Functional regeneration of axons occurs more frequently in the peripheral nervous system (PNS) than in the CNS because of two main reasons which are-
The environment in the central nervous system is not suitable for the regeneration or repair of axons.
The axons of the neurons in the CNS cannot upregulate genes for the growth or regeneration of the axons. Whereas the axons in the PNS can upregulate these genes to a much greater extent than the CNS.
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The central nervous system(CNS) and the peripheral nervous system(PNS), are the two branches of the nervous system in the body. The central nervous system consists of the spinal cord and the brain. While the peripheral nervous system consists of the nerves and neurons that arise from the spinal cord and the brain. It is difficult for axons to repair themselves in the central nervous system because these axons are present in and around the main structures that control the entire nervous system.
What happens during daytime is, oxygen that gets trapped between filaments of algae, moves them to the surface and during night as O2 is not produced, they slowly sink to lower depths, and you don't see them
