Hippocampus Circuits, Visual, Auditory and Somatosensory Pathways

Hippocampus

Cytoarchitecture:

• CA1: parvocellular; limit with HIPC and subiculum
• CA2: mixed
• CA3: magnocellular (CA2 + CA3 are within the hippocampus proper)
• CA4: limit with HIPC and the dentate gyrus

Afferent Pathways

1. Entorhinal cortex → perforant pathway → dentate gyrus molecular layer → CA3 → CA1 → subiculum
2. Amygdaloid complex → subiculum
3. Fornix

Efferent Pathways

1. Subiculum → alveus → fimbria → fornix → postcommissural fibers (subiculum) → medial mammillary nucleus, ventromedial hypothalamic nucleus, anterior thalamic nuclei; precommissural fibers (hippocampus) → septal nuclei, medial prefrontal cortex, preoptic nucleus, anterior hypothalamic nucleus, nucleus accumbens
2. Pyramidal cells of the hippocampus → alveus → …

Full Papez Circuit

Subiculum → mammillary nuclei → mammillothalamic tract → anterior thalamic nuclei → thalamocortical projections → cingulate cortex → cingulum → entorhinal cortex and subiculum

Intrinsic Hippocampal Circuit

Entorhinal cortex → molecular layer of CA4 and CA3 → fimbria → molecular layer → CA2 and CA1 → fimbria → CA2 and CA1 → molecular layer of the subiculum → (1) fimbria; (2) entorhinal cortex

Cerebrospinal Fluid (CSF)

Produced by the choroid plexus through passive movement (dependent on blood pressure) and absorbed at the arachnoid villi. Circulation passes through the foramen of Monro and the cerebral aqueduct.

Visual Pathways

Light → cones and rods → bipolar cells → ganglion cells → optic nerve → optic chiasm → optic tract → lateral geniculate nucleus (LGN)

• Contralateral eye fibers: 1, 4, 6
• Ipsilateral eye fibers: 2, 3, 5

Optic radiations then split into two beams:

1. Contralateral inferior quadrant of the visual field: LGN (dorsomedial) → internal capsule → superior lip of the calcarine sulcus (A17)
2. Superior quadrant of the visual field: LGN (ventromedial) → temporal lobe white matter (Meyer's loop) → inferior lip of the calcarine sulcus (A17)

A17 → A18 → visual association cortex

Taste Pathways

Cranial nerves VII, IX, and X (anterior two-thirds of the tongue and posterior tongue) → nucleus of the solitary tract → (1) ventral posteromedial nucleus (VPM) → gustatory cortex (parietal operculum); (2) parabrachial nucleus → amygdala

Auditory Pathways

Cochlea → cochlear nucleus → (1) ipsilateral: lateral lemniscus → inferior colliculus → medial geniculate body → auditory cortex (A1, A2, and surrounding areas); (2) bilateral: superior olivary complex → trapezoid body → lateral lemniscus; superior olivary complex → outer hair cells

Olfactory Pathways

Olfactory bulb glomeruli (with mitral and tufted cells) → piriform cortex, entorhinal cortex, amygdala, and inferior tubercle → orbitofrontal cortex (shared with taste)

Somatosensory Pathways

Epicritic Pathway (body)

Dorsal root ganglion → gracile and cuneate nuclei → medial lemniscus → ventral posterolateral nucleus (VPL)

Protopathic Pathway (temperature, pain, and crude touch)

Dorsal root ganglion → spinal cord → spinothalamic tract → posterior nucleus (Po), VPL, intralaminar nuclei

Epicritic Trigeminal Pathway

Trigeminal ganglion (Gasserian) → principal trigeminal nucleus → trigeminal lemniscus → ventral posteromedial nucleus (VPM)

Protopathic Trigeminal Pathway

Trigeminal ganglion (Gasserian) → spinal trigeminal nucleus → Po, VPM, intralaminar nuclei

Paleospinothalamic Pathway

Multisynaptic → spinal trigeminal nucleus → Po, intralaminar nuclei, medial posterior ventral nucleus (MPV)

Neurodevelopment

Induction, neurulation, proliferation, differentiation, migration, selective cell aggregation, neuronal maturation, selective neuronal death, and selective axon elimination.