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| Brain neuron |
Normal Neurotransmission:
The cells in the brain can send signals
from one region of the brain to another.
Nerve cells communicate with each other at a region between them called
a synapse. At the synapse, signals move
from the one neuron (sending cell) to another neuron (receiving cell). The small space between the two cells, the
synaptic cleft, is where communication takes place. The cell that will send the message is loaded
with sacs called vesicles that are filled with neurotransmitter molecules. There are many types of
neurotransmitters. The cells that
receive the message are coated with receptors specific for a particular
neurotransmitter. An electrical impulse
triggers the vesicles to release their content into the synaptic cleft. Once the neurotransmitter molecules are
released into the synaptic cleft, they collide with and lock into the receptor
molecules on the receiving cells. When
the neurotransmitter is docked, the receptor sets into motion a cascade of chemical
reactions resulting in the production of a second messenger molecule. Once the neurotransmitter has done its job it
is released from the receptor and travels back to the sending cell through
reuptake transporters. It is either
repackaged for reuse or broken down. Back
in the receiving cell, the second messenger initiates a nerve impulse which
travels down the axon of the neuron.
Once the impulse reaches the end of the neuron, vesicles with
neurotransmitter are stimulated and the process starts over again. The receiving cell becomes a sending
cell. An impulse will stop if there are
too few neurotransmitters to bind receptors or if the neurotransmitter is
inhibitory in nature, preventing further reaction. We are born capable of normal neurotransmission.
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| Synapse - vesicles release neurotransmitter |
Plasticity
Plasticity is the capacity of the nervous
system to develop new neuronal connections.
This includes the ability to change and adapt, especially the ability of
the central nervous system to acquire alternative pathways for sensory
perception or motor skills.
Neuroplasticity defines the ability of the nervous system to change its
capabilities by experience and plays a major role in compensating for the loss
of neurons with age.
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| Nerve cell |
Following birth, the brain of a newborn is
flooded with information from the baby's sense organs. This sensory information must somehow make it
back to the brain where it can be processed. To do so, nerve cells must make
connections with one another, transmitting the impulses to the brain. like a
basic telephone trunk line strung between cities, the newborn's genes instruct
the "pathway" to the correct area of the brain from a particular
nerve cell. For example, nerve cells in the retina of the eye send impulses to
the primary visual area in the occipital lobe of the brain and not to the area
of language production in the left posterior temporal lobe. The basic trunk
lines have been established, but the specific connections from one house to
another require additional signals.
Over the first few years of life, the
brain grows rapidly. As each neuron matures, it sends out multiple branches
(axons, which send information out, and dendrites, which take in information),
increasing the number of synaptic contacts and laying the specific connections
from house to house, or in the case of the brain, from neuron to neuron. At
birth, each neuron in the cerebral cortex has approximately 2,500 synapses. By
the time an infant is two or three years old, the number of synapses is
approximately 15,000 synapses per neuron. This amount is about twice that of
the average adult brain. As we age, old connections are deleted through a
process called synaptic pruning.
Synaptic pruning eliminates weaker
synaptic contacts while stronger connections are kept and strengthened.
Experience determines which connections will be strengthened and which will be
pruned; connections that have been activated (used) most frequently are
preserved. Neurons must have a purpose to survive. Without a purpose, neurons
die through a process called apoptosis in which neurons that do not receive or
transmit information become damaged and die. Ineffective or weak connections
are "pruned" in much the same way a gardener would prune a tree or
bush, giving the plant the desired shape. It is plasticity that enables the
process of developing and pruning connections, allowing the brain to adapt
itself to its environment.
Plasticity of Learning and Memory
It was once believed that as we aged, the
brain's networks became fixed. In the past two decades, however, an enormous
amount of research has revealed that the brain never stops changing and
adjusting. Learning is the ability to
acquire new knowledge or skills through instruction or experience. Memory is
the process by which that knowledge is retained over time. The capacity of the
brain to change with learning is plasticity.
At least two types of modifications occur in the brain with
learning. First, one sees a change in
the internal structure of the neurons, the most notable being in the area of
synapses. Second there is an increase in
the number of synapses between neurons.
Initially, newly learned data are
"stored" in short-term memory, which is a temporary ability to recall
a few pieces of information. Some evidence supports the concept that short-term
memory depends upon electrical and chemical events in the brain as opposed to
structural changes such as the formation of new synapses. One theory of short-term memory states that
memories may be caused by "reverberating" neuronal circuits. This means an incoming nerve impulse
stimulates the first neuron which stimulates the second, and so on, with
branches from the second neuron synapsing with the first. After
a period of time, information may be moved into a more permanent type of
memory, long-term memory, which is the result of anatomical or biochemical
changes that occur in the brain (ie learning).
Injury-induced Plasticity
During brain repair following injury,
plastic changes are geared towards maximizing function in spite of the damaged
brain. In studies involving rats in which one area of the brain was damaged,
brain cells surrounding the damaged area underwent changes in their function
and shape that allowed them to take on the functions of the damaged cells.
Although this phenomenon has not been widely studied in humans, data indicate
that similar (though less effective) changes occur in human brains following
injury.
We are a long way from being able to
safely manipulate neural growth and behavior.
Perhaps the intricacies and complexities of the human neural networks
make the task an impossible one. Or if not impossible, perhaps limited. But there are now known ways to influence,
beyond a doubt, improved function to some degree in some systems. Intuitively we have known this for
centuries. This is why we have practiced
anything to become “better”. Only now we
“know” more about why it
is worthwhile to practice. And perhaps
we are learning ways to manipulate practice.
The list of clinical examples is long, showing how healing, performance,
growth and development can all be enhanced through optimal neural
learning. Do the research if you find
yourself intrigued. If overwhelmed by
the concept, well, just keep practicing.
Don’t worry about why it works.
