(You don't really know where you are unless you know how you got here.)
The ancient Greeks such as Galen of Pergamum (b129AD, d~200AD) tried to understand anatomy and how a living body functions. Galen's writings were taught to succeeding generations with some of his views still a part of common culture today. Following the Dark Ages, those centuries when much of Greek understanding was ignored by European civilization, the Renaissance attempted to revive the best of ancient culture. Less than a century after Gutenberg's printing technology heralded a proliferation of books, Andrea Vesalius (←woodcut at left, b1514 ,d1564) studied medicine and Galen's anatomy in Belgium and France, finally receiving a doctorate at Padua, Italy, in 1537. After writing several preliminary works, Vesalius published in 1543 De Humanis Corporis Fabrica contrasting Galen's description of assumed human anatomy with facing page drawings of dissected human cadavers. Numerous discrepancies between text and diagrams were obvious. While it remained illegal to desecrate even the bodies of executed criminals, Vesalius' book based on such dissections revolutionize the understanding of human anatomy and the functions of its organs.
Two centuries later, European royalty often found it entertaining to be shocked by new machines, which when rotated, generated electric sparks. Luigi Galvani (b1737, d1798, portrait at right→), professor of Anatomy at the University of Bologna, investigated contractions of legs from dissected frogs caused by such sparks. Later he investigated a possible connection between what he called this animal electricity and proposals from people such as Benjamin Franklin that lightning storms might also involve electricity. To investigate, Galvani observed dissected frog legs hung out in a lightning storm. Disappointed after a day with no observed effects, Galvani returned the frog legs to hanging from metal hooks in a storage cabinet. There they quickly started to twitch!
After reading Galvani's 1791 reported findings, Alessandro Volta of Como, Italy (←portrait at left, b1745 ,d1827) suggested the twitches might have been caused by electricity generated between the metal of the hooks and a dissimilar metal of the cabinet. When he constructed a sandwich of two dissimilar metals to test this hypothesis, he was unsure if there was any electrical effect. So he tried to amplify any effect by stacking multiple layers of such metal sandwiches, interspersed by brine soaked cardboard. With such a battery, Volta found that when he brought a wire attached to the bottom metal near the top metal, a spark occurred. That 1800 discovery provided the first reliable device to generate a continuous flow of electric current, the electrical battery of multiple Volteic cells. That rapidly led to investigations and understanding of many electric phenomena and eventually to today's electronic age.
We now understand that each kind of atom has its own characteristic strength of attraction for its outer constituents called electrons. Atoms in metals, a subgroup of the known elements in which each readily conducts electricity, only weakly attract their outer-most electrons. Within a piece of metal with its large number of identical atoms, these outer valence electrons can move with little resistance as long as they remain near the metal atoms. If a few extra electrons are supplied to one point on a metallic body, the freely flowing sea-like valence electrons can move slightly (adjusting at the speed of light within the metal) so that the excess can be drawn off at some distant location nearly instantaneously. The spark from Volta's electric battery is caused by each metal having a different amount of attraction for its valence electrons, something we measure as electrical potential in units appropriately named Volts. When two different metals compete for electrons available from chemical reactions with the brine, the metal with the stronger attraction for additional electrons pulls some electrons away from the other metal.
The animal electricity investigated by Galvani flows along the outer membrane a particular kind of long, wire-shaped cell commonly called a nerve, or more formally a neuron. But unlike electric current flowing through metallic wires, the transmission speed is slower by more than 10-6, and does not involved the flow of weakly held electrons. Instead the transmission of the electrical signal along a nerve involves much slower chemical reactions and floods of ions and molecules. The process of animal electricity can be divided into at least four distinct functions:Most detection of environmental stimuli involve variations of G-proteins which are members of a still larger super-family of proteins, which thread 7 times across cell membranes. G-proteins are widely used by many body systems. Besides their use to detect odors and tastes, they are used as detectors for hormones such as adrenaline. In other organisms, some as primitive as yeast, the G-protein mechanism is used to detect and chose a mate! (See Biochemistry 3 for the basic chemistry of proteins and details about G-proteins and their sensory mechanism.)
A closely related group of proteins are similar but have only 6 similar membrane crossings. In place of the section between the fifth and last crossings (where the 6th crossing is missing) are 20 to 30 amino acids which help form a gated pore through the cell membrane which control the passage of ions and larger molecules. Each tube or channel through the membrane is not a single protein, but rather is constructed of four distinct channel-proteins, each contributing a quarter of the pore lining. To open a single pore, four cyclic-AMP molecules are required, one to activate each channel-protein. When one or two nucleotides are bound the channel only opens slightly (~1% of the maximum ion flood current). The channel opens significantly more with three nucleotides bound (~33%), and opens fully when four are bound. The segment of each channel-protein which attaches to the cyclic-AMP hangs below the pore-forming region inside the cell, much like the location where GDP attaches to the odor detector G-protein. (See details in the previous investigation). Similar channels have been identified in the light detecting rod and cone cells in eyes, the flagellum of mammal sperm, and within brain, liver and heart tissue.
The fourth crossing segment of all these proteins show an additional similarity to still another family of Voltage gated channel proteins which are activated by Voltage changes and function to magnify nerve signal strengths.
To most women who have given childbirth or anyone having experienced the tubal pain caused by appendicitis, or a kidney stone or gallstone, there seems to be little in common between those shrill pains and pleasure provided by other stimuli. In general pain is the device a body uses to signal that something is either wrong or in danger of injury or other system failure. Possible connections between pain and pleasure remains somewhat confusing. So our interest here will skirt that issue and consider how the pain is detected, and how that process works compared other sensory experiences. In the process we will take a still closer look at the construction and operation of the protein gates which control passage of ions and molecules into and out of cells such as neurons.
Cell membranes are composed of lipids. These molecules have long oily, electrically-bland, non-polar tails [colored tan in the ←structural formula to left and diagrams below↓ and elsewhere in these Biochemistry lessons]. As a result this portion of the lipids have low solubility in water. Each lipid also has an electrically-polar, water attracting carboxyl group on one end [colored blue-green in the diagrams]. These lipid molecules align in a double layer, forming membrane surfaces. Their water-loving carboxyl groups are attracted towards the aqueous solutions which are prevalent both inside and outside most cells. Generally the long, non-polar tails are flexible except where a double bond forces a stiff kink. As a result the tails from both layers intertwine creating the flexible strength suitable for life's packaging material.
There is a least one specialized type of nerve receptor, called nociceptors, responsible for initiating pain. The molecule-sized sensors in these cells are activated by harmful chemical or physical conditions. The first identified pain sensor imbedded in the outer surface membrane of such nociceptors is called TRPV1 (transient receptor potential cation channel, subfamily V, member #1), a protein string with 838 amino acids and molecular mass of 95 kDa. TRPV1 is a member of the family of proteins which have coiled sections which thread across the cell membrane 6 times (depicted in diagram above right→). Such proteins provide a gated channel which, when open, selectively allows electrically charged ions to pass through the outer membrane of the nerve cell. Compared to the above described G-proteins used as sensory receptors by other kinds of neurons, these nociceptors have gated channel proteins in the section between their 5th and final crossing sections in the same location where G-proteins have their sixth of 7 coiled sections crossing the cell membrane. So nociceptors contain specialized pain detector proteins which combine both sensor and gated-channel functions.
The flattened 2-dimensional diagram (above ↑) shows the various sections of one channel protein. In an actual cell membrane the coiled crossing sections clump together (←as in the 3-dimensional diagram at left of two channel proteins). Generally ion channels through cell membranes require four proteins to form the gated passageway. (←Here the front half of the cluster is not shown; these two proteins form the back half of the channel.) The more flexible, only partially coiled 20 to 30 amino acids between the 5th and the last crossing of each protein provide the gate function which blocks or permits passage of ions. The semi-transparent diagram below left (↵) provides a still less detailed image of how four TRPV1 proteins form the walls of the central channel through the cell membrane.
TRPV1 is sensitive to both heat and to capsaicin (a pungent extract of the Capsicum pepper family, formula right→). TRPV1 has a preference for allowing the passage of ions with an electric charge of 2+ (Ca2+ > Mg2+ > Na+ ≈ K+ ≈ Cs+). Ca2+ is especially important to TRPV1 function. Ca2+ originating outside the nerve cell helps to eventually desensitize the nerve, a process which enables the nerve to adapt to the particular chemical or physical cause of the pain, eventually reducing the nerve's response.
The question remains: How does the channel detect pain? Pain caused by heat may be the easiest to understand. It has been long known that the shape of a protein can be denatured by both heat and other harsh conditions. A protein's critical coiled and folded shape is maintained by weak chemical bonds between various functional groups along the long amino-acid chain. As temperature is increased, increasing vibrational motions can cause the dissociation of these weak bonds, resulting in the protein changing shape. In the case of TRPV1, a rise in temperature can cause enough changes near the pore to cause it to open, allowing ions to flood through setting off the nerve impulse that is subsequently interpreted by the brain as pain. Other mechanical threats such as pulling a hair or a puncture wound likely use those physical motions to initially open the pore triggering the pain.
The nociceptor's response to a chemical threat might be understand by recalling the description in Biochemistry 3 of the mechanism for smelling. The attachment of the odor molecule and subsequent reaction with a GTP energy carrying molecule causes a shape change ejecting the odor molecule and setting off a sequence of chemical changes which eventually causes nearby protein channels to open. In the pain mechanism, the shape change directly opens TRPV1's own channel. This immediate route for ion passage probably provides both faster and more sharp response than does odor or taste detection.
A tarantula native to the West Indies emits a venom which also activates TRPV1 channels causing searing pain. There is hope that by understanding such molecules which trigger the opening of pain channels in nerve endings, molecules might be found to block the channel. Such a molecule might alleviate the intense pain of burns and the sting due to venomous plants and animals.
As an example of how such knowledge is applied, an injectable local anesthetic QX-314 (shown at left above) blocks pain without causing the paralysis or numbness caused by earlier local anesthetics. The pain relief is provided by combining capsaicin with QX-314, a variation of lidocaine (the first amino amide-type anesthetic, developed in 1943; this variation has a third -CH2CH3 ethyl group on the positively charged amine tail in the structure as shown above). The combination blocks transmission of pain signals but does not block nerve pulses which control movements or non-painful sensations. The combination works by blocking Sodium ion channels in nerves that sense pain, thereby preventing the nerves from transmitting pain signals to the brain. Earlier forms of local anesthetics which are electrically neutral, pass by osmosis through the non-polar lipid membranes into nerve cells then plug the nerves' Sodium channels from the inside. However such anesthetics also enter all nearby nerve cells and so block all nerve signals including those which carry information unrelated to pain. Because of its positive electric charge, QX-314 is unable to get passively through the lipid membrane of nerve cells by osmosis. But the accompanying capsaicin provides QX-314 entry only to nociceptors by triggering opening of TRPV1 ion channels. After entering only nociceptors through these TRPV1 channels, QX-314 then plugs the numerous Sodium channels, blocking amplification of the Voltage spike needed to carry the pain message along the neuron. Because the TRPV1 ion channels are not found in the cell membranes of other kinds of neurons, such a combination of capsaicin and QX-314 selectively deactivates only pain signals.
Much of the biological value of proteins is related to their ability to maintain unique molecular shapes. Often a change in a protein's shape changes some of its chemical properties, thereby promoting or discouraging a particular chemical reaction. The above described opening of channel pores in the cell membranes or nerves is one example. We also use this to preserve food by canning. Sufficient heating of the food after packaging in a sealed container changes the shapes of proteins in any microorganisms present. If there is enough shape changes in the proteins which microorganisms needs to survive, the microorganisms die and become part of the food rather than cause the food to spoil. The change of molecular shape does not change the stored energy in the food (measured as Calories) or the contained amino acids and nucleic acids which we digest and reassemble into larger molecules needed for our own bodies. So while the cooking does sterilize the food and somewhat change its color and texture, cooking need not significanly change the nutritional value of the food. (Higher temperatures break the stronger chemical bonds when food is browned or burnt.)
Communicating technical information such as observations and findings is a skill used by scientists but useful for most others. If you need course credit, use your observations in your journal to construct a formal report.
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