Spinal Cord & Nerve Root Development
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The Internal Organization of the Neural Tube
- The sulcus limitans divides the neural tube into dorsal and ventral regions.
- Within the dorsal (aka posterior) region, lies the alar plate, which comprises the sensory neuronal populations of the neural tube.
- Within the ventral (aka anterior) region, lies the basal plate, which comprises the motor neuronal populations of the neural tube.
The peripheral extension of the dorsal/ventral – sensory/motor divisions
Dorsal nerver root ganglion
- The dorsal nerve root ganglion is a sensory ganglion (a cluster of pseudounipolar neurons) [it grows centrally into the alar plate].
Ventral nerve root
- The ventral nerve root is a motor root that emanates from the basal plate.
development of the neural tube and its folding
We see a coronal section through the germ disc.
- The notochord induces the overlying ectoderm to differentiate into the neural plate.
- Sonic hedgehog (Shh) is the primary mediator to signal this induction.
- When the neural plate has begins its folding, the neural groove forms the base of the developing neural tube and the neural crests comprise the neural fold tips.
- With further differentiation of the neural tube, it descends into the mesodermal layer and is separating from the ectoderm.
- The neural crests abut along the dorsal surface of the neural tube and migrate into the mesoderm: we learn the neural crest cell migration and differentiation elsewhere.
- The neural crests produce biological mediators: bone morphogenetic proteins (BMPs) and Wnt, which signal the dorsal neural tube to form the roof plate.
- The roof plate then uses these same mediators to form the bilateral alar plates.
- The notochord signals the base of the neural tube to form the floor plate (again, via sonic hedgehog), and the floor plate uses Shh to form the bilateral basal plates.
Alar and basal plates induction of the formation of the peripheral nervous system
- Ventrally, motor neurons from the basal plate produce ventral nerve roots which extend out peripherally. The mixed spinal nerves innervate myotomal muscle masses.
- Dorsally, lies the dorsal root ganglia, which form from neural crest cells. Eventually, these ganglia connect centrally with the alar plate and extend peripherally to help form the mixed spinal nerve.
- Neural crest cells migrate peripherally to form the sympathetic chain, a key component of the peripheral autonomic nervous system, which connects to the thoracolumbar spinal neurons.
Spinal Cord & PNS
- The ventral horn derives from the basal plate and produces motor innervation.
- The dorsal horn derives from the alar plate and receives sensory innervation.
- The ventral root carries motor fibers.
- The dorsal root ganglion houses the primary sensory neuron, is pseudounipolar –it sends fibers centrally toward the dorsal horn of the spinal cord via the dorsal nerve root, and also peripherally to merge with the ventral root to form the mixed spinal nerve.