The capacity for neural regeneration
So the eventual wiring of the adult brain in part reflects experiences during the long period of brain development that takes place after birth. And, to some extent, the brain responds to those experiences by making structural changes. However, once the mammalian brain is fully developed, the capacity to form new neurons is drastically reduced, though not totally lost (see below). Even before full development is reached, a lack of input during a critical stage can lead to a permanent loss of appropriate connection. For example, covering one eye during development can distort visual connections, leading to persistent impairment of adult vision that depends on that eye. As little as two weeks of occluded vision can induce these effects in human infants. This has implications for eye surgery procedures in children – for example, placing a patch over the eye after surgery could significantly impair the efficient wiring of the visual system. In this respect, the central nervous system differs from the peripheral nervous system, in which regeneration occurs regularly after injury. Areas of axonal loss can be reinnervated (i.e. the neural connections can be re-established) under some circumstances, to afford a complete recovery of function. But in the central nervous system, spinal cord damage, for instance, leads to permanent paralysis. Christopher Reeve, once the star of Superman films, is now confined to a wheelchair due to spinal cord damage sustained during a riding accident. It is possible that this difference between the central nervous system and the peripheral nervous system lies in the nonneuronal cells that are found alongside neurons. In the central nervous system, these are glial cells; in the peripheral nervous system, they are schwann cells. These non-neuronal cells provide the environment for the neuron, and can clearly secrete a variety of bioactive signalling substances. When peripheral nerves are cut, the portion of axon lying beyond the injury is cleared away, partly by the schwann cells, which form into cylindrical guides along the original path of the axon. New axon processes sprout and spread from the remaining stump, and if one of these processes enters the schwann cell guide tube, then its growth rate increases and it is led along the tube towards the nerve’s original target. Central nervous glial cells do not seem to have this ability to guide regenerating axons.
The question of whether it might be possible to induce the central nervous system to regenerate has taken a new turn since the early 1970s. At this time, it became clear that adult neurons can sometimes form new connections. If one input to a target area is lost, the remaining inputs sometimes send out new branches from their axons to colonize the vacant space (Raisman & Field, 1973). This is not necessarily an advantage. If normal function of the target area depends partly on interactions between two inputs, it may be worse off having a double signal from only one of them than having a normal signal from one and no signal from the other.
So the eventual wiring of the adult brain in part reflects experiences during the long period of brain development that takes place after birth. And, to some extent, the brain responds to those experiences by making structural changes. However, once the mammalian brain is fully developed, the capacity to form new neurons is drastically reduced, though not totally lost (see below). Even before full development is reached, a lack of input during a critical stage can lead to a permanent loss of appropriate connection. For example, covering one eye during development can distort visual connections, leading to persistent impairment of adult vision that depends on that eye. As little as two weeks of occluded vision can induce these effects in human infants. This has implications for eye surgery procedures in children – for example, placing a patch over the eye after surgery could significantly impair the efficient wiring of the visual system. In this respect, the central nervous system differs from the peripheral nervous system, in which regeneration occurs regularly after injury. Areas of axonal loss can be reinnervated (i.e. the neural connections can be re-established) under some circumstances, to afford a complete recovery of function. But in the central nervous system, spinal cord damage, for instance, leads to permanent paralysis. Christopher Reeve, once the star of Superman films, is now confined to a wheelchair due to spinal cord damage sustained during a riding accident. It is possible that this difference between the central nervous system and the peripheral nervous system lies in the nonneuronal cells that are found alongside neurons. In the central nervous system, these are glial cells; in the peripheral nervous system, they are schwann cells. These non-neuronal cells provide the environment for the neuron, and can clearly secrete a variety of bioactive signalling substances. When peripheral nerves are cut, the portion of axon lying beyond the injury is cleared away, partly by the schwann cells, which form into cylindrical guides along the original path of the axon. New axon processes sprout and spread from the remaining stump, and if one of these processes enters the schwann cell guide tube, then its growth rate increases and it is led along the tube towards the nerve’s original target. Central nervous glial cells do not seem to have this ability to guide regenerating axons.
The question of whether it might be possible to induce the central nervous system to regenerate has taken a new turn since the early 1970s. At this time, it became clear that adult neurons can sometimes form new connections. If one input to a target area is lost, the remaining inputs sometimes send out new branches from their axons to colonize the vacant space (Raisman & Field, 1973). This is not necessarily an advantage. If normal function of the target area depends partly on interactions between two inputs, it may be worse off having a double signal from only one of them than having a normal signal from one and no signal from the other.
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