Keywords
critical periods, long-term potentiation (LTP), long-term depression (LTD), hippocampus, anterograde amnesia, retrograde amnesia, Papez circuit, Wallerian degeneration, chromatolysis
Chapter Outline
Both Neurons and Connections Are Produced in Excess During Development, 156
Neurotrophic Factors Ensure That Adequate Numbers of Neurons Survive, 156
Axonal Branches Are Pruned to Match Functional Requirements, 156
Synaptic Connections Are Adjusted Throughout Life, 157
There Are Short-Term and Long-Term Adjustments of Synaptic Strength, 157
Multiple Memory Systems Depend on Adjustments of Synaptic Strength, 158
The Hippocampus and Nearby Cortical Regions Are Critical for Declarative Memory, 159
The Amygdala Is Centrally Involved in Emotional Memories, 160
The Basal Ganglia Are Important for Some Forms of Nondeclarative Memory, 160
The Cerebellum Is Important for Some Forms of Nondeclarative Memory, 161
PNS Repair Is More Effective Than CNS Repair, 161
Peripheral Nerve Fibers Can Regrow After Injury, 161
CNS Glial Cells Impede Repair After Injury, 161
Limited Numbers of New Neurons Are Added to the CNS Throughout Life, 161
The nervous system is a lot less able to repair itself after damage than some other organs are, but that doesn’t mean it can’t change. There’s extensive adjustment of connections during development, but even in adult brains, synapses all over the nervous system modify their strength over timescales ranging from seconds to years. Some of these modifications are the basis of normal learning and memory.
There is hope that enhancing adult plasticity , or reactivating developmental plasticity, will make much greater levels of neurological repair possible in the near future.
Both Neurons and Connections Are Produced in Excess During Development
Similar developmental processes are at work in the formation of animals with bodies as different as snakes, star-nosed moles, and humans, and each needs a nervous system matched to its body. For example, humans need more motor neurons in the spinal cord segments that supply limbs, but snakes do not. This could be done by starting out with a baseline number of neurons in each cord segment and adding more where needed, but in fact an exactly opposite approach is taken. Spinal cord segments, and all other parts of the CNS, start out with more neurons than they will ever need and the extras die during development. Similarly, each neuron starts out with more processes than it will ever need, and the extras get pruned away during development.
Collectively, these processes of developmental plasticity result in nervous systems that are matched to the bodies and environments in which they live. The downside of this strategy is that environmental abnormalities during development can lead to permanently “miswired” nervous systems.
Neurotrophic Factors Ensure That Adequate Numbers of Neurons Survive
A critical factor that determines whether a given neuron survives or dies during development is its success in accumulating neurotrophic factors of specific kinds, different kinds for different neuronal types ( Fig. 24.1 ). Neurotrophic factors are produced in limited amounts by target tissues (e.g., muscle, glands, other neurons), gobbled up by presynaptic endings, and transported back to the cell body. There they act to prevent apoptosis (programmed cell death) and to promote growth. Going back to the spinal cord example, more dorsal root ganglion cells and motor neurons survive at levels where there’s a lot of target tissue in the periphery (e.g., lower cervical) than at levels where there’s less (e.g., midthoracic). But this isn’t restricted to the spinal cord—throughout the nervous system, something like half of all the neurons produced during development die before birth.
Axonal Branches Are Pruned to Match Functional Requirements
Long after neurons finish competing with each other for survival, they continue to compete for neurotrophic factors in an effort to preserve their connections ( Fig. 24.2 ).
The best-known example is the innervation of skeletal muscle by lower motor neurons. Early on, individual muscle fibers receive inputs from multiple motor neurons. By about the time of birth, all but one input has been pruned away and the sole survivor develops into a single elaborate neuromuscular junction. Similar pruning goes on throughout the nervous system; depending on the area involved, this is a process that may continue well after birth.
Pruning of Neuronal Connections Occurs During Critical Periods.
Neuronal connections are pruned and refined during limited time windows called critical periods . These are periods during which patterns of connections are fine-tuned, largely completing the process of matching the nervous system to the body and environment; once they end, further change is much more difficult. The downside here is that the decreased plasticity makes it difficult to repair things after damage to adult nervous systems.
The timing of critical periods is roughly correlated with the complexity of neural functions. This makes sense because, for example, multimodal cortical areas can’t finish refining their connections until after unimodal areas are done. Some patterns of connections (e.g., innervation of skeletal muscle) are finalized at birth or earlier. Others (e.g., subtleties of language) continue for another decade or longer.
This has important clinical implications: abnormalities in eyes, ears, and social situations early in life need to be avoided or corrected to prevent permanent deficits.
Synaptic Connections Are Adjusted Throughout Life
Critical periods can’t be the end of the story, though, because changes in synaptic strength continue throughout life. Some of the changes last no more than a few minutes, but others last hours to years, long enough to play key roles in things like learning and memory. Changes in presynaptic or postsynaptic Ca 2+ concentration play an important part in many, but not all, of these changes.
There Are Short-Term and Long-Term Adjustments of Synaptic Strength
Cortical maps are adjusted throughout life.
Longer-term changes can involve almost any conceivable part of presynaptic or postsynaptic elements. One prominent example is the insertion or removal of postsynaptic transmitter receptors, resulting in long-term potentiation ( LTP ) or long-term depression ( LTD ). This can be triggered by postsynaptic Ca 2+ entry through NMDA receptors ( Fig. 24.4 ). NMDA receptors (named for N -methyl- d -aspartate, which binds to them) are glutamate receptors with some special properties. First, they only open when they bind glutamate and the membrane needs to be depolarized (depolarization removes a Mg 2+ block of the cation pore), making them great detectors of simultaneous activity at multiple synapses—something that could be a building block for memory formation. Second, they are less selective than other ion channels and let Ca 2+ through (in addition to Na + and K + ). Larger amounts of Ca 2+ entry cause LTP while small amounts or the lack of Ca 2+ entry cause LTD. Subsequent Ca 2+ -initiated communication with the nucleus can make these changes very long lasting.
