White Matter

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NEUROANATOMY

  • Human brain filled w/ nervous tissue, glial cells, and vasculature, weighing avg 1400 grams
  • Estimated 100 billion neurons, each makes contact w/ at least 10,000 others
  • About 50% of adult cerebral volume is occupied by white matter
  • Great majority of white matter in white matter tracts, though some found in gray matter
    • Distinction between gray and white matter therefore relative

Myelin

  • Word derives from Greek word for marrow (myelos), coined to indicate abundance of white matter in the core or “marrow” of brain
  • Universal fx is to insulate axons and thereby dramatically affect electrical properties
  • Myelin sheath is discontinuous, leaving Node of Ranvier uncovered, to permit more efficient axonal transmission

Glial Cells

  • Of 4 types of glial cells in CNS (oligodendrocytes, astrocytes, ependymal cells, microglia), olig and astrocytes important in structure and fx of white matter
  • Oligodendrocytes are responsible for formation of myelin in CNS (Schwann cells in PNS)
  • Astrocytes found throughout CNS – provide structural support to neurons; in white matter they make contacts at nodes of Ranvier and help to regulate ionic microenvironment

Blood Supply

  • Blood supply for white matter comes from many perforating arteries arise from larger arteries at base of brain
  • Most prominent is Lenticulostriate Arteries

White Matter Tracts

  • Axons in tracts as short as 1 mm (if intracortical) and as long as 1 meter (brain to cord)
  • Tract most common term, but also fasciculus, funiculus, lemniscus, peduncle, bundle
  • Coalesce w/ each other to form rich mass of white matter w/in each hemisphere above internal capsule called corona radiata; still higher is centrum semiovale (subjacent to cortex)
  • May have more white matter in right hemisphere, w/ biggest difference in frontal lobe
  • Three Major Groups of White Matter Pathways
    • Projection
      • Consist of long ascending (corticopetal) and descending (corticofugal) tracts
      • Includes thalamo-cortical radiations; corticospinal and corticobulbar tracts
    • Commisural
      • Connect two hemispheres
      • Corpus callosum biggest (made up of posterior splenium, central body, anterior genu, and ventrally directed rostrum)
      • Other commisures include: anterior (connects olfactory and temporal regions) and hippocampal/fornical (links the fornices)
    • Association
      • Connect cerebral areas within each hemisphere
      • Generally bidirectional
      • Short association fibers (U or arcuate fibers): link adjacent cortical gyri
      • Long ass’n fibers (all terminate in frontal lobe): arcuate fasciculus, superior occipitofrontal fasciculus, inferior occipitofrontal fasciculus, cingulum, uncinate fasciculus

NEUROPHYSIOLOGY

  • Brain electrical organ
  • Speed/efficiency of action potential propagation significantly influenced by degree and integrity of myelination of axons

Action Potential

  • Neurons in brain conduct impulses at velocity of 1 to 120 meters per second
  • Speed influenced by size of axon

Saltatory Conduction

  • Increase of neuronal conduction also conferred by saltatory conduction
  • Myelin sheath interrupted every 1-2 mm by unmyelinated segment called node of Ranvier, which permits the action potential to jump from one node to next

Clinical Neurophysiology

  • Function of white matter indexed w/ EPs or evoked potentials (EEG index of cortical fx)
  • Most familiar: visual, auditory, brainstem auditory (BAEP) and somatosensory
  • Special form of EP is event-related potential (ERP): in general, long-latency waves related to cognitive stimuli
  • Most familiar ERP is P300 (can be affected by both gray and white matter dysfx); thus, P300 and other ERPs are of research interest but don’t have routine clinical application yet

NEUROPATHOLOGY: NEURAL NETWORK DISCONNECTION

  • Interference w/ normal operations of distributed neural networks by white matter lesions may disturb all aspects of neurobehavioral function. Need to consider if lesion is:
    • Focal or diffuse
    • Static or progressive
    • Associated w/ edema or mass effect
    • Severe enough to cause axonal loss
    • Combined w/ cortical or subcortical gray matter pathology
  • Microscopic level burden of white matter pathology may involve any of 5 components:
    • myelin
    • oligodendrocyte
    • axon
    • vascular system
    • astrocyte

Pathogenesis of White Matter Disconnection

  • Slowed Conduction: if pt given enough time, accurate completion of tasks
  • Absent Conduction: lesion may be severe enough to completely block axonal conduction
  • Focal Neural Network Disruption: e.g., auditory verbal agnosia, conduction aphasia
  • Diaschisis: remote effects of an acute lesion on other regions of the brain; remote area itself intact, so recovery from acute insult eventually leads to partial or complete return of some functioning in temporarily deactivated region
  • Wallerian Degeneration: axonal injury in white matter tract removed from the primary site of pathology; thus, severe white matter lesion may also produce damage in other regions of the tract; may proceed distally thru length of axon or proximally toward cell body; can occur over period of months to years
  • Transsynaptic Degeneration: secondary damage can be seen in neurons linked to those that undergo primary injury; similar to Wallerian degeneration but extends beyond initially damaged neurons; essentially refers to neuronal loss and reactive gliosis in neurons deprived of synaptic input by lesions in adjacent neurons