I think therefore, I am, says the famous philosopher Rene Descartes. The very notion that we exist simply because we can doubt our own existence. In doing so, we express thoughts therefore are conscious of our own thoughts because we can think about them logically. This leads one into the realm of consciousness, and the very nature of human existence. We derive our consciousness as self-aware beings as being able to respond mentally and physically to our surroundings with the first influencing the latter, but how does this happen? How does information inside our brains correlate with the material world and allow us to respond to it by means of conscious thought? Does the brain work like a computer or does the biology of neurochemistry and physics affect how our mind works and how consciousness is attained? We shall see.
Although science has a few distinct theories on how the brain creates thought, we shall first examine how neurons work to send signals to one another. Neurons share information using electrical signals by changes in voltage across membranes as a function of time. Over time, chemicals of varying positive(Sodium, Potassium, and Calcium) and negative(Chloride) charges move in and out of the neuron across the synapse via channel gates and work downhill across neural networks of dendrites to send information based on the input stimuli. The chemical signal is turned into electric signals and vice versa, allowing neurotransmitters to be released across synapses. But how does the neuron know when to fire?
The average voltage of a neuron cell at rest is approximately - 65-70mV(millivolts). Dendrites receive signals from other neurons via neurotransmitters which bind to receptors on the dendrite which produces a chemical signal. As the binding takes place, it opens ion channels that allow charged ions to flow in and out of the cell of the neuron which converts the chemical signal into that of an electrical signal. If there is net influx of positive charges in the neuron it is known as an excitatory post-synaptic potential which leads to depolarization. In contrast, if the cell has a net flux of negatively charged ions, it's known as an inhibitory post-synaptic potential. Because the neuron is connected to countless other dendrites this can shift the polarity of the neuron towards depolarization as more and more positively charged ions enter the cell. If the voltage in the neuron reaches -55mv, it triggers voltage gates of Sodium channels to open, which then triggers more channels to open at the Axon Hillock, causing a chain reaction in the axon itself. This chain reaction is the action potential, or when the neuron fires. When this happens the cell becomes positively charged, reaching up to 40mv. At this point the Sodium ion voltage gate becomes inactivated, and they no longer flow into the cell. After the Sodium ion voltage gate channels become inactivated, Potassium voltage gates channels open up, causing positive charge to leave the cell which causes the cell membrane to repolarize. During this repolarization phase, a Sodium/Potassium pump begins functioning. This pump moves three Sodium ions out of the cell and two Potassium ions into the cell. This is known as the absolute refractory period and keeps the action potential, or firing of the neuron from happening too close together and keeps the flow of electric charge in one direction. The combination of the voltage gate channels of Potassium leaving the cell and the actions of the Sodium/Potassium pump leads to a period of over correction. This causes the cell to lose charge known as the relative refractory period when the overall charge of the cell is less relative to the initial resting cell membrane. When the Potassium ion voltage gated channels close, the neuron goes back to resting membrane potential again.
As we move this story forward, axons connect neurons and act as wires that send electrical signals from one neuron to the next at their synapses. Myelian Sheath acts as a protective layer around the axons much like insulation in a conductive wire. Between the myelian are what is referred to as Nodes of Ranier. These nodes contain very high densities of voltage gated sodium and potassium unlike the myelinated segments which don’t allow the ions to pass through. Since the action potential can’t be regenerated in the myelin, the action potential or firing takes place from node to node known as saltatory conduction. As depolarization takes place( positive charges move from outside to inside the axon), it opens the gates to the next local voltage gated channel further moving the electrical signal along. This leads to further action potentials taking place until the signal reaches the synapse and the original process takes place all over again. In the end, it's a continuous loop of chemical→electrical→chemical→electrical signals that guide information transfer from one neuron to another.
Now that we understand the transmission of chemical and electrical signals in the neuron of the brain, we can discuss thought. Thought doesn’t arrive from simply one neuron, but a collection of billions of cells across trillions of synapses. As the neurons interact with sensory input(external stimuli or the environment), they combine past experiences(memory), emotions, and attention to form thought. These factors integrate with brain circuits of the cortex, thalamus, and hippocampus to form thought. Thought doesn’t just come from chemicals or the voltage differences within the neuron cell and its axons and dendrites, but the brain’s information processing.
Populations of neurons form patterns of activity that represent sensory features, memories, or abstract concepts. Due to the plasticity of the brain, these patterns change over time as the brain: compares incoming stimuli with stored memories(hippocampus, cortex), weighs options(prefrontal cortex, basal ganglia), and evaluates importance via emotion/reward circuits(amygdala/ dopamine system).
Communication between areas of the brain often involves rhythmic oscillations(brain waves) that help coordinate timing. When thousands or millions of neurons fire in a coordinated rhythm, their tiny electrical currents add up to produce oscillations or waves in the brain’s overall electric field. Based on these waves, it dictates the state of being of the person from deep sleep to being at attention.
Now let's compare a computer to that of the human brain and its neural networks. Information flows through both the brain and a computer, but through different mediums. For the brain, information flows through neurons and neural networks while information in a computer flows through transistors. Although similar, information flows much faster in a computer than that of the human brain. In addition, you can compare the logic gates of computer to that of channel gates of a neuron. Neurons can have action potentials or no action potentials just as logic gates are either 0 or 1. Both take the sum of inputs, and if the action exceeds a threshold, fires an output spike. Nonetheless, the brain and the computer are inherently different. The brain is able to change, grow, and adapt over time while a computer is set based on instructions given to it. To change the computer, you must change the code, while the human brain rewires itself as we learn more external information. In the end, the brain is an information processor, but not a computer.
And so remains the burning questions. Is consciousness derived from neurochemistry and physics, or is there a kind of dualism that separates the mind and conscious thought from the inner workings of the brain and its neural function? Some say consciousness pervades all matter throughout the cosmos, and its thought that brought everything into existence. Are neuro-networks entangled allowing quantum processes to take place inside the brain, or is there too much decoherence or background noise that breaks down entanglement? These are the burning questions that lead me to sit and ponder the nature of consciousness, the human brain, and the universe.
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