“Neurons that fire together wire together” As an engineer, I had pictured the brain as hardware and the mind as software that runs on top. But this representation changed 3 years ago after I read some articles about neuroplasticity, and since then I started my research in Neuroscience and have recently attended a computational neuroscience course at MIT. In this article, I will present the engineering architecture of the brain and how the maps in the brain change due to new experiences.
As you see in the architecture, I positioned the Amygdala in the centre of the brain. Although it’s a small part of the big brain, but it’s the most important as it receives information from our 5 senses that were defined by Aristotle and responds accordingly. This Amygdala is coded differently between humans and the embedded code can change over time due to new experiences. There is a very complicated system in the background that is processing the code and taking decisions, this system is not only composed of the amygdala but also the neocortex. Why I am studying neuroscience today and why I did engineering yesterday? The answers lie in the Neocortex, the frontal cortex around the limbic system, and the limbic system itself that includes Amygdala.
Trying to understand how the brain works, I will start first with the basics of Neurons and synapses. Neurons are discrete cells, a leaky bag of charged liquid made of dendrites that receive inputs from other neurons. These dendrites lead to cell body which sustains the life of the cell and contains its DNA. Information flows from the dendrites to the axon via the cell body as shown in the architecture. The Ionic channels allow ions to path through, which give rise to action potential and spikes. Brain information is stored in the physical and chemical structure of neurons and synapses. Neurons don’t use LinkedIn to connect with each other, they use synapses. When the dendrites receive a lot of excitatory signals, will fire a signal through the axon to a dendrite of another neuron but it doesn’t touch the dendrite because of the synapse that separates neurons from each other The synapses are the basis for memory and learning in the brain, all the memories and learning are stored in the synapses. The synapse release neuro-transmitter which causes ionic channels to open or close which in turn change the membrane potential. So memories are stored in the synapse but what about perceptions? This is beyond the synapse; it’s a network of neurons that give rise to perception, behavior and consciousness.
To get a glimpse of how the brain generates behavior, let’s look at the computational models. There are three models used in computational neuroscience: The descriptive model for characterizing what nervous system do through encoding and decoding, the mechanistic model for determining how they function by simulating a network of neurons and the interpretive models for understanding why do brain circuits operate the way they do. In all these models, the receptive field is the base. When we look at an object, the information from the retina is passed to the Lateral Geniculate Nucleus LGN and this passes information to the back of the brain to an area called the primary visual cortex V1. The mechanistic model shows how do receptive fields (LGN cells) constructed using the neural circuitry of the visual cortex to form V1 cell and the interpretive model shows why are the receptive fields in V1 shaped in specific way. After knowing the what, how and why, we can go further to look at how neurons are connected to form recurrent network using Eigen vectors and Eigen values and then how these connections can be adapted using synaptic plasticity allowing the brain to learn about the world from its inputs and change the brain map accordingly.
After learning how the brain generates behavior, we need to understand the neurotransmitters. When the neuron fires an electric signal, it causes a release of neurotransmitter, a chemical that travels throughout the brain and binds to receptors that are attached to other neurons. I will start by the Noradrenaline, the Neuro transmitter responsible for the sympathetic nervous system. It controls the heart, vision lungs, vessels every single function in our body. Now the connection between prefrontal cortex and hippocampus is a fibre optic of noradrenaline neurons in the locus coeruleus deep inside the skull. Testosterone, on the other hand, is associated with aggressiveness and pursuit of dominance. Oxytocin is released from the pituitary gland that secretes prolactin and this affects fear. Vasopressin, on the other hand, affects relationship. Finally, the Dopamine which plays an important role in the reward motivated behavior, and Serotonin, a neurotransmitter that affects mood, that’s why antidepressant medications like Prozac and Zoloft act on serotonin receptors. People with a variant of dopamine regulating gene DRD4 take more risks in life than people a variant of serotonin regulating gene SERT.
More about dopamine – Looking at the visual system, the network of neurons in the prefrontal cortex is the basic for paying attention and they are sensitive to dopamine only. When dopamine is released, it unlocks these neurons which they start firing tiny electrical impulses that stimulate other neurons in their network. Taking the auditory system, I went to a rock festival for one of the best guitar players kurt vile, I liked some of his plays and didn’t like the others; so how the brain distinguish between consonant and dissonant sounds? When the sound comes to the eardrum, it will be split into 2 processing circuits, 1 circuit that process speech which decompose the signal to identify vowels and consonants that make the words, and 2nd circuit that process music and separately analyse pitch, timbre and rhythm; the 2 signals connect with the frontal lobe that do temporal pattern matching like in functional programming.
So what’s next? What we know about the brain is very limited and am not sure if the scientific understanding of neurons and synapses is correct. Just imagine there are 500 synapses in 1 red blood cell and there are 100s of billion cells; So you have billions of billions of synapses. To simulate one brain only, it needs trillions of computers and that’s mathematically impossible. Moreover, every brain is different and neurons in one brain are different than neurons in another. Adding more complexity, the brain is only 15% neurons and most of these are in the cerebellum not the cerebrum, 85% of the cells in the brain don’t use synapses (no dendrites, no axons) to communicate, don’t make electrical impulses, and these cells communicate with other cells that have neurons and use synapses. On a scale from 1 to 10, we are still at level 1 of brain discovery and it might take 10,000 more years to get an answer of how exactly the connections are built in this almond shape structure that’s named brain and that we carry with us every day.