Nervous System Development

What is neurulation?

The process that creates the neural tube, which forms the central nervous system (brain + spine); begins around day 18 & two types (primary neurulation & secondary neurulation)

Primary Neurulation

Neural plate cells invaginate and pinch off from the surface to form a tube (most of the neural tube forms this way); uses ectoderm cells

Neural Plate: forms brain and spinal cord

Epidermis: forms skin

Neural crest: forms diverse cell types

Secondary Neurulation

A solid row of cells coalesces and hollows out to form a tube; occurs in tail bud and forms spinal cord of sacrum (uses mesoderm cells); not complete until week 8

Neural Crest Cells

Originate from dorsal-most cells of neural tube; undergoes epithelial-to-mesenchymal transition & migrate throughout body → contributes to diverse cell types: cranial, cranial & caudal, caudal

Phases of Nervous System Development

Neurulation (creation of neural tube)

→ Proliferation (massive cell division; creates new neurons)

→ Migration (movement and organization of neurons within tube)

→ Synaptogenesis (creation of neural networks and connections between neurons)

→ Apoptosis (normal cell death and synaptic pruning)

→ Myelination (wrapping of neuron axons with myelin sheath)

Stages of Circuitry Maturation

Pre-reflex stage (initial growth of neurites: motor, sensory, interneurons)

→ Closure of primary reflex circuit

→ Connections with other axons (descending from brain down the spinal cord)

→ Completion of complex circuits and myelination