Thursday, July 23, 2015

The Brain, Anatomy and Evolution

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Brain, by Pearson Scott Foresman

M: We cannot see the mind but we can see the brain. To understand the mind, we can start by examining the brain from the outside for a broad view, or from the inside for a close view, and from the time side for a chronological view.

From the outside, the human brain has 4 regions: 1. the big brain (cerebrum), 2. the small brain (cerebellum), 3. the brainstem (midbrain + pons + medulla), 4. the interbrain (diencephalon, the region between cerebrum and brainstem, not labeled).

The cerebral cortex, or the outer layer of the big brain cerebrum, is what commonly pictured as the brain. But it is actually only part of the inseparable central nervous system that also includes the spinal cord.

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Nervous System Diagram by William Crochot.

The brainstem goes down and becomes the spinal cord. The spinal cord is inside the spine vertebrate. At each segment of the vertebral column, the spinal cord sends out spinal nerves that connect to peripheral nerves that branch out to nerve endings that connect to all the organs (heart, lungs…), muscles, and circulatory and gland systems (blood, hormone, lymph). The nerve endings at the skin give us sensations of outside temperature, pressure, itchiness, pain, and others. The other four sense organs (eyes, ears, nose, tongue) connect directly to the brain via cranial nerves without going through the spinal cord.

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Tree fern (Dicksonia antarctica) by Evelyn Simak.

P: I see it the other way around. Instead of the complex brain connecting down to the spinal cord and spreading out as simple nerves, it is the root-like peripheral nerves gathering together to become the trunk-like spinal cord, and then sprouting out a growth called the brain, like a bud coming out of a botanical branch that is rooted under the earth. The tip of a tree fern even looks somewhat like to a brain. This order of progression is similar to Lamarck's theory that organisms evolve up the ladder of complexity and organization instead of down to the lower rungs of simplicity.

M: Maybe there is something valid in what you say. We can always boldly assume, but need to carefully verify. Evolution of the brain is not straightforward like everything else. It’s hard to know what a pre-brain was like because of the punctuated equilibrium situation. In punctuated equilibrium, the transitions are step-like - changes taking place quickly and fundamentally. Birds evolved from pterosaurs, a species of the dinosaurs. But who knows what pterosaurs really looked like. You can hardly guess how a turkey’s head look like from its skull. So it is also hard to guess the evolution of central nervous system from the fossil skeletons. Besides, which ones are the pre-human skeletons anyway?

P: There is a YouTube video showing the growth of the city of Las Vegas from satellite photos year-by-year. Some comments say that it looks like cancer. That is what makes me think of the brain as a tumor growing out of the spine. I don’t mean the brain is harmful to the body like a tumor. Just that it is a phenomenal growth fueled by available resources. Some say the brain uses more energy than any other organ in the body. But the brain also helps the body to take in more food and energy and use them efficiently. So it is a square deal.

M: If the resource income is greater than the expenditure, then there will be growth. Cities can grow because there are jobs and economic opportunities that attract people to move in and stay. It is all about resource development and usage. How does the growth of brain help the body develop resource income and use that efficiently?
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Circular flow of income and expenditure
by Bureau of Economic Analysis (BEA), U.S. Department of Commerce

P: Are we talking about economics now? Actually, come to think of it, I do believe that economic advantages are what drive the brain to grow so large. The dictionary says ‘economics’ means laws of household, and ‘ecology’ is the study of environment. The prefix eco- is household or environment, the suffix -logy study, and -nomics law. I didn’t see how economics and ecology are related. But now I see that they are both about what is going on in an environment of inhabitants. Ecology describes how the inhabitants interact with each other hierarchically and establish qualitative relationships like predator-prey or boss-worker. Economics describes how the inhabitants trade with each other and build quantitative relationships like producer-seller or marketer-shopper. Anyway, it is a simple way for me to think of them.

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Cross section of jellyfish, by Mariana Ruiz Villarreal LadyofHats.

What is the economic advantage of a big brain? The economic advantage is that the brain can help the body get more food (and other things) than the body can without the brain. It does so by using information from the senses (eyes, ears, nose, tongue, skin) to control and coordinate muscular/gland activities, and thereby capture more food. Look at how the primitive humans hunted with weapon and in group, not to mention that they made tools and built vegetable garden or animal farm. That takes much coordination and planning from the brain. Simple living beings like the jellyfish don’t have nearly as much development and advantage. They have no brain, only some loose network of neurons around the skin and the digestive chamber.

Our spinal cord probably shares similar functions as the jellyfish’s nerve net. It gets sensory information from the skin and reacts in a primitive way, such as the kicking reflex when a knee is tapped, or the secretion of digestive juices when food is in the alimentary canal.

But with the brain comes into the picture, some of the simple reflex reactions can advance to more finely tuned secondary controls. One example is the autonomic nervous system. This is an energize/relax control system based on the nerves coming from the brainstem and diencephalon. The autonomic nervous system is to maintain homeostasis of organ functions like breathing, digestion, circulation, metabolic rates. It is a secondary control in the sense that it is indirect. Direct control can be nerve impulses causing organ to contract. Indirect control can be nerves stimulating endocrine glands like pancreas or adrenal glands to release hormones into the bloodstream. The hormones then reach the organs to induce muscular or biological reactions for holistic homeostatic balancing.

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PID Controller, by Arturo Urquizo.

Secondary controls are to help primary controls to reach the goal more precisely. They evolve later after the primary controls. So the brain comes after the spinal cord, which comes after ganglia, which comes after neural net like those in jellyfish. Speaking of controls, engineers often use PID controllers for precision process/plant control. PID stands for proportional-integral-differential. Proportional control is a simple primary control based on the current sensor datum. Integral-differential controls are complex secondary controls. They are based on the sums and the trend of sensor data. By controlling the trend of data history, a finer, more stable and less oscillating control is reached than simple primary controls. This may be the case of cerebellum, the little brain, which maintains body balance and coordinates motor movements with sensory inputs.

Towards the end of the cerebellum video, it shows a loop where nerve impulses travel around the motor cortex, cerebellum, and bicep muscle to actuate a motion. With our negative feedback loop model, we can describe how the movement control works. First, visual information sets a reference target position for the hand to move to. Then, muscle and vision give information of where the hand position is. The governor, whether it is the cerebellum or the motor cortex of the cerebrum or both, is acting on the difference of the hand position and the reference target position and minimizing that difference.

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The governor iteratively moves the muscles to reduce positional differences between the limb and the target. This is the movement sequence a baby learns to get hold of something or to walk. It is a primary proportional control. To maintain balance while walking is a secondary control on top of the primary control. It deals with reducing the shift of center of gravity while walking. With practice a baby will develop muscle memories for such movements so he can do it without looking or paying attention.

The sequence of primary negative feedback control is also how the missile system works. A radar senses a target’s position and sets that as a reference position. Other sensors calculates the position of the missile and the difference between it and the target. The governor, a computer getting the sensory difference data, directs the missile in the direction of the target to cut down their positional difference, till the difference becomes zero and the target is reached.

Movement control can be based on vision or echolocation (bat, radar, sonar). A blind baby can learn to walk with a cane without seeing. So it can be based on the sense of touch as well. What is special is that practice can turn movement controls into muscle memory. And movement controls can have a secondary component that is to govern the trend of positional differences. It is the memory of trends that account for how Michael Jordan doing jump shots or Tom Brady throwing football. It is to aim at a (relatively) moving target by throwing the ball to a position not where the target is at currently, but to where it will be in the future by the trend of movements. That is a secondary control.

The cerebellum regulates body balance and motor movements. What about the big brain cerebrum, does it regulate the senses? After all, eyes, ears, nose, tongue all have threshold mechanisms that are controlled by the brain. It regulates vocal communication and the formation of perceptions as well. Sight, hearing, smell, taste, speech, body language are all manufactured in the brain. Whatever the cerebrum is about, the evolution of the senses/brain must offer some advantages (economic and otherwise) to the whole body so that the body can nurture in return the growth and specialization of brain/senses, thereby ensure the survival of both parts.

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Iguana parietal eye by SurreyJohn.

M: Speaking of vision, there may be a clue to the genesis of the eyes. That is the parietal eye. Some fish, amphibians, and reptiles have a parietal eye, or third eye, on their head. This eye is sensitive to light and can provide visual cues to the brain. But it is not functioning like the regular eyes. An 8-week old human embryo has a pre-brain in an exposed pre-skull. The skull roof, the parietal bones, has not formed yet. It will form later to cover up the brain. However, some species have parietal bones not fused together during formation, leaving an opening at the top of the head after birth. The parietal eye is located at that opening. Lizards, frogs, and lamprey eels have such third eyes.

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Human embryo 8 weeks by Anatomist90.

In fact, all human sense organs other than the skin are located at the cavities of the skull. This raises the possibility that the origin of sense organs in the head starts from some cranial nerves being exposed to the outside environment where the skull bones are not formed to enclose the brain and shut out direct contacts. If the brain is, like you said, the latest growth in the central nervous system, then these cranial nerves are extended from the brainstem. The brainstem grows out of the spinal cord. The spinal cord aggregates from nerves of the rest of the body. Somehow, the exposure of the newly formed cranial nerves to the outside environment might have started a differentiation process that ended up as sense organs and the brain.

Actually, I think that is a good possibility. To illustrate this differentiation possibility, let me use a story of some explorers discovering an uninhabited bay that is filled to brim with fish and sea creatures. People hear about this discovery, so a few of them come and settle here to harvest the fruits of the sea. The bay is so abundant that more and more people arrive. And differentiation begins to go into high gear. The bay becomes lined with fishing boats and fishermen. Behind the bay grows mixed population and buildings and interlocking businesses. They pop out like mushrooms, serving the needs of the fishing industry and the community. And it keeps growing and differentiating until the whole place and interactions are stabilized. In this analogy, the explorers are the exposed cranial nerves. The bay is the outside environment. The fishing boats / fishermen are the sense organs. The bay area community growths are the morel-like cerebrum and cerebellum.

This story is like conjectures made by detectives to piece together clues to reconstruct what has happened at the crime scene. It is not the truth, but it may be a possibility that can lead to other possibilities that are closer to the truth. Anyway, how can some exposed nerve cells evolved into sense organs like the eyes? Well, it can possibly happen by a differentiation process of positive feedback or schismogenesis. What that positive feedback process needs is a difference as the trigger, and a circuit of active systems that can amplify and feedback that difference mutually and continuously.

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Positive Feedback leads to Differentiation.

The contact between the nerve cells and the outside environment can introduce such a difference. Einstein’s thesis on the Photoelectric Effect shows that light quanta can induce electric conduction in unconnected metals. In the absence of light, unconnected metals cannot conduct electricity. By this photoelectric effect, nerve cells closer to the outside light source can have a different electric impulse conduction than the nerve cells behind them. This photoelectric difference may be the trigger. And even if the photoelectric effect has not made the difference, then some other yet discovered factor(s) must have, since all living things exposed to a new environment do undergo changes. And those changes can lead to other changes, and on and on till stability is reached.

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Brain Vesicle Development Differentiation by OpenStax College

Once a difference exists, a positive feedback mechanism can start to enhance and reinforce it, provided that energy supplies are available to the systems to keep the feedback loop going, and the difference feedbacks do not reach the level of saturation or system fatigue. The spinal cord nerves are surrounded by nutrients-rich blood vessels. Blood cells are even made in the vertebrate bone marrow! That takes care of energy supply for the feedback systems. In the analogy of a bay turning into a fishing village, resources are pouring in to make the fishing technology more specialized and complex for greater harvest yield, along with creating a growth of people and accommodations and organizations to service the fishing technology. If a similar economic advantage holds true in the evolution of eyes, then nourishment would have made the exposed nerve cells more efficient at being photoreceptors, while also created growth of additional nerve structures such as brain lobes to service the information coming from the photoreceptors. Anyway, economic advantages of having sharp sensors are self evident. Look at the spies of the CIA or the Wall Street. They are highly specialized sensors in the ecology of national security or the stock market.

I agree with you that the evolution of senses is directed by economic advantages and differentiation. Or at least partly so. The economic advantages lie in the ability to perceive and to respond to changes in the environment. Sense organs are the first step to establish connections between the outside world and the inside organs. The connections have stimulated more neural growth to handle perceptions and responses. That goes on at the cell level in the brain as well. They are synapses of the neurons, which are what we will look at from the inside.