  8. after the synapse, the 4th order neuron projects to the primary sensory cortex in the postcentral gyrus of the parietal lobe
  • Lesion of the spinothalamic tract results in the loss of pain and temperature sensation below the level of lesion on the contralateral side

Dorsal spinocerebellar tract (afferent)

  • conveys somatic sensation of proprioception, touch and pressure from the skin, muscle spindles, and joints
  • Goes to the cerebellum for coordination of equilibrium during on-going movement
  1. 1st order neuron, with its cell bodies in the DRG, enters the dorsal horn
  2. synapses with 2nd order neuron in dorsal horn
  3. axon of 2nd order neuron travels to ipsilateral lateral funiculus
  4. ascends in the dorsal spinocerebellar tract in the lateral funiculus
  5. reaches medulla oblongata
  6. enters cerebellum via inferior cerebellar peduncle