Neocortex Overview and Evolution

• Completion of Class 6 (Chapter 7) on forebrain overview
• Covered limbic system, transitioning to neocortex discussion
• Neocortex definition: connections to hypothalamus distinguish limbic system
• Two major learning types evolved in early olfactory system before neocortex:
  • Object preferences (striatal dependent) - identification of good/bad objects
  • Place learning (hippocampus/medial pallium dependent) - memory of good/bad places
• Both learning types remain important in mammals including humans

Neocortical Pathways and Functions

• New sensory pathways with neocortical expansion:
  • More rapid conduction than spinal reticular and spinothalamic pathways
  • Information sent to forebrain/neocortex
• Dorsal column pathway structure:
  • Primary sensory neurons from skin (Pacinian corpuscles, fine touch)
  • Axons ascend to dorsal column nuclei at top of spinal cord
  • First synapse in pathway, conducts rapidly via large myelinated axons
  • Decussation at dorsal column nuclei
  • Medial lemniscus pathway through hindbrain/midbrain to thalamus
  • Thalamic cells connect to somatosensory neocortex
• Descending pathways from cortical regions:
  • Motor cortex pathways to spinal cord (branches to striatum, thalamus, midbrain, hindbrain)
  • Most axons terminate on interneurons in ventral horn
  • Few direct connections to motor neurons
  • Somatosensory cortex also has descending pathways terminating in dorsal horn
• Bird equivalents: Wulst structure in advanced birds (owls, raptors)
  • Motor wulst projects to spinal cord
  • Subcortical neostriatum/nidopallium with neocortex-like connections

Neocortical Characteristics and Functions

• Enhanced sensory/motor acuity
• Fine movement control
• Fine object discrimination
• Object separation from background
• Image formation of specific objects
• Unique capabilities:
  • Anticipation and planning through image formation (posterior neocortex)
  • Movement planning using internal world models/simulations

Embryological Development Stages

• Post-fertilization sequence: morula → blastula → gastrulation → neurulation
• Gastrulation: formation of elementary canal (mouth to anus)
• Neurulation: nervous system formation
• Cellular events in development:
  • Contraction (contractile proteins like muscle)
  • Adhesion (cell adhesion molecules in membranes)
  • Movement (via filopodia - filamentous feet)
  • Growth (primarily cell proliferation via mitosis)

Neural Tube Formation Process

• Three germ layers: ectoderm (outer), mesoderm (muscle/skeleton), endoderm (internal organs)
• Notochord: cartilaginous structure along back, precedes neural tube
• Induction discovery: Hans Spemann (Nobel Prize) and Hilda Mangold (died at 26, no Nobel)
• Neural plate formation: thickened ectoderm cells above notochord
• Neural groove formation through differential contraction
• Neural crest cells: form on sides, don't join tube, create peripheral nervous system
• Tube closure begins at cervical spinal cord region
• Closure progresses rostrally and caudally
• Failure to close: spina bifida (caudal), anencephaly (rostral)

Neural Tube Structure and Differentiation

• Roof plate: top, remains one cell thick
• Floor plate: bottom, remains one cell thick
• Walls thicken through proliferation
• Alar plate: upper/dorsal part
• Basal plate: lower/ventral part (motor neurons form here)
• Sulcus limitans: groove separating alar and basal plates

Molecular Induction Factors

• Sonic Hedgehog protein: from notochord, ventralizing influence
  • Induces nervous system formation
  • Promotes motor neuron differentiation in ventral neural tube
• Dorsalizing factors: BMP-4 and BMP-7 (bone morphogenetic proteins)
  • Secreted by ectoderm adjacent to neural tube
  • Induce dorsal neural tube development

Cell Division and Migration

• Two cell division types:
  • Symmetric division: perpendicular to ventricular surface, even protein distribution (Notch/Numb), both cells remain stem cells
  • Asymmetric division: uneven protein distribution, one cell remains stem cell, other becomes postmitotic and migrates
• Cell cycle: nucleus moves away from ventricle for DNA synthesis, returns for mitosis
• Two migration types discovered:
  • Nuclear translocation: nucleus moves away from ventricle, loses attachments (described by Cajal, Morest)
  • Radial glial guidance: cells use elongated glial cells as ladder (discovered by Rakic in primates)
• Resolution of migration controversy: Morest and Domesick study of shepherd's crook cells in tectum
  • Demonstrated axon formation before cell body migration
  • Proved nuclear translocation occurs in brain, not just spinal cord
  • Rakic agreed both migration types exist