Animal Physiology
Many small organisms obtain oxygen by diffusion through their body surfaces, without having any special respiratory organs and without circulating blood. Larger and more complex animals often have specialized surfaces for gas exchange and also a blood system to transport oxygen more rapidly than diffusion alone can provide. (p. 16)
Comment: Simple organisms only need a market for gas exchange. Complex and large animals need more regulated systems.
Although a small organism can get enough oxygen by diffusion through the surface, this is usually not true for larger organisms. ... We find specialized respiratory organs with greatly enlarged surfaces. Often these organs also have a thinner cuticle than other parts of the body, thus facilitating gas exchange.
Comment: Thinner cuticles also makes microbes easy to enter the body. This is why respiratory systems are easy to be invaded by pathogens. This is also why auto immune disease often occur in respiratory systems, such as nose allergy and asthma. It is a tradeoff.
The water may leave the gill having lost as much as 80 or 90% of its initial oxygen content. ... In contrast, mammals remove only about one-quarter of the oxygen present in the lung air before it exhaled. (p. 21)
Comment: Oxygen is scarce in water and abundant in air. That is why oxygen absorption is more efficient (and more expensive) in water.
Small animals have a higher metabolic rate per gram than large animals. We may think low fixed cost systems have higher discount rates.
Study oxygen dissociation curve (p. 69) more. Higher affinity makes lung get oxygen easier. Lower affinity makes oxygen part easier into the tissues. This is a tradeoff. Some factors, such as carbon dioxide level can affect this tradeoff. Think more about this problem. Put it into discussion between use to use and easy to store in my Economy of Nature paper.
The toadfish, whose curve lies to the extreme left, is a slow-moving and relatively sluggish bottom-living fish. It is often found in less well-oxygenated water, and it is highly tolerant of oxygen deprivation. The hemoglobin of the toadfish has a particularly high oxygen affinity, and this is in accord both with the environment in which the fish normally lives and with its relatively slow metabolic rate, the demands on the loading environment apparently more important than the unloading in the tissues. (p. 77)
Comment: When environment is poor, loading is more important than unloading. When environment is rich, unloading is more important. In our human society, at least in wealthy countries, unloading is much more important than loading because of abundant resource consumption. This causes the economic theory mainly focused on customer satisfaction and the end result, hence the theory of market economy and neoclassical economics. In our theory, both fixed cost and variable cost are considered. Add this into Economy of Nature.
The comparison between closed systems and open systems in circulation systems. Vertebrate are closed while many invertebrates are open. In closed systems, distribution to different organs well regulated. In open systems, distribution of blood less readily regulated. (p. 93) This means that complex systems are often more regulated than simple systems. Apply this to Market and Regulation.
The major evolutionary change in lungfish is that, in addition to gills, they have lungs as respiratory organs. The gills in part receive blood that has already passed through the lungs. If the gills were similar to the gills of ordinary fishes, this might be a disadvantage, for a lungfish swimming in oxygen-depleted water would then lose oxygen from the blood to the water that flows over the gills. The lungfish gills, however, have degenerated, and some of the gill arches permit a direct through flow of blood. (p. 98)
Comment: Only very few species have both lungs and gills. This suggest a hybrid system may only have very small niches. This suggests that hybrid cars may only have very limited niches. Add this to Economy of nature.
Earlier in this chapter we discussed how an increased oxygen capacity of the blood, caused by the presence of a respiratory pigment, reduces the volume of blood that must be pumped to supply oxygen to the tissues. Crabs provide a good demonstration of this principle because of the wide species- to- species variations in the hemocyanin content of their blood. Figure 3.26 shows that higher the hemocyanin content of the blood is, the less blood is pumped to supply the tissue with a given amount of oxygen.
The same principle applies, of course, to the demand on the circulation of all animals that have a respiratory pigment: The higher the oxygen capacity of the blood, the less volume needs to be pumped. There is a trade-off here between the cost of providing the respiratory pigment and the cost of pumping, and the question is, Which strategy pays best? It seems that for highly active animals a high oxygen capacity is most important; for slow and sluggish animals it may be more economical to avoid a heavy investment in the synthesis of high concentrations of a respiratory pigment. (p. 120)
Comment: This is another example of fixed cost, variable cost trade-off. For high output systems (highly active animals) investment is fixed cost (respiratory pigment) is favored while for low output systems (slow and sluggish animals) high variable cost (more pumping) is preferred. Pumping is variable cost compared with respiratory pigment because respiratory pigment lasts longer.
The rate of metabolic reaction is greatly influenced by the temperature. An increased temperature speeds the rate. Heat usually also inactivates enzymes; at temperature above 50C or so, most enzemic actions are completely destroyed. Therefore, a moderate increase in temperature gives an increased rate of reaction, but when the temperature increased further, thermal destruction of the enzyme catches up with the accelerated reaction rate. At some temperature we will observe a maximum, but because thermal destruction increases with time, we will find that the optimal temperature depends on the duration of the experiment. In a long lasting experiment the temperature effect on the enzyme will have proceeded further, and the observed temperature optimum will therefore be lower; in a short experiment with the same enzeme the temperature optimum will be higher. Thus the temperature optimum is not a specific characteristic of an enzeme; it depends on the duration of the experiment. (p. 139)
Comment: Optimum is duration dependant. We can use it in duration and optimum part. Add to Economy of Nature.
The second advantage of the ruminant system is that the mechanical breakdown of the food can be carried much further, for coarse and undigested particles can be regurgitated and masticated over and over again. This difference is clearly visible if we compare the fecal material of cattle (ruminants) and horses (nonruminants). Horse feces contain coarse intact fragments of the food; cow feces are well-ground-up, smooth mass with few large visible fragments. (p. 147)
Comment: Horses are much stronger and faster than cattle.
For a system to be strong and fast, it will not digest low quality food. So
level f waste (low quality food) is much higher in horses. This can be
understood from Carnot's principle. When the temperature of heat sources
decreases somewhat after initial use, one can either recycle the lower
temperature heat source for further use, or dump the lower temperature heat
source away and use more high heat sources. If we recycle, the engine efficiency
will be lower but the total energy efficiency will be higher. If we do not recycle,
the engine efficiency will be higher and the total energy use will be higher as
well. In an affluent society, horses will be preferred over cattle. In a stable
and saturated society, cattle will be preferred over horses. This is indeed the
case. Cattle were the predominant animal of field labor in the old world while
horses were the predominant filed animal in the
Not all proteins have the same turnover rate. In humans, for example, the half-life of serum proteins is approximately 10 days, but connective tissues has a low rate of protein turnover. Once these latter structural proteins are formed, they remain relatively stable compared with proteins of metabolically active tissues such as blood, liver, muscle, and so on. (p. 152)
Comment: More use leads to quick wear and replacement. So total output of a production system is restrained.
Because fat yields more than twice as much energy as carbonhydrate, it is better suited for energy storage. However, there are exceptions to this rule. For animals that do not move about, such as oysters and clam, weight is of minor consequence, and glycogen is used for storage. Also many intestinal parasites, such as the roundworm Ascaris, store glycogen.
In these animals glycogen is probably a more suitable storage substance than fat, for they are frequently exposed to conditions of low or no oxygen, and in the absence of oxygen, glycogen can yield energy by breakdown to lactic acid. Therefore, as bivalves frequently closed their shells for long periods and devoid of oxygen, it is advantageous for these animals to store glycogen rather than fat. Sessile animals and parasites have little need for weight economy, and from this viewpoint the use of glycogen instead of fat is no disadvantage.
For animals that move about, however, weight economy is of great importance. Just how important this is for migrating birds, which may fly nonstop for more than 1000 km. To have enough fuel, migrating birds may carry may carry as much as 40 to 50% of their body weight as fat, if the fuel were heavier, the weight could be excessive, and long distance migration would be impossible because much of the available energy would be merely go into carrying the heavier body aloft.
Storage of glycogen involves more weight, not only because of the lower energy content of carbohydrate compared with fat, but also because glycogen is deposited in the cells with a considerable amount of water. It has been estimated that the deposition of glycogen in the cells of liver and muscle is accompanied by about 3 g water for each gram of glycogen stored. ...
The simplest way of expressing the weight of fuel relative to the energy value is to compute the weight per energy unit, or the isocaloric weight. For the fuels we have discussed, the isocaloric weights, in grams of fuel per kilocalorie, are as follows:
Fat 0.11
Protein 0.23
Starch 0.24
Glycogen + water ~1.0
Expressed in this way, it is immediately apparent that, with the extra weight of water, glycogen is some 10 times as heavy to carry as the same energy in fat.
In spite of this weight difference, glycogen is an important storage form for energy. Its advantage is twofold: It can provide fuel for carbohydrate metabolism very quickly, whereas the mobilization of fat is slow; and, perhaps more importantly, glycogen can provide energy under anoxic conditions. This is common during heavy muscular exercise when the blood does not deliver sufficient oxygen to meet demands. For long-term storage of large amounts of energy, however, glycogen is unsuited, and fat is the prefered substance. (p. 173)
Comments: I deduced most of the results before i learned about the facts. Add plants into the discussion as well. Discuss why fat cannot be mobolized quickly. Fat is hydrophobic, which helps it to lower volume in storage becuase it will not attach water, unlike glycogen. But the reaction rate is much lower because of the lack of surrounding water, which is a good solute. This also explains why heat engine is more useful than fuel cell, for gases mixture is much quicker than separation of electron, which is much more precise.
Reread physiological time (p. 207) and apply it to discuss fixed cost and duration, fixed cost and discount rate. Reread Metabolic rate and body size (p. 192) and discuss fixed cost and variable cost. Essentially reread whole Chapter 5 and incorporate into Economy of Nature. It should be expanded into a book.
Within the temperature range an animal can tolerate, the rate of oxygen consumption often increase in a fairly regular manner with increasing temperature. In general, a rise of 10C in temperature causes the rate of oxygen consumption to increase about twofold or threefold. (p. 219)
Comment: This is the same for human society. The increase of spending also increases the energy consumption. Empirical measure of spending and energy production. How to count the imbalance of power between trading partners?
The fact that the so-called primitive groups --- insectivores, marsupials, and especially monotremes --- consistently have low body temperatures raises some interesting evolutionary problems. These groups are considered very ancient and presumably have had more time than more recent groups to evolve toward a higher body temperature, if this is 'desirable" or advantageous. Have they remained in their more "primitive" stage because they have lacked the capacity to evolve in this direction? They have certainly been successful as witnessed by their ability to survive for so long.
The fact is that we do not fully understand the advantage of any given body temperature. In any event, it would be a mistake to interpret a low body temperature as a sign of "primitive" and thus inadequate temperature regulation. It has been said that the egg-laying echidna is halfway to being a cold-blooded animal and is unable to regulate its body temperature adequately. In fact, the echidna is an excellent temperature regulator and can maintain ts core temperature over a wide range of ambivient temperature down to freezing or below, although it has poor tolerance to high temperature. (p. 245)
Comment: Our theory provides a simple understanding of temperature regulation. Higher temperature represents higher fixed cost and low variable cost. Whether a system will evolve toward higher temperature is determined by whether such evolution will help improve return from such a change. Specifically, how much the increased temperature will help efficiency in catching prey and avoiding predators. For mammals of low temperature, their prey may be insects or other animals that do not run very fast. So low temperature (30 C) is sufficient for catching such slow moving prey. Avoiding predator faster may not fully compensate the cost of increasing body temperature. Anyway, our theory turn the discussion into a quantitative measure.
Diurnal animals show a temperature peak during the day and a minimum at night; nocturnal animals show the reverse pattern. However, these daily cycles are not caused directly by the alternating periods of activity and rest, for they continue even if the organism is at complete rest. (p. 243)
Comment: This shows animals keep their body temperature higher when there is higher chance to be active. Higher temperature helps maintain the state of readiness. Lower temperature helps reduce fixed cost. The temperature of human beings are higher at night. This suggests humans were nocturnal in our evolutionary past. Human eyes are dilated when eye muscles are relaxed. This also suggest that humans were nocturnal in the past.
A comparison of science and religion
Science is less reverent to its founders than religion. This caused the basic ideas to be drifted away from the ideas of the founders more easily than religion. For example, Jevons' The Coal Question is largely ignored by the current neoclassical economists, although Jevons is regarded as the founder of the neoclassical economics. Similarly, Walras' theory of value is rarely mentioned. The lack of respect to the works by Jevons and Walras is an important reason why the economic theory is in such a sorry state.
Religion, on the other hand, pays much more respect to their founders. All Christians are required to read Bible. This is why the ideas of founders can persist for such a long time. There are similar forces that drive religions away from the founders. Religious reforms are often attempts to restore the original teachings of the founders. For example, Protestant Reform was an attempt to go back to the teaching of Bible directly instead of listening to the religious authority. Today, the preaching of most pastors are very different from the New Testament. For example, many pastors related to economic prosperity of Western countries to Christianity. Few pastors would talk about the The Blessed are the meek, the pure in heart, the persecuted.
The founders often experience great difficulties in their life, which gave them great insight. The insights cannot be obtained by many followers, especially when the religion becomes dominant. The followers become persecutors instead of being persecuted.
Torpid and the cost of arousal. The cost of arousal is high, which prevent systems to switch from one state to another frequently. Check pages for that.
As a result, the tuna can maintain muscle temperature as much as 14C warmer than the water in which it swims.
The advantage of keeping the swimming muscles warm is that high temperature increases their power output.* A higher power output gives the tuna a high swimming speed relatively independent of water temperature, and this in turn enable the tuna to pursue successfully prey that otherwise swim too fast to be caught, such as pelagic fish (e.g., mackerel) or squid. The greatest advantage is probably that it makes the tuna relatively independent of abrupt changes in water temperature as it moves rapidly between the surface and deeper cold water. ...
The tuna enjoys an additional advantage from the substantial temperature differences between various parts of the body. Separate heat exchanges permit the digestive organs and the liver to be kept at a high temperature. The high power output of the muscles requires a high rate of fuel supply, and this puts a premium on a rapid rate of digestion, which is most readily achieved by a high temperature in the digestive tract.
Swordfish and the closely related marlins and sailfish have the remarkable ability to keep eyes and brains warm, again by using countercurrent heat exchangers to prevent the loss of heat. These predatory fish have some of the largest eyes of any animal. A large swordfish has eyes the size of grapefruit, which undoubtedly is helpful when it pursues prey in deep cold water where the light is minimal.
*The force exerted by a contracting muscle is relatively independent of temperature, and the work performed in a single contraction (work = force X distance) is therefore also temperature-independent. However, at higher temperature the muscle contracts faster, and because the number of contractions per unit time increases, the power output (work per unit time) increases accordingly. (p. 287)
Most insects become increasingly sluggish and are unable to fly at low temperature. Some insects, however, can warm up their flight muscles and be active in quite cold air. The flight muscles are located in the thorax and can produce large amount of heat by a process similar to shivering in vertebrates. Heating of the flight muscles before take off occurs mainly in large insects such as locusts, large moths, butterflies, and bumblebees, and also in wasps and bees that are strong and rapid fliers.
A bumblebee must have a thoracic temperature of at least 29 to 30C before it is able to fly. If the flight muscles are at lower temperature, their speed of contraction is too slow to support flight, which requires a wing-beat frequency of 130 Hz.
The maintenance of a high thoracic temperature enable the bumblebees to forage for nectar at temperature as low as 5C. However, remaining warm on a continuous basis is not feasible unless the bee can find food at a rate at least equal to the rate at which fuel is consumed. A bumblebee weighing 0.5g may have an oxygen consumption of 50 ml o2 per hour, which corresponds to the use of 60 mg sugar. This investment in fuel may be worthwhile, for at low temperature the competition for the available food supply is reduced because many other nectar-feeding insects are inactive.
The advantage of a small body size, relative to mammals, is not only that warmup can be achieved rapidly, but also that cooling is rapid. Therefore, the body temperature can easily be adjusted to the energy supplies, and the insect can choose to keep warm only when the energy supplies warrant the expense; in other words, it can be utterly opportunistic about being "warm-blooded". (p. 291)
Why is
Comment: It is the muddy water that is most resistant to further pollution. Similar, humans who have more social experiences are more resistant to external influences.
Although as a rule marine invertebrates are osmoconformers, this does not mean that their body fluids have the same solute composition as sea water. On the contrary, they maintain concentrations of salts that are out of equilibrium with the environment, and this requires extensive regulation. (p. 305)
Comments: Even the simplest animals regulate and is out of the equilibrium with the environment. More complex animals generally require more regulation and are more out of equilibrium with the environment. The same is true in social systems.
The concentrations of ions inside the cells are usually very different from those outside; for example, sodium and chloride concentrations are usually low inside the cells and are high outside, and potassium is usually high inside and low outside. However the cells are isosmotic with the surrounding tissue fluid and blood, ALTHOUGH THE CONCNETRATION OF INDIVIDUAL SOLUTES DIFFER. (p. 308)
Comment: systems try to be isosmotic with their environment as much as possible to reduce energy expenditure. At the same time, they have to be different from the environment to maintain gradient, which is necessary for directional movement. The balance between the two should be studied in details. Learn why the sodium and
potassium are distributed in such a way.
I am now reading Chapter 8: water and Osmotic regulation. It discussed how animals try to maintain osmotic balance with the environment. But it does not mention that inequilibrium is the fundamental driving force of life. If we understand that, all the associated cost of maintaining the inbalance becomes crystally clear to understand.
In a new paper, in the section of detailed study on various factors, the first part should be the relation between fixed cost and variable cost. Put a lot of notes from this book into the part. Discuss osmotic regulation. Explain why animals maintain different level of osmotic concentration from the environment. Specifically explain why marine animals maintain more dilute fluid than the environment, because more dilute fluid is of higher free energy. Read more about free energy.
The double arrow at the gills indicates the elimination, by active transport, of sodium and chloride. The excretion of sodium and chloride in the urine is of minor importance because teleost urine is more dilute than the body fluids. However, the kidney plays a major role in the excretion of divalent ions, magnesium and sulfate, which makes up roughly one-tenth of the salts in sea water. These ions are not eliminated by the gills, which seem to transport only sodium and chloride. (p. 319)
Further study: Find out the exact biophysical properties of gills and kidneys how they are experts at transport single and bivalent ions.
It is certain that fish adapted to sea water are relatively permeable to ions, and those adapted to fresh water are relatively impermeable. ...
The advantage of a low permeability to ions in fresh water is obvious, but it is difficult to see the advantage of a higher permeability in sea water. marine fish must expend work to maintain their osmotic steady state in sea water, and it seems that a low permeability would reduce this work. It takes fish several hours to return to the higher permeability in sea water, and we can only wonder why they do not permanently retain the low permeability that seems to be within their physiological capacity. (p. 320)
Further study: Think about it.
The transport mechanism is located in the skin of the adult, and the frog skin has become a well-known model for studies of active transport.
Pieces of frog skin can be readily removed and used as a membrane separating two chambers, which can be filled with fluids of various concentrations. By analyzing the changes in the two chambers, the transport function by the skin can be studied. Such isolated pieces of skin survive for many hours. This apparatus for the study of active transport processes was originally devised by the Danish physiologist Hans Ussing and is known as the Ussing cell.
When frog skin in the Ussing cell separate two salt solutions of the same composition, a potential difference of about 50 mV is rapidly established between the inside and outside of the skin. The inside is positive, and it is therefore postulated that the potential is caused by an inward active transport of the positive sodium ion. When the potential difference has been established, the chloride ion passes through the skin by diffusion, accelerated by the electric field. An enormous amount of evidence has accumulated to indicate that this interpretation is correct, and the fact that the transport is active is clear from the developed potential and from the fact that metabolic inhibitors prevent the formation of a potential and inhibit the transport. (p. 322)
Comment: Learn more about it! It may be crucial in my further understanding.
Plants and invertebrate animals, when eaten, present roughly the same osmotic problem as the drinking of sea water. Marine teleost fish, as we have seen, contain much less salt, and animals that subsist primarily on fish have a less severe salt problem than those feed on plants or invertebrates. (p. 344)
Comments: Those who regulate themselves better become more favorite food than others. So it is often beneficial to remain humble so we are less targeted.
No specific excretory organs have been identified in coelenterates and echinoderms. This is curious, for fresh-water coelenterates are distinctly hypertonic to the medium, and they undoubtedly gain water by osmotic influx. How the excess water is eliminated is not known. Echinoderms, on the other hand, have no problem of osmoregulation, for echinoderms do not occur in fresh water and the marine form are always isomotic with sea water. (p. 357)
Comment: Even echinoderms are isomotic with sea water, there is still need for excretion for the exact composition of body fluid is different from the sea water. If no specific excretory organ can be identified, skin must be the organ.
Even for the very simple organisms, sodium, potassium pump is present (p. 359). Therefore this pump must be of fundamental importance to most organisms. Find out the exact physiology significance of the pump.
I would like to reorganize the system of knowledge of physiology from the function point of view. First identify the needs of animals. Then find out how organisms find out the solutions to these needs and the corresponding energy cost.
In mammals and birds, the excretory fluids in kidneys are more concentrated than in blood. This can be understood as the metabolic rates in mammals and birds are very high. So metabolic end products are high and the need to discharge waste is high. This makes it economical to invest in kidney function more.
Sometimes, nitrogen is discharged. Other times, nitrogen is recycled. Probably they are determined by the relative cost and benefit. Maybe we can analyze more about it.
The most striking feature of nucleic acid metabolism is that the "higher" animals ... completely lack the enzymes necessary to degrade the purines. Among the "lower" animals we find an increasing complexity in the biochemical and enzymatic systems for further degradation of the purines. Thus the "lowest" animals in this case possess the most complete enzyme systems. (p. 386)
Reread Chapter 9.
Chapter 11: Control and Integration
It is obvious that physiological processes must be controlled and not allowed to run wild. In the preceding chapters we have often mentioned regulation and control, but we have not discussed the mechanisms of regulation.
To regulate means to adjust an amount, a concentration, a rate, or some other variable, usually in order to reach or keep it at some desired level. For example, we take it for granted that respiration must provide oxygen at the rate it is used by the organism. Similarly, all physiological processes must be controlled and integrated.
Integration means putting parts together. In physiology the word means to control all functional components so that they merge into a smoothly operating organism where no single process is permitted to precede at its own independent rate. (p. 465)
For every three sodium ions that are extruded, two potassium ions are taken up. Sodium pumps appeared to be universally present in the cells of all animals. (p. 476)
Comment: The difference between sodium and potassium ions that are transported build up the potential over the membrane. Is there a need to utilize the permeability of potassium ions of the membrane to explain the electric potential? That sodium pump is universal indicates that the build up of gradient is the universal characteristic of life.
In spite of the unusual ion concentration in some insects, both the resting and the action potentials of their nerves are similar to those in other animals. How is this possible when the ion concentration that set up these potentials are absent?
It is easier to understand this intriguing situation when we learn that the entire central nerve system of insects is surrounded by a nerve sheath that separate the nerve from immediate contact with the extracellular fluids. This sheath consists of an outer noncellular layer and an inner membrane, the perineurium, which is a single layer of specialized cells. The sheath acts as a barrier that separates the surface of the axon from the hemolymph and restricts the movement of materials between hemolymph and the fluid at the neuronal surfaces. The sodium concentration in the fluid bathing the neuronal surfaces may readily be an order of magnitude higher than in the hemolymph. (p. 480)
Comment: Animals, if necessary, will pay a high cost to maintain the efficiency of the communication system. This shows that the so called importance and uniqueness of information society is simply unfounded.
Hormone means messenger in Greek.
The unexpected fact that several messenger substances known from vertebrates occur widely among invertebrates as well as in unicellar organisms suggests that these substances are evolutionarily much more ancient than was previously thought. The hormones and endocrine systems as we know them in vertebrates appear t have forunners in some of the simplest organisms known --- organisms without glands, nerves, or circulatory systems. It now appears that vertebrate endocrine systems may be highly specialized forms of general biological mechanisms of very ancient origin. (p. 521)
Comment: Communication has to be very ancient, for otherwise how can organism, even uniceller organisms, coordinate themselves.
The sensory neurons of the retina are highly sensitive to light, those of the ear to vibrations at frequencies in the range we identify as "sound", and so on. However, all sensory neurons respond in the same way; they translate the stimulus into nerve impulses that, via the appropriate sensory nerves, are transmitted to the central nervous system.
We now come to the intriguing fact that the nerve impulses carried in the different sensory nerves are all the same fundamental nature. For example, the optic nerve carries the same kind of nerve impulse as the auditory nerve. Both carry similar action potential; it is the central nervous system that sorts out what the original stimulus was.
If the auditory nerve is stimulated artificially, the induced nerve impulse are perceived by the central nervous system as "sound", and artificial stimulus of the optic nerve is perceived as "light". Most of us have experienced this kind of interpretation; we know that mechanical pressure on the eyeball is perceived as light and that a sharp blow on the eye makes us "see stars", although no light is involved.
... we say that the sensory neurons are the transducers that receive the external information and encode it as impulses in the sensory nerves. In the central nervous system the nerve signals are again decoded and the pertinent information is integrated and utilized. The most interesting events are the transducing and encoding of information on the one hand, and the decoding and processing on the other. ... The astounding fact is that these various operations are all carried out by nerve cells that function in the same fundamental way, based on the electric potential across the cell membrane and the generation of action potential. (p. 535)
What humans receive as light lies within a very narrow wavelength band, 380 to 760 nm, out of the wide spectrum of electromagnetic radiation that ranges from the extremely short gamma rays to longwave radio waves.
All other animals' visual sensitivity lies within or very close to the same range of wave lengths as humans'. The remarkable fact is that not only animals, but also plants, respond to light within this same range. This includes both photosynthetic and phototropic growth of plants.
The reason for this universal importance of a very narrow band of electromagnetic radiation is simple. The energy carried by each quantum of radiation is inversely related to wavelength. Therefore, the longer wavelength do not carry sufficient energy in each quantum to have any appreciable photochemical effect, and the shorter wavelengths (ultraviolet and shorter) carry so much energy that they are destructive to organic materials. The universal biological use of what we know as "light" is a result of the unique suitability of these particular wavelengths. (p. 548)
Comments: I have an alternative explanation for the visible light spectrum. The visible light spectrum is exact the range of maximum intensity of light from the sun. So the reason for this could be an economical one. It is perceivable that if the range of maximum intensity of the sun is somewhere else, the organisms may evolve other ways to utilize light at different wavelengths. One way to check my explanation is to see if the earliest life forms are photosynthetic. I suspect the earliest lifeforms utilize energy from the early hot earth and hence utilize energy with lower temperature and hence higher wavelength. This is also consistent with my thought that life start from low fixed cost systems.
The range is not exactly the same for all animals. The vision of insects, for example, extends into the near ultraviolet range, to slightly shorter wavelengths than the vertebrate eye. This is evident from the ability of honeybees to distinguish any spectral color between 313 and 650 nm from white color, an ability that is unrelated to light intensity and therefore must depend on wavelength discrimination.
The mammalian retina is sensitive to ultraviolet light, but these wavelengths do not penetrate to the retina, primarily because of a slight yellowness of the lens, which acts as a filter. Persons whose lens has been surgically removed because of cataract can perceive light in the near ultraviolet that previously was invisible to them.
The narrowing of the band of wavelengths that can be perceived is probably advantageous. A lens, if made of a uniform material, refracts short wavelength radiation more strongly than longer wavelengths. This means that various wavelengths cannot brought into focus simultaneously. In man-made lengths this difficulty is known as chromatic aberration. It can be corrected for by the use of composite lens consisting of several elements with different refractive indexes. For an eye that does not have color correction, the simplest way of reducing this difficulty of simultaneously focusing different wavelengths is to narrow the wavelength band that is permitted to enter. (p. 550)
From biology, genes are more proper units to consider than individuals. Similarly, in human societies, the benefit of individuals are more proper units than companies or societies. Or put it other way, we should adopt a population approach such as n ecology instead of measuring profit or GDP.
We saw in Chapter 11 that an action potential is always of the same magnitude, irrespective of the strength of the stimulus that produces the action potential, once the threshold is exceeded. If the action potentials are always equally large, how is it possible to convey information about variations in the strength of stimulus --- information that obviously is of the greatest interest?
The answer is that a change in frequency of the action principles in the axon can be used as an indicator of stimulus strength. This is illustrated in Figure 13.21, which shows that a pressure-sensitive skin receptor cell in the finger of a person responds to an increased stimulus strength with increasing frequency of the action potential in its axon. This particular receptor had a threshold of about 0.5 g, and therefore showed no response to a force of 0.2 g. A force of 0.6 g, however, gave a clear response, which increased in frequency with the application of increasing forces. Thus, we can say that the magnitude of a sensory stimulus is coded and transmitted as a frequency-modulated signal. (p. 563)
Comment: This is a good example how information processing is coded. This also shows that the human communication functions just like the digital communication systems with code 0 and 1.
If all information available to the sensory organs were transmitted to the central nervous system, the mass of signals would be overwhelming and probably utterly unmanageable. However, a great deal of screening, filtering, and processing takes place before the signals are passed on, beginning at the sensory neuron and continuing at several levels on the way to the brain. The filtering networks pass on only selected portions of the information they receive. Furthermore, they carry out certain steps of processing that improve on the information that is transmitted to higher levels. (p. 563)
Lateral inhibition ... The overall effect is that the transmission between brightly and dimly lit receptors is emphasized. As a result, the messages in the optic nerve give a correct picture of the edge, but with an emphasized contrast between the two zones. (p. 565)
It has been observed in the scallop, which has well-developed image forming eyes along the edge of mantle, that the retina has two layers. One layer, as expected, responds to light as a normal stimulus. The other layer of the retina, however, behave in a very different way; it does not respond to increased light, but is sensitive only to a decrease in the intensity of illumination. The importance of this phenomenon can easily be imagined. When a shadow suddenly falls on an animal, it often means the approach of a predator, and in this case, a decrease in stimulus intensity is far more important to the animal than any other information. (p. 565)
Comment: The above two quotations show that systems evolve toward usefulness, not necessarily truthfulness.
The eye of a cat, and of many other mammals, has about 100 million receptor cells in the retina. The optic nerve carries about 1 million axons. This immediately tells us that the brain can not receive separate information from eachindividual receptor cell; a great deal of sorting and processing take place before the information is sent to the central nervous system. (p. 566)
The meaning of this highly complex signal evaluation or processing is obvious. It is of no particular interest to record and inform the brain about the general intensity of uniform light; the details of light and dark contrast are of much greater importance. What we have is a case of selective destruction of information, carried out in the eye. The eye selects what kind of information is to be transmitted in the optic nerve and thus reduces the information load on the brain. (p. 568)