Neuroanatomy Lecture Notes

Neuroanatomy Lecture Notes

Objectives for Development of the Nervous System

  1. Describe the stages of neurogenesis and the relevance of critical periods.

  2. Describe the formation of the notochord, neural plate, neural folds, neural tube, and neural crest.

  3. List the major functional regions in the adult that derive from the cephalic vesicles.

  4. Describe the basic functional organization of the neural tube.

  5. Describe the development of the Peripheral Nervous System (PNS) and how its connections are formed.

  6. Describe major abnormal developmental conditions with examples.

Importance of Development

  • Provides framework for understanding the topographical anatomy of the nervous system.

  • Insight into the pathogenesis of developmental abnormalities.

    • 3 Main types of abnormalities:

    1. Genetic (inherited)

    2. Environmental (maternal/teratogens)

    3. Idiopathic (unknown, combination of genetic and environmental)

    • Abnormalities classified based on the major processes occurring:

    1. Induction

    2. Proliferation

    3. Migration

    4. Maturation

Neurogenesis

  • Involves multipotent stem cells capable of unlimited self-renewal into any cell type.

  • Lineage of cells during neurogenesis consists of:

    • Self-renewing stem cells

    • Precursor (progenitor) cells (limited self-renewal)

    • Specialised cells including:

    • Neurons

    • Oligodendrocytes

    • Astrocytes

Stages of Neurogenesis

  1. Neural Induction

    • Determination of neuronal precursors (stem cells) through growth factors from ectoderm.

  2. Proliferation

    • Increase in cell number.

  3. Migration

    • Neural cells migrate to their final locations; guided by

      • Radial glial cells, ECM factors, and axon elongation guidance molecules.

  4. Maturation

    • Includes synapse formation and refinement driven by activity-dependent processes.

Development Timeline in Human Cortex

  • Neurogenesis occurs between 6-20 weeks and final neuron numbers established by 25 weeks.

  • Migration: Begins approximately at week 8 and ends at week 29.

  • Maturation: Includes axon and dendrite growth, starts around week 20.

  • Myelination: Predominantly postnatal, beginning in the second trimester up to 4 years of age.

Synaptogenesis in the Cortex

  • The number of synapses (in billions) varies over the lifespan:

    • Prenatal to year 1: Rapid increase, then pruning occurs in subsequent years (up to ~42% cell death).

Critical Periods in Development

  • Defined as stages when the nervous system is particularly sensitive to environmental stimuli, notably growth factors or teratogens.

  • If required stimuli are not received during these periods, certain functions may never develop adequately.

Examples of Critical Periods

  • Prenatal: Major defects like neural tube defects and cerebral palsy.

  • Postnatal: Examples include monocular deprivation in visual cortex and obstetric brachial plexus palsies.

  • Notable CNS plasticity and recovery are highest during early postnatal years.

Development Stages

  • Fertilization leads to:

    • Zygote (1 cell) → Cell division → Blastula (ball of cells) → Gastrulation (week 3).

    • Formation of 3 germ layers:

    1. Ectoderm: Skin and nervous system.

    2. Mesoderm: Skeletal muscle and connective tissues.

    3. Endoderm: GI, respiratory, and genitourinary systems.

Neurulation

  • Notochord induces formation of the neural tube via thickening of the dorsal midline neural plate, leading to neural folds and groove formation.

  • As the tube forms, it separates from the skin, leaving behind cells forming the neural crest.

  • The entire nervous system derives from the neural plate.

Neural Tube Differentiation

  • Differentiation: Rostral end → brain (primary and secondary vesicles), while the rest develops into the brainstem and spinal cord.

  • Tube closure starts in week 4 and is complete by the end of week 4, closing cranially and caudally.

Spinal Cord and Peripheral Nervous System Development

  • Separate cells from ectoderm during neurulation give rise to PNS neurons, spinal and cranial ganglia, and Schwann cells.

  • Spinal cord basic organization undergoes transverse differentiation, creating sensory (dorsal) and motor (ventral) roots.

Connections in the Spinal Cord

  • Dorsal root ganglion (DRG) cells form before spinal cord cells.

  • Afferent ingrowth occurs in a specific order, regulated by guidance molecules and growth/ transcription factors.

Early Brain Differentiation

  • Week 3: Formation of the rostral neuropore and primary vesicles.

  • Week 5: Appearance of secondary vesicles, including:

    1. Prosencephalon (forebrain) - Telencephalon & Diencephalon.

    2. Mesencephalon (midbrain).

    3. Rhombencephalon (hindbrain) - Metencephalon & Myelencephalon.

Cervical and Cephalic Flexures

  • Development involves specific flexures:

    1. Cervical - Early in week 4.

    2. Cephalic - Responsible for the rotation of the forebrain.

    3. Pontine - Develops later, allowing for transverse differentiation.

Pharyngeal Arches Development

  • Two specializations occur:

    1. Pharyngeal arches: Contain artery, cartilage, nerve, and muscle to innervate specific head and neck structures.

    • Several cranial nerves are involved:

      • 1st Arch (CN V) - Mastication

      • 2nd Arch (CN VII) - Facial expression

      • 3rd Arch (CN IX) - Others involve laryngeal and pharyngeal plexus (CN X-XI).

    1. Special sense apparatus: Eye, ear, balance, taste, and smell receptors, derived from placodes.

Brainstem Development

  • Changes occur from dorsal-ventral relationships to lateral-medial relationships due to pontine flexure.

  • Alar plate develops sensory nuclei; basal plate develops motor nuclei.

Cerebellar Development

  • Rhombic lip forms the cerebellar plate and develops into three lobes (vermis & two hemispheres).

  • Fissures develop during the fifth month:

    • Posterolateral fissure separates lobes, while primary fissure divides into anterior and posterior lobes.

Cerebral Cortex Development

  • Rapid growth from the 2nd to 3rd month involves the alar plate and lamina terminalis fusing to form diencephalon and telencephalon.

  • Cell proliferation leads to the formation of gyri and sulci.

Types of Cortex

  • Variance in structure based on evolutionary development:

    • Paleocortex (olfactory), Archicortex (hippocampus), and Neocortex (6-layer structure).

CNS Structures Derived from Neural Tube and Crest

  • Primary Vesicle Derivatives:

    • Prosencephalon: Telencephalon and Diencephalon (e.g., Cerebral hemispheres, thalamus, hypothalamus).

    • Mesencephalon: Midbrain.

    • Rhombencephalon: Metencephalon (pons, cerebellum) and Myelencephalon (medulla).

  • The neural crest gives rise to PNS ganglia.

Key Developmental Events Timeline

  • Weeks 3-6: Key events include the appearance of the neural groove, primary vesicles, neural tube closure, and sensory ganglia development.

  • Weeks 12-16: Neuronal proliferation and migration, glial differentiation, and corpus callosum development. \n- Weeks 16-42: Gyri and sulci form, ongoing neuronal proliferation, and myelination processes start.

Developmental Abnormalities

  1. Neural Tube Defects:

    • Spina bifida (failure of posterior neuropore closure).

    • Types include:

      • Closed or open types.

      • Anencephaly: Failure of anterior neuropore to close, leading to severe brain development issues.

      • Cranium Bifidum (Encephalocele) involves herniation due to neural tube defect.

  2. Proliferation and Migration Defects:

    • Microcephaly (insufficient neuron migration).

    • Heterotopia (neurons migrating incorrectly).

    • Fetal Alcohol Syndrome: Affects brain structures leading to a range of developmental deficits.

  3. Blockage of CSF:

    • Congenital obstructive hydrocephalus due to stenosis.

    • Arnold Chiari Malformations: Herniation of structures.

    • Dandy-Walker Syndrome: Associated with malformations or agenesis.

Conclusion

  • Integration of neural development knowledge is essential for understanding normal and abnormal development, with implications across multiple domains of neuroscience, healthcare, and education.