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Example research essay topic: Central Nervous System Peripheral Nervous System - 1,382 words

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The ability to regenerate the tissues of the human central nervous system (CNS) is one of the greatest projects undertaken by biomedical engineers today. With this eventual technology, permanent paralysis and blindness due to CNS injury will be a thing of the past. Central nervous system injuries will be repairable, an idea that was, until recently, just a fanciful dream, something out of a science fiction novel. In the last few years, however, giant strides have been made to make the idea of CNS regeneration a reality within the grasp of engineers and doctors alike.

This technology has advanced to the point where successful tests are being performed on lower level adult mammals. If all continues to go well, human implementation may soon follow. The axons of the central nervous system in adult mammals do not regenerate spontaneously after injury, mainly because of the presence of oligodendrocytes that inhibit axonal growth. These glial cells block the growth of the axons in the central nervous system, preventing any kind of regeneration within the CNS.

What was discovered, through experimentation, was that lower non-mammalian vertebrates could regenerate their central nervous system after injury. Regeneration of the optic nerve occurs spontaneously in fish. This phenomenon has been correlated to the presence of factors that are toxic to oligodendrocytes. This substance is closely related to interleukin- 2. Lower level mammals, on the other hand, are, like humans, unable to regenerate their CNS. The same experiment performed on the fish above yielded completely different results when done on adult mammals.

Severing of the optic nerve near the eye is followed by a loss of retinal ganglion cells combined with a failure of axons to regrow into the brain. Further experimentation found that by manipulating the environment around the injured retinal ganglion cells, increases the survival rate of neurons, and make lengthy axonal regeneration, that restores nerve function to the injured area, possible. This discovery suggested that that injured nerve cells in the mature mammal CNS are influenced by interactions with their immediate environment. In certain conditions, injured central nervous system neurons can resemble normally developing neurons, and return to a functioning state.

The restoration of connections in the injured CNS of adult mammals is aided by a guided channeling of the injured axons along a transplanted segment of peripheral nerve. These neurons recover their capacity to form synapses along their former channel. These peripheral nerve grafts increase the survival rate of severed neurons in adult rats twenty percent. Some of these neurons returned to a fully functional form, making complete synaptic connections with other neurons.

To explore further the capacity of damaged CNS neurons to initiate and sustain fiber growth, PN grafts were first applied to the spine of adult rats. After six to forty two weeks, the range in which the CNS and PN grafts have been known to integrate, the rats's pines were crushed. Investigated four to eleven weeks later, it was shown that these grafts had significantly helped the regeneration of the spinal cord. The number and distribution of neurons in the crushed areas of the rats's pines was found to be similar to that of the unbrushed regions. This suggests that central neurons whose axons are grafted with peripheral nerve cells are capable of renewed growth after injury. Under these experimental conditions, CNS neurons respond to injury in a similar manner to peripheral nerve cells.

Another hypothesis was made, that suggested that axons could only regenerate when their growing tips are surrounded by extracellular fluid containing proteins from the blood. An experiment was done on fetal rat explant's to test the hypothesis. The explant's were cultured in serum medium for ten days, followed by an eight day period in a serum free medium. It was found that all explant's cultured in serum medium for ten days showed a greater than seventy seven percent viability. The explant's that were kept in the serum for eight more days retained their viability rate, while the viability rate for the explant's that were placed in the serum free environment dropped to seven and a half percent. Electron microscope analysis, showed that tissue viability was above seventy five percent in all explant's, indicating that serum is important only to axon growth and not neuron survival.

This data strengthened the hypothesis that blood derived proteins were needed for prolonged regen! eration. There are certain cells, found in the peripheral nervous system, that undertake a broad field of tasks in the peripheral nervous system. Called Schwann cells, they regulate ensheathment and myelin ation, they are involved in extracellular matrix production, and they are also instrumental in the promotion of peripheral nervous system regeneration by remy elating axons and restoring electrophysiological conduction.

Along with astrocytes, which provide nutritional proteins for the regeneration of axons, these two cells are primarily responsible for peripheral nervous system regeneration. The conjecture that was made next was that these Schwann cells were the reason that the peripheral graft experiments went so well. The Schwann cells and astrocytes were helping to rebuild not only the peripheral nerves that had been crushed, but the CNS neurons as well. Another experiment was done on rats to determine whether or not the Schwann cells were responsible for the regeneration in the peripheral nervous system. Semipermeable nerve guidance channels were prepared, inserted to connect to ends of a severed peripheral nerve, and the seeded with astrocytes, Schwann cells, or a mixture of the two. The astrocytes alone, impeded regeneration, while the Schwann cells increased the amount of growth.

The combination of the two worked as well, provided that the Schwann cells out numbered the astrocytes. Taking this into account, the latest move has been to attempt to create nerve guidance channels for the central nervous system. Using the all the previous research in the field, biomedical engineers have designed and created a device that is being tested in animals right now. Using a nerve guidance channel filled with agarose hydrogel, a gel-like medium ideal for nerve regeneration and excellent at conducting electricity, this channel is inserted into the body at the site of CNS injury.

Each separated end of the nerve is enclosed in the guidance channel, and then the channel is seeded with Schwann cells, astrocytes, proteins to nourish the growing nerve, and interleukin- 2, to destroy the inhibiting Oligodendrocytes. If this works, we may soon be able to cure paralysis and some types of blindness. Mankind will make another great stride forward in the field of medicine, curing another seemingly incurable affliction. This technology will be an achievement of utmost magnitude and importance, and we will all benefit from the realization of something that, until recently was just a pipe dream. List of Works Aguayo AJ, et al. Degenerative and Regenerative Responses of Injured Neurons in the Central Nervous System of Adult Mammals.

Philosophical Transactions of the Royal Society of London-Series B: Biological Sciences. 331 (1261): 337 - 43, 1991 March 29. Aguayo AJ, et al. Synaptic Connections Made by Axons Regenerating in the Central Nervous System of Adult Mammals. Journal of Experimental Biology. 153: 199 - 224, 1990 October. Bray GM, et al.

The Use of Peripheral Nerve Grafts to Enhance Neuronal Survival, Promote Growth and Permit Terminal Reconnections in the Central Nervous System of Adult Rats. Journal of Experimental Biology. 132: 5 - 19, 1987 September. Bunge RP. The Role of the Schwann Cell in Trophic Support and Regeneration. Journal of Neurology, 242 (1 Supplement 1): S 19 - 21, 1994 December. David S, Aguayo AJ.

Axonal regeneration After Crush Injury of Rat Central Nervous System Fibres Innervating Peripheral Nerve Grafts. Journal of Neurocytology. 14 (1): 1 - 12, 1985 February. Eitan S, et al. Identification of an Interleukin 2 -like Substance as a Factor Cytotoxic to Ologodendrocytes and Associated with Central Nervous System Regeneration. Proceedings of the National Academy of Sciences of the United States of America. 89 (12): 5442 - 6, 1992 June 15. Guard V, Aebischer P, Bunge RP.

The Astrocyte Inhibition of Peripheral Nerve Regeneration is Reversed by Schwann Cells. Experimental Neurology. 126 (1): 44 - 60, 1994 March. Oorschot DE, Jones DG. Tissue Culture Analysis of Neurite Outgrowth in the Presence and Absence of Serum: Possible Relevance for Central Nervous System Regeneration. Journal of Neuroscience Research. 15 (3): 341 - 52, 1986.


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Research essay sample on Central Nervous System Peripheral Nervous System

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