Saturday, 8 November 2008

Lean, Mean, Thinking Machine

At birth children's brains are in a surprisingly unfinished state . Newborns have all the genetic coding required to guide their brain development. What's more, they have nearly all of the billions of brain cells,or neurons, they will need for a lifetime of thinking, communicating, and learning. But these neurons are not yet linked up into the networks needed for complex functioning. It is like having billions of telephones installed around the nation, but not yet completely connected to each other. If children have more synapses than they will have as adults, what happens to the trillions of excess connections? The answer is they are shed as children grow.

Before the age of 3, synapse production is by far the dominant process; from 3 to 10 the processes are relatively blanced, so the number of synapses stays about the same. But as children near adolescence, the balance shifts, and the shedding of excess neurons moves into high gear.

Neurons do not exist in isolation. The Hebbian rules follow the use it or lose it principle and simply state that neurons that fire together, wire together. The functional property of the neuron is dependent on that neuron becoming incorporated into a functional neural circuit, a network of neurons interconnected in different regions of the brain, such that the combined activities of many neurons in different parts is necessary to produce a given behaviour. It is the constant repetition of these behaviours which reinforce the synaptic connections between neurons. So as we grow older the neurons not only fire faster, but their signals become clearer. Faster neurons are more likely to fire in sync with each other - becoming better team players - wiring together more and forming groups of neurons that give off clearer and more powerful signals. This is a crucial point, because a powerful signal has a greater impact on the brain.
Faster neurons ultimately lead to faster thought -no minor matter- because speed of thought is a crucial component of intelligence. Speed of thought is essential to our survival. Events often happen quickly, and if the brain is slow, it can miss important information. Essentially survival and growth of neurons depends on exchange between pre-synaptic neurons and target neurons in which activation is exchanged.

Between the ages of 2 and 16 years of age there is actually a loss of about 50% of the synapses. So you can see the fight for survival is very competitive. Only the regularly used synapses survive, with unused synapses disappearing trough a process sometimes referred to as 'shedding', and is an on-going part of something referred to as competitive plasticity in the brain.

Every neuron has an axon (usually only one) the axon is an output fiber that sends impulses to other neurons. Each neuron also has many dendrites - short, hair-like 'input' fibers that recieve impulses from other neurons. In this way, neurons are perfectly constructed to form connections. The process of dendritic growth is much slower than axonal growth and involves much more branching and elaboration. At some stages of dendritic growth there appears to be an overabundance of denritic branches. Some of these excess or unused branches are eventually lost in a process referred to as pruning.

Myelin is a lipid layer that surrounds only the axons of many neurons. Myelination acts rather like electrical insulating tape being wrapped around a bare wire. Myelin has two important advantages: fast conduction speed and energy efficiency. For axons larger than a minimum diameter (roughly 1 micron), myelination increases the conduction velocity of an action potential typically tenfold. Without myelination, the long axons of our nervous system would be turtle-like in their ability to relay information. Indeed, some tragic diseases like multiple scerosis, result from a loss of myelination and lead to a disruption between mind and muscle. Most of the neurons in the brain are interneurons. When connecting to each other interneurons often lack myelinated axons, their tight proximity makes the accelerated speed and efficiency of myelination unnecessary. When looking at the surface of the brain these bunches of unmyelinated neurons appear as a great gray mass - hence the name 'gray matter' for these parts of the brain. 'White matter' refers to the tracts of fatty myelinated axons that stretch down the spinal cord or connect distant parts of the brain. Importantly, some of these primary cortical areas are the first to under go myelination at birth, reaching maturity before the infant is 18 months, and it includes the auditory cortex, the visual cortex and the motor cortex. Myelination slows down after 3 years of age, but continues into adult life. This not only increases the efficiency at which an infant is able to become mobile, but also in how fast they can process the outside world into information the mind can utilise.

Given a complete overview of the brain, it emerges as a machine that is continously being made more efficient throughout our lives. The workings of the brain are funnelled to adapt their immediate enviroment. Signals run faster, and more smoothly. Relationships that are important between different parts of the brain grow stronger, while those that are not needed are quite ruthlessly discarded. The brain has been designed to percieve the outside world with greater efficiency. The metabolic rate slows down as the body no longer needs to find all that extra energy to feed an inefficient brain. So what of the effects of the metabolic rate, if any, on the rate of perception, I wonder? What would it mean for a 2 year old child's experience of the speed of light, compared to the experience of a fully grown adult's?

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