Question

Grafting peripheral nerves onto the two parts of a cut spinal cord in rats was found to restore some function in the hind limbs. Apparently, when the white matter of the peripheral nerve was joined to the gray matter of the spinal cord, some regeneration of central neurons occurred across the two spinal cord sections. What component of the peripheral nerve probably contributed to the regeneration? Discuss the factors that promote and inhibit central neuron regeneration.

Short answer

In short, the Schwann cells in the white matter of the peripheral nerve are the component that likely contributes to the regeneration of central neurons in a cut spinal cord. Factors promoting central neuron regeneration include neurotrophic factors, Schwann cells, and extracellular matrix components, while inhibiting factors include myelin-associated inhibitors, glial scar formation, and inflammatory response and immune system activity.

Step-by-step solution

Identify the component of the peripheral nerve that contributes to regeneration

The peripheral nerves have two main components - Schwann cells and axons. Schwann cells, which produce myelin sheath and secrete neurotrophic factors, are crucial for the regeneration processes. When peripheral nerves are grafted onto the cut spinal cord in rats, the Schwann cells in the white matter of the peripheral nerve likely contribute to the regeneration of central neurons.

Factors that promote central neuron regeneration

There are several factors that promote central neuron regeneration: 1. Neurotrophic factors: These factors, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3), stimulate the growth, survival, and differentiation of neurons during development, and they also play a critical role in neuronal regeneration. 2. Schwann cells: Schwann cells not only produce the myelin sheath for neuron insulation but also secrete neurotrophic factors and provide structural and metabolic support to regenerating axons. 3. Extracellular matrix (ECM) components: The extracellular matrix is crucial for providing physical support to regenerating neurons and facilitating the migration of Schwann cells and other cells that help in regeneration.

Factors that inhibit central neuron regeneration

There are several factors that inhibit central neuron regeneration: 1. Myelin-associated inhibitors: The central nervous system (CNS) contains inhibitors, such as Nogo, the myelin-associated glycoprotein (MAG), and the oligodendrocyte myelin glycoprotein (OMgp) which interact with specific receptors on neurons and block axonal regeneration. 2. Glial scar formation: After an injury, reactive astrocytes form a dense, fibrous network known as a glial scar which acts as a physical barrier and releases inhibitory molecules that obstruct axonal regeneration. 3. Inflammatory response and immune system: Inflammation caused by CNS injury may initiate a cascade of events that contribute to scarring and retraction of damaged axons, further impeding axonal regeneration. In conclusion, the component of the peripheral nerve that most likely contributes to the regeneration is the Schwann cell, which secretes neurotrophic factors and provides physical support for regenerating neurons. Factors that promote and inhibit central neuron regeneration include neurotrophic factors, extracellular matrix components, Schwann cells, myelin-associated inhibitors, glial scar formation, and inflammatory response and immune system activity.

Key concepts

Schwann Cells

Schwann cells are a special type of glial cell found in the peripheral nervous system (PNS). They play a critical role in the regeneration of neurons. One of their main functions is to produce myelin, a protective sheath that wraps around nerves, allowing efficient signal transmission.
These cells also secrete neurotrophic factors, which are essential for nerve regeneration and growth.

• Myelin Production: Schwann cells create the myelin sheath that insulates peripheral nerves, similar to how insulation surrounds electrical wires.
• Structural Support: They serve as a skeletal support to the axons attempting regeneration, creating pathways for the growth of new nerve fibers.
• Secretion of Neurotrophic Factors: By releasing these factors, Schwann cells encourage the survival and growth of adjacent neurons.

In cases of nerve damage, such as a spinal cord injury, the introduction of Schwann cells can assist central nervous system wiring to reconnect, offering a potential pathway to partial recovery.

Neurotrophic Factors

Neurotrophic factors are proteins that play a crucial role in the maintenance, development, and function of neurons both during and after neural growth. They act as signals that inform neurons about their health status and environment. In the context of neuron regeneration, they are indispensable.

• Growth Stimulation: Factors like the Nerve Growth Factor (NGF) and Brain-Derived Neurotrophic Factor (BDNF) aid in the elongation of axons and dendrites, essential components of neurons.
• Neuronal Survival: They prevent neuron death, ensuring that neurons not only grow but also thrive in challenging environments.
• Signal Transmission: These factors enhance synaptic plasticity, aiding in the strength and adaptability of neural connections.

Neurotrophic factors, often secreted by Schwann cells or introduced through therapy, are vital in promoting the growth and survival of neurons, especially after injury.

Glial Scar Formation

After a nervous system injury occurs, the body's natural response involves forming a glial scar. This scar, primarily composed of astrocytes, is a double-edged sword in neural recovery.

• Protective Barrier: The glial scar acts as a shield against further damage or infection by isolating the injury site.
• Barrier to Regeneration: The same scar becomes a physical and chemical barrier that obstructs the growth of new axons across the injury site.
• Inhibitory Signals: It releases molecules that actively inhibit axon growth and nerve repair, complicating healing processes.

While the formation of a glial scar prevents immediate danger from further damage, it poses significant challenges to neuronal regeneration, necessitating additional strategies to facilitate recovery.