10. Regeneration and Repair of Nervous Tissue

Neural Plasticity

The nervous system's ability to change based on experience.

Neural Plasticity: Neuron Changes

Neural plasticity can involve the growth of new dendrites, synthesis of new proteins, and changes in synaptic connections with other neurons, driven by chemical and electrical signals

Neural Regeneration

Neural regeneration is the ability of neurons to replicate or repair themselves after damage.

PNS: Neural Repair

Damage to dendrites and myelinated axons in the PNS may be repaired if the cell body remains intact and Schwann cells remain active.

CNS: Neural Repair

Little or no repair of damaged neurons occurs in the CNS; even if the cell body remains intact, a severed axon in the CNS cannot normally be repaired or regrown.

Neurogenesis

The birth of new neurons from undifferentiated stem cells.

Neurogenesis: Epidermal Growth Factor (EGF)

Epidermal growth factor (EGF) is a hormone-like protein that can stimulate cells to divide and develop into neurons and astrocytes.

Neurogenesis: EGF: Other Functions

Before its effects on neural cells were discovered, EGF was known to trigger mitosis in nonneuronal cells and promote wound healing and tissue regeneration.

Neurogenesis: CNS Limitation

Neurogenesis is nearly absent in most other regions of the brain and spinal cord.

Neurogenesis: Inhibitory Factor (Neuroglia)

Neuroglia, particularly oligodendrocytes, release inhibitory influences that limit neurogenesis and regeneration in the CNS.

Neurogenesis: Inhibitory Factor (Fetal Development)

The CNS lacks many of the growth-stimulating cues that were present during fetal development, limiting neurogenesis and regeneration.

CNS: Myelination

Axons in the CNS are myelinated by oligodendrocytes rather than Schwann cells, which inhibit the regeneration of neurons and axons.

CNS Myelin: Axonal Growth

The inhibitory effect of CNS myelin may help stop axonal growth once an axon reaches its correct target region during development.

Astrocytes: Scar Tissue

Astrocytes form scar tissue after CNS injury, creating a physical barrier that prevents damaged axons from regenerating, making injuries to the brain or spinal cord usually permanent

PNS: Nerve Repair

Axons and dendrites associated with a neurolemma may repair if the cell body is intact, Schwann cells are functional, and scar tissue does not form too quickly.

Axon Injury: Cell Body

When an axon is damaged, changes occur in the neuron's cell body and in the axon distal to the injury; changes may also occur proximal to the injury.

Chromatolysis

The breakdown of Nissl bodies into fine granular masses occurs about 24–48 hours after injury to a peripheral neuron.

Wallerian Degeneration

The degeneration of the distal portion of a damaged axon and its myelin sheath occurs after swelling and fragmentation around the third to fifth day after injury.

Wallerian Degeneration: Neurolemma

Although the distal axon and myelin sheath degenerate, the neurolemma remains intact.

Macrophages: Nerve Repair

Macrophages remove debris from the damaged nerve following chromatolysis, increasing the synthesis of RNA and protein to support axon rebuilding and regeneration.

Regeneration Tube

Formed when Schwann cells grow toward each other across the damaged area, guiding a new axon from the proximal portion across the injury into the distal region

Axon Regeneration: Large Gaps

New axons cannot successfully grow across an injury if the gap is too large, as such the injury gap becomes filled with collagen fibers for the new axon to grow across it