Cerebral White Matter Fibers

Types of Cerebral White Matter Fibers

• Association (Arcuate) Fibers: connect one part of the cerebral cortex to another part in the same hemisphere
• Commissural Fibers: cross the midline and connect one part of a hemisphere to the corresponding part of the opposite hemisphere
• Projection Fibers: connect the cerebral cortex to the subcortical nuclei in the cerebral hemispheres and with nuclei in the diencephalon, brainstem, and spinal cord

Association Fibers

• Short Association Fibers: U-shaped, pass from one part of a gyrus to another part of the same gyrus or adjacent gyrus of the same lobe
• Long Association Fibers: run long distances between cortical areas of different lobes, forming visible nerve bundles
  • Superior Longitudinal Fasciculus: connects the anterior part of the frontal lobe to the occipital and temporal lobes
  • Cingulum: within the white matter of the cingulate gyrus, connects frontal and parietal lobes with the parahippocampal gyrus
  • Inferior Longitudinal Fasciculus: from the occipital to temporal lobes
  • Uncinate Fasciculus: from inferior frontal lobe gyri to temporal lobe

Commissural Fibers: Corpus Callosum

• Rostrum: tapered anterior end connecting the orbital cortices
• Genu: sharp backward bend from the rostrum, connecting frontal cortices
• Body (Trunk): largest, convex upwards, connecting parietotemporal cortices
• Splenium: thick rounded posterior end, connecting occipital cortices
• Isthmus: the area between the body and splenium
• Fibers in the Sagittal Plane:
  • Tapetum
• Fibers in the Horizontal Plane:
  • Commissural fibers
• Fibers in the Coronal Plane:
  • Supracallosal gyrus or Indusium Griseum
  • Medial and Lateral Longitudinal Striae (of Lancisi)
• Median Relations:
  • Dorsally: Inferior Sagittal Sinus in the lower free border of the falx cerebri
  • Caudally: Inferior Sagittal Sinus joins the Great Cerebral Vein → Straight Sinus → Transverse Sinus
  • Ventrally: Septum Pellucidum, Interventricular Foramen (of Monro), Fornix (& its Commissure), Pineal Body
• Paramedian Relations:
  • Dorsally: Pericallosal Branch of ACA, Cingulate Gyrus
  • Caudally: Post.Horn of Lateral Ventricles
  • Ventrally: Ant.Horn & Body of Lateral Ventricles

Midline Commissurotomy (Corpus Callosotomy) & Split-Brain Syndrome

• Prevents epileptic seizures from spreading from one half of the brain to the other

Internal Capsule

• Parts:
  • Anterior Limb
  • Genu
  • Posterior Limb
• Fibers:
  • Descending Tracts:
    • Anterior Thalamocortical Fibers
    • Corticonuclear & Corticospinal Fibers
  • Ascending Tracts:
    • Optic Radiation
    • Auditory Radiation

Visual Pathways

• Medial smaller root of the Optic Tract forms the Superior Brachium which passes to the Midbrain and synapses in the Superior Colliculus (general light reflex) and Pretectal Nucleus (pupillary light reflex) to mediate the light reflexes

Auditory Pathways

• Cochlear Fibers end in the Cochlear Nuclei and 2 sets of fibers arise
• Dorsal Fibers cross the Pontine Tegmentum to the Contralateral Lateral Lemniscus
• Ventral Fibers synapse bilaterally in Superior Oliuary Nucleus Complex to the Bilateral Lateral Lemnisci
• The Lateral Lemniscus ends at the Inferior Colliculus of the Midbrain
• The Medial Geniculate Body (MGB) receives the Inferior Brachium from the Inferior Colliculus of the Midbrain
• The Auditory Radiation arises from the MGB, passes through the Sublentiform Part of the Internal Capsule to reach the Auditory Cortex (Temporal Lobe)

The Limbic System

• Definition: a group of structures that lie in the border zone between the cerebral cortex and the hypothalamus, concerned with regulating behaviors related to the objectives of primitive life (Sustenance, shelter, and sex)
• Cortical Areas: Anterior Thalamic Nucleus (Emotion), Mammillary Bodies (Facial Recognition)
• Functions and Disorders:
  • Amygdala: signals the cortex of motivationally significant stimuli, related disorders: Anxiety, Autism, Depression, Post-Traumatic Stress Disorder, Phobias
  • Hippocampal Formation: Learning (Conversion of short-term memories to long-term memories), related disorders: Anterograde Amnesia, Alzheimer's Disease, Navigational Disability
  • Septal Nuclei: Reward & Pleasure Center, related disorders: Neuropsychiatric Disorders (Schizophrenia, Bipolar, OCD)
  • Mammillary Bodies: Formation of Recognition Memory, related disorders: Prosopagnosia (Face Blindness)
  • Insular Cortex: Social Emotions, related disorders: Addictive Behavior