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Blood-brain Barrier and the Sodium-potassium Pump

Jocelyn Brown-Eaton
The blood-brain barrier and the sodium-potassium pump have many similarities and differences. Similarities include the fact that they both function to maintain a balance and that they both are selectively permeable. Differences includes the way the mechanisms carry out those functions and what kind of balance they maintain. The blood-brain barrier is a mechanism that isolates the central nervous system neurons from chemicals coming from the rest of the body. It is made up of the walls of brain capillaries that are tightly joined together, other capillaries in the rest of the body do not align themselves so close together and they do allow chemicals to pass from the blood into the areas of the body they are flowing through. In contrast, the sodium-potassium pump is a protein in the membrane of cells that helps maintains the difference of electrical charges inside and out of the cell, keeping the cell polarized along with the difference in permeability of sodium and potassium within the rest of the membrane (Khan Academy 2010). The resting potential is maintained before an action potential arrives and then is restored when the action potential is over. Comparatively they are both maintaining balances. The blood-brain barrier is balancing chemicals and protecting the brain neurons from harmful substances since these neurons do not regenerate, but the sodium-potassium pump is keeping an ionic balance. Selective permeability is also a similarity of the two mechanisms. The blood-brain barrier is only a barrier for water soluble molecules and selectively allows lipid soluble molecules to pass, while the sodium-potassium pump only deals with sodium and potassium. The sodium-potassium pump takes in two potassium ions for every three sodium ions it pushes out. Transporter proteins control the movement of these substances. The difference is that with the blood-brain barrier there is a separate protein that actively transport the selected chemicals, while the sodium-potassium pump is a protein in itself. There are areas of the blood-brain barrier that are more permeable than the rest in order to allow the function of those specific parts of the brain. One such area is the area postrema. The area postrema detects toxins in the body and initiates vomiting.
Khan Academy. (2010). Correction to Sodium and Potassium Pump Video. [Online Video]. 11 July 2010. Available from: https://www.youtube.com/watch?v=ye3rTjLCvAU. [Accessed: 25 February 2017]
Before an action potential arrives, there is a balance between the extracellular fluid (on the outside of the cell) and the intracellular fluid (on the inside of the cell). This difference in the electrical charge is called the membrane potential. The membrane potential is created by diffusion of ions and electrostatic pressure. Diffusion refers to the process of molecules evenly distributing themselves. Molecules push away from areas that they are more concentrated in. Electrostatic pressure is the force that comes from the attraction or repulsion of ions. Positive charges repel other positive charges, negative charges repel other negative charges, and positive charges attract negative charges. The ions involved in these forces are organic anions, potassium ions, chloride ions, and sodium ions. Organic anions (A-) are negatively charged and found in intracellular fluid. These ions remain in the intracellular fluid because the membrane is impermeable to them. Potassium ions (K ) are positively charged. They try to get out of the membrane because of diffusion, there is a higher concentration of them inside than out. Electrostatic pressure, however, pushes back against them because extracellular fluid is more positively charged inevitably keeping the ions where they were. Chloride ions (Cl-) are negatively charged. They try to get into the membrane due to diffusion but electrostatic pressure keeps them where they are as well. Sodium ions (Na ) are positively charged and get pushed into the membrane due to diffusion. Unlike the other ions sodium is not pushed back by electrostatic pressure. Instead, they are attracted in because the intracellular charge is more negative. The sodium-potassium pump helps maintain the resting potential, which is on average -70 mV. The sodium-potassium pump trades three sodium ions to the outside of the cell for two potassium ions to bring into the cell. During an action potential, a signal is sent to the membrane the membrane to become more permeable to sodium ions increasing the intracellular charge. The membrane potential reaches its threshold and a depolarization spike occurs. Depolarization is when the internal polarization of the cell increases; when it gets closer to zero. Voltage dependent sodium channels, triggered by the depolarization, open allowing sodium to enter at a faster rate. At a higher level of depolarization voltage dependent potassium channels open and potassium flows away from the more positively charged interior. Voltage dependent potassium channels are less sensitive than the sodium channels are. Next sodium channels close and go into a refractory state, preventing them from opening again until the resting potential is restored. The cell goes through hyperpolarization, where the intracellular charge drops in order to get back to normal. When hyperpolarization goes lower than the resting potential it is called the undershoot. When the undershoot is reached it signals the potassium channels to close and resting potential is closer to normal. After that all passes the sodium potassium pumps slowly help the resting potential return and everything is back to its original state.
Neurotransmitters open ion channels in two ways, directly and indirectly. Directly opening the ion channels occur when there are ionotropic receptors. When a neurotransmitter binds to an ionotropic receptor the ion channel immediately opens and let ions flow freely through. With metabotropic receptors, when a neurotransmitter binds to its binding site it starts a “chain of chemical events” (Carlson and Birkett, 2017). These chemical events involve the G protein being activated, which in turn activates the second messenger system. The second messenger travels to the nearby ion channel and signals it to open. Metabotropic receptors got their name because they require extra steps that uses up some of the cell’s metabolic energy. The important differences between ionotropic receptors and metabotropic receptors are the speed of effect and the duration of effect after their activation. Ionotropic receptors are faster because when a neurotransmitter binds to it the ion channel is opened immediately and triggers a postsynaptic potential. The whole process happens very quickly. Metabotropic receptors are slower because the signal to the ion channel is transferred between a sequence of different molecules to get to the ion channel and activate it. This process causes a delay in effect, they take longer to begin but they also last longer. Serotonin has both ionotropic and metabotropic receptors. All but one receptor, the 5-HT3 receptor, are metabotropic. The 5-HT3 receptor is ionotropic and it controls a chloride ion channel, therefore producing inhibitory postsynaptic potentials. This receptor plays a role in nausea and vomiting. Because ionotropic receptors act quickly, if the receptor is bound to by an agonist, which would open the ion channel, it would induce vomiting or nausea right away. An example of this would be when a person smells something rotten and immediately feels nausea. Antagonists of this receptor are used to treat the side effects of chemotherapy and radiation treatments. Serotonin is used for mood regulation, and that happens in the metabotropic receptors. This means that the effects take longer but will last longer. If this happened rapidly then there would be no transitions between our moods. It allows the drugs for mood regulation (like SSRIs) to have compound effects and build up in our system by inhibiting the reuptake of serotonin.
Carlson, N. R., Birkett, M. A., (2017). Physiology of Behavior, 12th Edition. [BryteWave]. Retrieved from https://shelf.brytewave.com/#/books/9780134517858/

Natural Selection and Genetic Drift | Experiment

Camouflage Lab
Eduardo Pérez
Introduction In 1859, an English scientist named Charles Darwin published his book, On the Origin of Species. This book described his Theory of Evolution, the process by which populations of organisms change over time to adapt to their environment. Over the years, the Theory of Evolution has become one of the most well-supported and widely accepted scientific theories out there.
The main purpose of this experiment is to show how natural selection and genetic drift look like when they are put into play. According to Dennis O’Neil, anthropology professor at Palomar College, natural selection is a series of events by which some organisms are born with random variations of a specific genetic trait that gives those organisms an advantage in “staying alive long enough to survive and successfully reproduce”. [HS1]Over time, these organisms will have more offspring, causing a shift in the population to that trait (O’Neil 2013). An example of natural selection is the finches of the Galapagos Islands. Each island has different food sources, and each species of bird has slightly different beaks that are better suited for consuming their food source. In his book, Life: The Science of Biology (2014), author David Sadava describes genetic drift as the “random fluctuation of gene frequencies in a population due to chance events.” An example of genetic drift would be an oil spill in a river populated by fish. The surviving fish will repopulate the river with their offspring who share the same genetic variations.
In this experiment, small beads were put on a colored mat to represent mussels in their environment. In the first part of the experiment, one team member was assigned as the “Oystercatcher” and they selected beads one by one and removed them from the environment to represent natural selection. In part two, beads were randomly removed by a pencil wrapped in tape (a piece of driftwood calling with mussels and killing them) which represented genetic drift. Both parts of the experiment were repeated for three “generations” after the surviving mussels repopulated the environment. The question being tested in this experiment was: How do natural selection and genetic drift affect populations of organisms? I hypothesized that the blue and red beads would be the most commonly selected and removed in the first part of the experiment, and that the beads would be removed in equal numbers by the pencil wrapped in tape.
Materials and Methods The two most important materials used in this lab were the small colored beads, and the mat. Blue, white, green, and purple beads were used to represent mussels with different traits. Ten beads of each color were placed in the “environment” to start the experiment. The environment for the mussels was represented by the mat with a random background printed on it to camouflage the beads. For the second part of the experiment, a pencil was wrapped with masking tape (sticky side out), and used to represent a log crashing into the environment. The pencil was rolled along the mat to randomly pick up beads.
To start off the experiment the person designated as “oystercatcher” removed beads one at a time from the mat and placed them in petri dishes (independent variable). The oystercatcher was instructed to pick the first beads they saw, and to look away from the mat between selections. After 30 beads were removed and placed into a petri dish, the survivors were counted (dependent variable). The numbers of each color of bead was recorded, and that number of beads (x) plus 3x beads were added back to the mat to represent the repopulation of the species based on the number of survivors. These steps were repeated two more times, and the data recorded each time.
In part two of the experiment, the pencil wrapped in tape was rolled along the mat to randomly select and remove beads until 30 beads were removed (independent variable). Then the same procedure used in part one to repopulate the environment was used in part two (dependent variable). These steps were repeated two more times, and the data was recorded.
Results I. Population of mussels over 3 generations after natural selection from Oystercatcher.
Oystercatcher Data
Survivors
Total
Generation 1
7 green
7×3 = 21
21 7 = 28
28
0 blue
0x3 = 0
0 0 = 0
0
2 white
6×3 = 18
2 6 = 8
8
1purple
1×3 = 3
1 3 = 4
4
Generation 2
10 green
10×3 = 30
30 10 = 40
40
0 blue
0x3 = 0
0 0 = 0
0
0 white
0x3 = 0
0 0 = 0
0
0 purple
0x3 = 0
0 0 = 0
0
Generation 3
10 green
10×3 = 30
30 10 = 40
40
0 blue
0x3 = 0
0 0 = 0
0
0 white
0x3 = 0
0 0 = 0
0
0 purple
0x3 = 0
0 0 = 0
0
When the beads were removed by the oyster catcher, the blue beads were completely removed from the map in just one generation, and the purple and white beads were also driven extinct, but not until the second generation, leaving only green beads at the end of the three generations.
II. Population of mussels over three generations after genetic drift from log colliding with habitat.
Oystercatcher Data
Survivors
Total
Generation 1
2 green
2×3 = 6
6 2 = 8
8
2 blue
2×3 = 6
6 2 = 8
8
2 white
2×3 = 6
6 2 = 8
8
4 purple
4×3 = 12
12 4 = 16
16
Generation 2
2 green
2×3 = 6
6 2 = 8
8
1 blue
1×3 = 3
3 1 = 4
4
4 white
4×3 = 12
12 4 = 16
16
3 purple
3×3 = 9
9 3 = 12
12
Generation 3
1 green
1×3 = 3
3 1 = 4
4
1 blue
1×3 = 3
3 1 = 4
4
7 white
7×3 = 21
21 7 = 28
28
1 purple
1×3 = 3
3 1 = 4
4
When the beads were removed by the log, the survivors were more random and more equal than when removed by the oyster catcher. By the end of the experiment however, a majority of the survivors were yellow beads.
III. Population of mussels over 3 generations after natural selection from Oystercatcher.
IV. Population of mussels over three generations after genetic drift from log colliding with habitat.
Discussion In part one of the experiment, where the beads were selected and removed by the oystercatcher, the blue beads were immediately driven extinct, and the white and purple beads were driven extinct in only one more generation. This left only green beads by just the third generation. These results show that in natural selection, organisms chances of survival are based on how fit they are to survive in their environment. In this experiment, the blue beads did not blend into their environment very well, and they were eliminated immediately. The purple and white beads were also poorly camouflaged, and were eliminated very quickly as well. Even by the third generation, where there were only green beads left, the oystercatcher had a hard time finding 30 beads to remove, because the green beads were much more difficult to see in the environment. These findings could be applied to a real life environment, and used to predict how well certain organisms have adapted to their environment, and how an entire population will change over time because of natural selection.
In part two of the experiment, the number of survivors was much more equally spread out between the different colors of beads. Although there were definitely more yellow beads than anything else by the end of the experiment, this outcome would be different every time you repeat the experiment, based on the survivors from earlier in the experiment. These results are consistent with the principles of genetic drift, where organisms are eliminated randomly from a population based on random occurrences like natural disasters and diseases. If for example, lightning struck an area with a high concentration of a particular type of mussel, over time, the number of that mussel would decrease because there are fewer mussels to reproduce.
This experiment was limited to the use of basic lab materials in a lab setting, but it accurately represents data that would be collected from an actual environment out in nature. This experiment was only able to demonstrate the effects of color and camouflage on the survival rates of an organism, but in reality, there are many other genetic variations which contribute to the fitness of an organism to its environment. An elephant, for example, may not be particularly well camouflaged, but its sheer size and strength help it to survive. Further research could be done to demonstrate the effects of other forces of evolution, as this experiment only involved genetic drift and natural selection.
Conclusion The data in this experiment supports the hypothesis that the blue and purple beads would be the most commonly eliminated by the oyster catcher, but the yellow beads were also driven extinct, leaving only green beads. The data somewhat supports the hypothesis that the beads would be removed in equal numbers by the log, although the population shifted to a majority of yellow beads by the end of the experiment. The same experiment could be repeated several times to obtain more data to prove or disprove this hypothesis.
References O’Neil, D. (2013). Early Theories of Evolution: Darwin and Natural Selection. Retrieved August 29, 2016, from http://anthro.palomar.edu/evolve/evolve_2.htm
Sadava, D. E. (2014). Life: The science of biology (10th ed.). Sunderland, MA: Sinauer Associates.
[HS1]Are these his exact words? If not, remove the quotes

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