So, what is the physical seed of thought? What is the source of our emotions, or decision-making, our passions, or pains, and everything else? Well, it’s the brain, and it’s set to be the most complex mechanism in the known universe! Ever wondered what goes on inside the brain, how does it work? How is it able to store and process information as fast and as accurately? What is the secret behind this supreme organ: the human brain.
It’s the neurons that are responsible for transporting information from one part to the other in a brain. They make the brain, a piece of gross looking meat work and tick. The brain is made up of millions of neurons, scientists still don’t know how many. But they do have some idea as to how they work and allow us to think. It’s not an accident that they call the biological study of thought as neuroscience.
There’s a lot of chemical stuff happening in the human brain but we’re going to focus on the neurons.
Neurons are cells within the nervous system that transmit information to other nerve cells, muscle, or gland cells. Most neurons have a cell body, an axon, and dendrites. So you can see the dendrites. Dendrites receive signals from other neurons. Either excitatory, like pluses, or inhibitory, minuses. Then they get to the cell body, which sums up these pluses and minuses. When you reach a certain threshold, a certain amount of pluses, there’s neural firing. Firing takes place through the axon, and the axon is much longer than the dendrites. In fact, for some motor neurons, it’s very long indeed. There are axons running from your spinal cord, all the way to your big toe. You could think of it of the relative sizes of things in terms of a basketball, and a 40-mile garden hose. Surrounding the axon is what’s called a myelin sheath. The myelin sheath is- you can think of it as insulation, as fatty tissue like insulation on a wire. So, the information comes through the dendrites and summed up in the cell body, and it’s transmitted through the axon. So, what neurons do, is they sum up and transmit information, and we know that there’s a lot of them. By some estimates, it’s 100 billion, or the estimates tend to be very different and very rough, but there are billions upon billions of neurons, and each connects to thousands, maybe tens of thousands of other neurons. About 100 billion neurons are each firing off 5-50 messages (signals) per second. So you can imagine the degree of complexity in its structure and working it’s difficult to imagine in any other machine or any other organ. The way the human brain works is close to a miracle.
How do neurons communicate?: Your brain receives information about where you are in space, or the brightness of the light, and responds accordingly. So, the brain controls everything: your emotions, your perception, your movements and You!
There are sensory neurons, which take in information from the environment, from the external world. There are motor neurons, which go from the brain out to your motor control. So, if you touch something hot, and you feel the pain, that is sensory neurons, if you rent your hand back, or you reach for something, that’s motor neurons. Finally, there are interneurons, which connect different neurons without making contact with the external world, either through sensation or through motor action. Now, the main thing to think about for neurons and neuron firing is that it’s all or nothing. It’s like firing a gun or sneezing. Neurons either fire or they don’t. Excitatory neurotransmitters cause the signal to propagate – more action potentials are triggered. Inhibitory signals work to cancel the signal. The way we get to the continuity of experience is that neurons can code for intensity in different ways. So, one way is in terms of the number of neurons that fire. If x neurons correspond to a mild experience, x times 10 neurons may correspond to an intense experience. Another factor is the impulse frequency of individual neurons, an individual neuron might denote a mild sensation by doing fire, fire, fire, fire. Well, it might denote an intense situation with fire, fire, fire, fire, fire, fire.
There is a gap between the axon terminal of one neuron, and the dendrite of another one. A very tiny gap, typically of like 1/110,000 of a meter wide. This gap is known as a synapse. When one neuron fires, the axon releases neurotransmitters, these are chemicals that shoot out over that gap and affect dendrites and other neurons. The effect of these neurotransmitters could be excitatory, which is that they raise the energy, so they increase the likelihood of a neuron firing, or inhibitory. So that they bring down the likelihood of a neuron firing. What’s interesting is that different neurons shoot out different neurotransmitters. So, they have different effects on other neurons that they made contact with.
Neurotransmitters: Neurotransmitters are synthesized by neurons and are stored in vesicles, which typically are located in the axon’s terminal end, also known as the presynaptic terminal. The presynaptic terminal is separated from the neuron or muscle or gland cell onto which it impinges by a gap called the synaptic cleft. In milliseconds, they travel from one end of a neuron to the other neuron. In many instances, the change in permeability results in depolarization, causing the cell to produce its own action potential, thereby initiating an electrical impulse. In other cases, the change leads to hyperpolarization, which prevents the generation of an action potential by the second cell.
The commonly known neurotransmitters are Dopamine and Serotonin, which are also found in drugs and medications. High levels of dopamine consumed through drugs can make you feel ‘high’ and happy. Some anti-depressants contain serotonin, as a lack of serotonin production can make an individual feel sad or depressed. Therefore, your emotions, your activity level, and energy are dependent on the production of neurotransmitters. You, your joys, your sorrows, your memories, and your ambitions, your sense of personal identity and free will, are in fact no more than the behavior of a vast assembly of nerve cells and their associated molecules and chemicals.
There’s a quote by Thomas Huxley: “How is it that anything so remarkable as a state of consciousness comes about as a result of irritating nervous tissue. That question is just as unaccountable as the appearance of the genie when Aladdin rubbed his lamp.” What he’s trying to say that, it’s like magic that a physical structure irritated by neurotransmitters swooshing back and forth and electrical signals running across neurons, how that gives rise to feelings. What neuroscience tells us is that our decisions, our activities are nothing more than neural firings, communicated through the transmitters.