Artificial Consciousness/Neural Correlates/Neural Models/Biological Model
The Biological Model of the Neuron
[edit | edit source]Neurons like every other cell in the human body, start out as stem cells, and differentiate according to patterns laid down in the DNA and signals that are transferred from cell to cell, and are later produced in glands that guide the general development of the body. This pattern is echoed in other multicellular animals, as well, and in fact there is some overlap between the developmental stages that such organisms go through, that suggest that there is some commonality in the development across species. We don't know all the rules for how DNA translates into the phenotype of any particular species, but this is an interesting topic for research to this day.
From the point of view of Biology, a Neuron is first of all a cell, which means that it has to do all the things that cells do, in order to stay alive. It is only once these basic cellular functions are out of the way, that energy can be spent on doing the things the cell is specialized for, propagating signals, storing memory and processing the signals. Neurons are however not alone in their need to do functions for survival first of the cell, and second of the organism, but are helped by a special type of helper cell called a Glial Cell. Probably this is because the roles of propagations of signals, storage of memory, and processing of signals require just a little more from the cell, that it would be able to achieve unassisted, at least in high demand areas such as the brain.
A Cell, must be able to protect its internal environment from attack, loss of nutrients due to diffusion, build up of wastes produced inside the cell, punctures, and damage of its external protective layer, starvation when food supplies are scarce, etc. It must, at least in a multi-cellular animal, be able to communicate with other cells in order to know what particular DNA configuration is correct for it, and be able to signal for help when it is hungry, when it is sick, or when it is dying.
Internal to the cell, there must be a structure or framework on which to attach, the parts of the cell, so that they don't damage each other, and to give it some structural strength when it is deformed. In order to digest food particles, it needs to have some way of forming vessicles, so that food can be ingested, it needs to have a chemical mechanism that breaks down food into smaller chemical structures, and rebuilds them into useful elements, including some way of efficiently turning glucose into chemical fuels such as ATP. It also needs to have some way of getting rid of larger waste particles such as viruses, or indigestible chunks it mistook for food. Mechanisms such as attaching of vessicles to the cell wall, and breaking them open to release their contents back into the extracellular fluid are important.
Although the natural shape of a cell, is spherical, some cells have to build structures within the body that hold it together, and support its weight. Other cells have to extend over distances or distribute themselves or their signals among a number of other cells. To do this they need to be able to adjust their structure, by extending some of the framework more and retracting other parts of the framework, in order to build a specific shape to the cell. Within the cell, such structures are used to separate the two halves of the cell during cell fission. Although a neuron will not have to secrete bone calcium like a bone cell, almost everything I have discussed is as true of Nerve cells as other cells in other places in the body.
Cells in general are complex, and Nerve cells are not simpler, however some of the functions that normal cells do, have been reworked in neurons, in order to allow them to do their job. Mechanisms like Ion channels that in the normal cell, are merely ways of attracting especially valuable ions from the extracellular fluid, and allowing them to penetrate the cell wall, or ways of secreting simpler Ionic wastes, take on new roles in the Neuron, pumping electrostatic charge either into or out of the cell, creating currents that develop a voltage within the cell called the Action Potential. The cell wall that was created to keep the extracellular fluid from diffusing nutrients away from the cell, has a capacitive effect that keeps the Voltage from leaking out of the cell, allowing it to build up until it finally exceeds the membranes dielectric constant, and overcomes the capacitance of the membrane, forcing it to depolarize. This in turn causes reflexive turning on of the Potassium ion channels in the cell, causing a characteristic spike that we call the firing of the neuron. In all probability this reflexive pattern exists not only in neurons but in other cells as well, as a protective mechanism to repair the polarity of the cellular membrane when it is breached, but is noticed in Neurons because the function is exercised every time the neuron fires.
The growth of extensions of the cell, becomes a mechanism whereby cells connect, the attachment of vessicles to the Cellular Membrane a method by which special Neuro-Transmitters are secreted. Some functions are new, like the complex ion channels which trigger faciliation as well as long-term memory formation, the particular permease molecules that detect specific Neuro-Transmitters etc. but many of the mechanisms that the neuron uses are reworked from normal cellular processes.
It is not what is done that is different it is when it is done, and why it is done that has changed.