(USMLE topics, neurology) Types of neuroplastic changes, mechanism, phantom limb phenomenon, and relation to age.
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Neuroplasticity is the ability of the brain to change, or rewire, throughout a person’s life. It is the basis of learning and brain repair after injuries. The brain consists of billions of neurons. Neurons communicate with each other through a space between them, called a synapse. This communication is made possible by chemical messages, or neurotransmitters. Basically, the pre-synaptic neuron releases a neurotransmitter, which binds to, and activates a receptor on the post-synaptic neuron. A typical neuron can have thousands of synapses, or connections, with other neurons. Together, they form extremely complex networks that are responsible for all brain’s functions. Synaptic connections, as well as neurons themselves, can change over time, and this phenomenon is called neural plasticity, or neuroplasticity. Neuroplasticity is activity-driven and follows the “use it or lose it” rule: frequently used synapses are strengthened, while rarely used connections are weakened or eliminated; new activities generate new connections.
Changes in synaptic strength can be temporary or long-lasting depending on the intensity and reoccurrence of the signal the synapse receives. Neurons can temporarily enhance their connections by releasing more neurotransmitter, activating a new receptor, or modifying an existing receptor. This is the basis of short-term memory. Long-term memory retention requires strong or sustained activities that produce structural changes, such as growth of new dendritic spines and synaptic connections, or even formation of new neurons. Structural neuroplasticity may also result in enlargement of the cortical area associated with the increased activity, and shrinkage of areas that receive less or no activity. For example, in right-handed people, the hand motor region on the left side of the brain, which controls the right hand, is larger than the other side.
Neuroplastic changes can also be functional, meaning neurons may adopt a new function when they are sufficiently stimulated. This is how the brain survives injuries, such as strokes. Healthy brain tissues can take over the functions of the damaged area during post-stroke rehabilitation. Some stimuli, such as stress or physical exercise, can cause certain neurons to switch from one neurotransmitter to another, often converting them from excitatory to inhibitory or vice versa. This neurotransmitter switching is thought to be the basis of behavioral changes induced by such stimuli.
An intriguing example of neural plasticity is the phenomenon of phantom limb sensation, in which patients who have lost a limb through amputation can still feel the limb. For example, patients may feel that their lost arm is being touched when their face is touched. Because incoming sensory signals from the arms and face project to neighboring regions in the somatosensory cortex, it is plausible that sensory inputs from the face spill over to the now inactive arm region that no longer receives any inputs, tricking the brain’s higher centers into interpreting that the sensation comes from the absent arm.
The plasticity of the brain is not limited by age, but is much more remarkable in children as their young brain is still developing. Neuroplasticity is essential for normal brain development, it helps create functional brain circuits and is the basis of learning. This is why acquiring a new skill, such as speaking a language or playing a musical instrument, is much easier in childhood than in adulthood. But changes brought about by neural plasticity can also be negative/maladaptive and have unfortunate consequences especially if happen in childhood. Childhood traumas are more likely to have long-lasting effects into a person’s life.
Neuroplastic changes happen all the time, but their magnitude depends on the amount of activity the brain receives. More practice leads to more learning. Keeping the brain busy is the way to keep it healthy and effective.