Textbook Question
What is the basis for the functional classification of neurons?
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12. The Central Nervous System
Introduction to the Central Nervous System
3:02 minutesProblem 13
Textbook Question
Why can't most neurons in the CNS be replaced when they are lost to injury or disease?
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Understand the structure of neurons in the central nervous system (CNS): Neurons are specialized cells responsible for transmitting signals in the CNS. They consist of a cell body, dendrites, and an axon, and are highly differentiated, meaning they are specialized for their function and do not divide like other cells.
Learn about the lack of mitotic activity in mature neurons: Unlike many other cell types in the body, mature neurons in the CNS are post-mitotic, meaning they have exited the cell cycle and cannot undergo mitosis to produce new neurons. This is a key reason why they cannot be replaced after injury or disease.
Explore the role of neural stem cells: While the CNS does contain neural stem cells in certain regions (e.g., the subventricular zone and hippocampus), their ability to regenerate neurons is limited and often insufficient to replace the large number of neurons lost due to injury or disease.
Understand the inhibitory environment of the CNS: The CNS has a unique environment that actively inhibits regeneration. Factors such as the presence of glial scars, inhibitory molecules like Nogo-A, and the lack of growth-promoting signals prevent neurons from regenerating effectively.
Consider the implications for treatment: Since neurons cannot be replaced easily, research focuses on alternative strategies such as promoting neural plasticity, using stem cell therapy, or developing treatments to bypass damaged areas and restore function.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Neurogenesis
Neurogenesis is the process by which new neurons are formed in the brain. In the central nervous system (CNS), this process is limited compared to other tissues, primarily occurring in specific regions like the hippocampus. The restricted capacity for neurogenesis in the CNS contributes to the difficulty in replacing lost neurons after injury or disease.
Glial Cells
Glial cells, or neuroglia, are non-neuronal cells in the CNS that support and protect neurons. While they play crucial roles in maintaining homeostasis, forming myelin, and providing support, they do not have the ability to differentiate into neurons in response to injury. This limitation further complicates the regeneration of neuronal populations in the CNS.
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Regenerative Capacity
The regenerative capacity of the CNS is significantly lower than that of other tissues in the body. Factors such as the presence of inhibitory molecules, the formation of scar tissue, and the lack of a conducive environment for growth hinder the ability of neurons to regenerate after damage. This limited regenerative capacity is a key reason why most CNS neurons cannot be replaced following injury or disease.
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