Final answer:
Brain and nerve cells do not regenerate after injury because they are terminally differentiated and cannot divide. While the brain has some degree of neuroplasticity, it does not replace large numbers of neurons, making serious injuries often permanent.
Step-by-step explanation:
Generally, brain and nerve cells, or neurons, do not regenerate after injury because they are considered to be terminally differentiated. This means they typically cannot re-enter the cell cycle to divide and produce new cells. Neurons in adult humans have a limited ability for neurogenesis, the production of new neurons from neural stem cells, which does not occur at a scale significant enough to fully repair damage. While the brain exhibits some degree of neuroplasticity, allowing for the creation of new synapses and changes within the existing nervous system, this plasticity is not enough to regenerate large numbers of neurons or repair severe damage.
Recent research has explored potential treatments that could enhance neurogenesis or promote neuroplasticity to aid recovery from nerve damage. Yet, these are still in experimental stages, and current therapeutics cannot typically replace dead or severely damaged neurons. Mild brain injuries like concussions tend to heal on their own, while more serious injuries can cause lasting disabilities. The best prognosis for nervous system injuries is to employ preventive measures to avoid the injury in the first place.