Internal Cerebrum (Association, Commissural, Projection Fibers, Basal Ganglion)

Main ideas / concepts covered

• Regional anatomy of the brain (starting point)

  • The central nervous system has two parts:
    • Encephalon (brain)
    • Spinal cord
  • The encephalon is divided into:
    • Brainstem: medulla, pons, midbrain/mesencephalon
    • Cerebellum
    • Diencephalon
    • Telencephalon (cerebrum) — the video’s main focus (“blue area”)

• View/section used to orient structures

  • A coronal section is introduced to orient the viewer:
    • Pons and midbrain are part of the brainstem
    • Diencephalon is visible
    • The remaining tissue anterior to these is the telencephalon (cerebrum)

• Cerebrum organization

  • The cerebrum has two hemispheres:
    • Right hemisphere
    • Left hemisphere
  • Each hemisphere is divided into:
    • Pallium (outer layers)
      • Cerebral cortex (surface; nerve cell bodies)
      • Underlying white matter formed by myelinated axons
    • Subpallium (deep region)
      • Basal ganglia (deep gray matter nuclei within the telencephalon)

• What the video focuses on next

  • White matter (3 fiber systems):
    1. Association fibers
    2. Commissural fibers
    3. Projection fibers
  • Then the anatomy of the basal ganglia (deep gray matter structures)

---

Detailed methodology / “list of structures” format (as presented)

A) Three types of cerebral white-matter fibers

1) Association fibers (within the same hemisphere)

• Connect different structures in the same hemisphere

• Divided into short and long association fibers:

  • Short association fibers

    • Arcuate fibers (described as forming a “U shape”; also called “U-fibers” in some sources)
    • Connect neighboring gyri/areas

  • Long association fibers

    • Superior longitudinal fasciculus
      • Connects frontal lobe ↔ occipital and temporal lobes
    • Inferior longitudinal fasciculus
      • Connects temporal lobe ↔ occipital lobe
    • Uncinate fasciculus
      • Connects frontal lobe ↔ temporal lobe
      • Runs along the bottom of the lateral sulcus
    • Cingulum (seen best on the medial surface)
      • Part of the limbic system
      • Communicates signals among:
        • Frontal lobe and parietal lobe via the cingulate gyrus
        • Eventually to the parahippocampal gyrus
        • Also sends fibers toward the temporal lobe
      • Related to the Papez circuit
        • Connected to emotional/episodic memory
        • Circulating around the subcortex to link prior episodes with the frontal lobe to trigger emotional responses

2) Commissural fibers (between hemispheres)

• Cross over the midline to connect corresponding structures between left and right hemispheres

• Three named sets:

  • Corpus callosum

    • Primary commissure connecting the hemispheres
    • Components/parts:
      • Splenium (posterior thickest end)
      • Truncus (anterior portion)
      • Genu (bent “knee”-like part)
      • Rostrum (narrow forward continuation)
    • Named frontal vs occipital extensions:
      • Anterior fibers into frontal lobe: minor/frontal forceps
      • Posterior fibers toward occipital lobe: major/occipital forceps
    • After crossing, fibers spread into the cortex (described as “radiation of corpus callosum”)

  • Commissure of fornix

    • Fornix connects parts of the limbic system, especially the hippocampus (memory/learning) to other limbic components (including mammillary bodies)
    • Fornix organization:
      • Crus of fornix
      • Body of fornix
      • Column of fornix
      • Commissure of fornix lies between the paired portions and connects right and left fornix

  • Anterior commissure

    • Small bundles that cross the midline
    • Located in front of the column of fornix and the 3rd ventricle

3) Projection fibers (to/from the cortex)

• Ascending and descending tracts
• Serve as sensory and motor pathways

• Key routing structure: Internal capsule

  • Described as the “highway” where cortical fibers travel to and from deeper brain structures

• Internal capsule anatomy (3 parts):

  • Anterior limb (crus anterius)
  • Genu (“knee”)
  • Posterior limb (crus posterius)

Tracts mentioned by internal capsule part

• Anterior limb

  • Frontopontine tract (a corticopontine tract)
    • Frontal lobe → synapse with pontine nuclei (pons) → pontocerebellar tract → cerebellorubral tract
    • From red nucleus, either:
      • to thalamus, or
      • down as rubrospinal tract (extrapyramidal; supports voluntary muscle movement)

• Genu

  • Corticonuclear tract
    • Primary motor cortex → synapses with cranial nuclei in brainstem
    • For voluntary control of muscles in the head and neck

• Posterior limb (most tracts)

  • Corticospinal tract

    • Originates from pyramidal cells of primary motor area
    • Descends to spinal cord for voluntary skeletal muscle control

  • Thalamocortical fibers (thalamus → cortex via posterior limb)

    • From VPL (ventroposterolateral) nucleus
      • Receives sensory input for trunk and limbs (via gracile fascicle and cuneate fascicle)
      • Supports epicritic sensibility (proprioception, touch, vibration)
      • Also receives input from spinothalamic tract (pain/temperature/pressure/touch)
    • From VPM (ventroposteromedial) nucleus
      • Receives facial sensory information (including pain, touch, proprioception)
      • Receives gustation (taste) via cranial nerves facial, glossopharyngeal, vagus

  • Corticopontine tracts from other lobes (extrapyramidal; improve precision of movement)

    • Temporopontine tract
    • Parietopontine tract
    • Occipitopontine tract
    • Follow the same general loop as frontopontine:
      • pontine nuclei → cerebellum → red nucleus → thalamus or rubrospinal
    • Note: frontopontine is specifically stated as passing through the anterior limb

  • Visual pathway

    • Optic nerve → optic chiasm → lateral geniculate body
    • LGB → occipital lobe (primary visual cortex)
    • Fibers called optic radiations
    • Optic radiations pass through the posterior limb of internal capsule

  • Auditory pathway

    • Cochlea (hair cells) → cochlear nerve → cochlear nuclei (pons)
    • Cross to other side → trapezoid body → lateral lemniscus → inferior colliculi
    • Through brachium of inferior colliculus → medial geniculate body → primary auditory cortex (superior temporal gyrus)
    • Auditory fibers between medial geniculate body and auditory cortex are called auditory radiations
    • Auditory radiations also pass through the posterior limb of internal capsule

---

B) Deep gray matter: Basal ganglia (anatomy and function)

What basal ganglia are for (functional overview)

• Work together to help voluntary movement
• Conceptual model described:
  • Cerebral cortex (especially primary motor area) plans movement and sends motor output via corticospinal tract
  • The basal ganglia “modify” the motor plan through direct and indirect pathways
  • Output returns to the cerebral cortex to start, stop, or modulate movement

Structures listed as part of basal ganglia network

• Principal nuclei (main input/output nuclei):
  • Caudate nucleus
  • Putamen
  • Globus pallidus externus
  • Globus pallidus internus
• Grouping terms:
  • Striatum = caudate nucleus + putamen
  • Lentiform nucleus = putamen + globus pallidus
• Functionally connected structures (included in the network discussed):
  • Thalamus, specifically:
    • Ventral anterior
    • Ventral lateral nucleus
  • Subthalamic nucleus
  • Substantia nigra
• Other closely connected structure:
  • Claustrum
• Limbic connection mentioned:
  • Amygdaloid body (amygdala), described as near the end of hippocampus and associated with basal ganglia in the video discussion

Claustrum location and role (as described)

• Located lateral to the putamen, between the putamen and the insula
• A strip of gray matter
• Role described: connects basal ganglia with different cortical areas

Caudate nucleus anatomy (brief)

• Head toward the frontal lobe
• Body within the parietal region
• Tail extending backward
• Connected with the putamen
• Neighboring structures referenced:
  • Globus pallidus (lateral/medial relations as described)
  • Thalamus medially to globus pallidus internus

---

Speakers / sources featured

• Speaker: “Meditay” / “Meditay here” (video host/lecturer delivering the explanation)