Piero Scaruffi(Copyright © 2013 Piero Scaruffi | Legal restrictions )
These are excerpts and elaborations from my book "The Nature of Consciousness"
The Life Of Neurons
In 1891 the Spanish anatomist Santiago Ramon y Cajal proves that the nerve cell (the neuron) is the elementary unit of processing in the brain, receiving inputs from other neurons via the dendrites and sending its output to other neurons via the axon. In 1905 Keith Lucas demonstrated that below a certain threshold of stimulation a nerve does not respond to a stimulus and, once the threshold is reached, the nerve continues to respond by the same fixed amount no matter how strong the stimulus is.
A brain is made of neurons (nerve cells) which communicate via junctions called synapses. Neurons are the largest cells in the human body. Neurons are extremely simple units that can be viewed as switches. What creates the complexity of the brain is the synapses that connect the neurons. A human brain has about 100 billion neurons (50% in the cortex), with an average of 10,000 synapses per neuron, which yields about 500 trillion synapses. Neurons die and are born all the time. Synapses are destroyed, created and modified even more rapidly.
The Italian biologist Luigi Galvani originally suggested that nerve cells were conductors of electricity already in 1771, but it wasn't until 1924 that the electrical activity of the brain was recorded (by Hans Berger, the first electroencephalograms). In 1921 Otto Loewi demonstrated chemical transmission of nerve impulses, proving that nerves can excite muscles via chemical reactions (notably acetylcholine) and not just electricity. In the 1940s the Australian neurologist John Eccles, the German neurologist Bernard Katz and others clarified how neurons communicate via chemicals, generally referred to as "neurotransmitters" (originally discovered in 1921 by the Austrian pharmacologist Otto Loewi).
Under appropriate conditions, a neuron emits an action potential, which a synapse converts into a neurotransmitter and sends to other neurons. More precisely, neurotransmitters are synthesized by the cell's body and stored in the synapses until a nerve pulse is generated. This chemical messenger can either excite (start firing an action potential of its own) or inhibit (stop firing the action potential) each receiving neuron. Neurons are binary machines: either they fire, or they don't. If they fire, they release always the same amount of neurotransmitter. Each neuron can synthesize and therefore release only one kind of neurotransmitter. There are about fifty kinds. Each neurotransmitter has a particular effect on receiving neurons and can therefore yield a different "pathway" within the brain.
Neurotransmitters do matter. Each seems to have a different kind of message. For example, endorphins are related to pain. Each neuron only releases one kind of chemical (which can contain more than one neurotransmitter, but is always the same combination). A neurotransmitter can be received only by appropriate receptors. De facto, each neurotransmitter creates a sub-net of neurons. Each seems to contribute to different aspects of mental life.
The “intelligence” of the brain is due to the high number of connections, which cause a simple signal to generate a very complex chain reaction of activation of neurons around the brain.
This intelligence would be pointless if it didn't relate to the rest of the body. The nervous system extends throughout the body via nerve fibers. There are two kinds of nerve fibers and they are both one-way only: "afferent" nerves connect the senses to the brain, and "efferent" nerves connect the brain to the muscles.
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