Brain Structures and Neural Pathways: An In-Depth Look

Posted on Jan 8, 2025 in Human Biology

Hypothalamus

Afferent Connections:

  • Medial Forebrain Bundle: Continuation of the longitudinal dorsal fasciculus.
  • Solitary Tract Nucleus.
  • Fornix: From the hippocampus to the mammillary nucleus.
  • Thalamus: From the retina and cortex.

Efferent Connections:

  • Telencephalon: Amygdala and hippocampus.
  • Thalamus: Anterior nucleus.
  • Pituitary: Downstream efferent pathways (hypothalamic-spinal, hypothalamic-bulbar, hypothalamic-periaqueductal; mammillotegmental tract).

Pituitary Gland

Irrigation:

  • Neurohypophysis: Inferior pituitary artery.
  • Adenohypophysis: Superior hypophyseal artery.

Fascicles:

  • Hypothalamic-Pituitary Fascicle: Transports vasopressin and oxytocin.
  • Tuberoinfundibular Fascicle: Transports releasing and inhibiting factors.

Circumventricular Organs: Subfornical organ, vascular organ of the lamina terminalis, subcommissural organ, pineal gland.

Pallium and Subpallium

  • Medial Pallium: Hippocampus.
  • Dorsal Pallium: Neocortex.
  • Lateral Pallium: Olfactory cortex.
  • Ventral Pallium: Claustrum, amygdaloid complex.
  • Subpallium:
    1. Ganglionic Eminence:
      • Lateral: Striatum (caudate nucleus + putamen).
      • Medial: Medial globus pallidus.
    2. Septal Eminence: Septal nuclei, amygdaloid complex.

Basal Ganglia

  • Dorsal Striatum: Caudate nucleus, putamen, globus pallidus.
  • Ventral Striatum: Nucleus accumbens, olfactory tubercle.
  • Ventral Pallidum and Substantia Innominata.

Direct Pathway:

Cortex (+) → Striatum (-) → Substantia Nigra Reticulata (-) → Intralaminar Nuclei (VA + VL) (+) → Globus Pallidus Medial (-) → Thalamic Fasciculus → VA + VL (+) → Intralaminar Nuclei (-) → Thalamostriatal Pathway → Striatum (-) → Thalamocortical Pathway → Cortex.

Indirect Pathway:

Neostriatum (-) → Globus Pallidus Lateral (-) → Subthalamic Nucleus (+) → Globus Pallidus Medial (+) → Substantia Nigra Compacta → Cortex (via corticobulbar, corticopontine, corticospinal, and corticoreticular pathways).

Nigrostriatal Pathway:

Substantia Nigra Compacta (dopamine) → Striatum → D1 Receptors: Excitatory (direct pathway) → D2 Receptors: Inhibitory (indirect pathway).

Amygdaloid Complex and Septum

General Connections: Olfactory tract, fibers from the stria terminalis, ventral amygdalofugal pathway, fibers of the substantia alba.

Afferent Connections: Olfactory bulb → associative cortical areas, subiculum of the hippocampus (via amygdalofugal pathway) → central nucleus + basolateral nucleus; locus coeruleus, substantia nigra compacta, and ventral tegmental area → central nucleus.

Efferent Connections:

  1. Corticomedial Nucleus (via amygdalofugal pathway) → Hypothalamus + Thalamus.
  2. Basolateral Nucleus → Cortex + Dorsomedial Nucleus of the Thalamus.
  3. Central Nucleus (via amygdalofugal pathway) → Hypothalamus + Thalamus + Brainstem (periaqueductal gray matter, reticular formation, dorsal motor nucleus of the vagus nerve).

Cortex Types

  • Allocortex:
    • Archicortex (medial pallium): Hippocampus, dentate gyrus, subiculum, indusium griseum.
    • Paleocortex (lateral pallium): Olfactory cortex (prepyriform cortex).
  • Isocortex: (dorsal pallium) Neocortex.
  • Mesocortex and Periallocortex: Transition between allocortex and isocortex.

White Matter

  • Internal Capsule: Projection fibers that ascend and descend from the cortex.
  • Commissural Fibers: Fibers that connect the hemispheres (fornix commissure, anterior white commissure, corpus callosum).
  • Association Fibers of the Cerebrum:
    • Superior Longitudinal Fasciculus (Occipital ↔ Frontal).
    • Arcuate Fasciculus (Temporal ↔ Frontal).
    • Inferior Longitudinal Fasciculus (Occipitotemporal ↔ Frontal).
    • Cingulum (Anterior Cingulate ↔ Parahippocampal Cingulate).

Cortical Layers

  • I: Molecular (few neurons).
  • II: External Granular (small granular cells + small pyramidal cells).
  • III: External Pyramidal (small to medium pyramidal cells).
  • IV: Internal Granular (stellate cells).
  • V: Internal Pyramidal (medium to large pyramidal cells).
  • VI: Multiform (pyramidal cells + fusiform cells).