Wikipedia:Reference desk/Archives/Science/2017 September 9

= September 9 =

Action potential in rabbit's sciatica
I am looking for a picture of afferent and efferent action potentials in rabbit's sciatica. Is anything like this available on the web? Thanks --AboutFace 22 (talk) 00:19, 9 September 2017 (UTC)
 * Do you mean the rabbit sciatic nerve? In any case you'll need to be a bit more clear.  It might help to give us a little information about what you would like to use this for. Looie496 (talk) 00:41, 9 September 2017 (UTC)
 * Sciatica is a pain symptom in the leg, associated with the Sciatic nerve. Afferent and efferent nerve fibers carry nerve impulses repectively towards and away from the Central nervous system i.e. brain and spinal chord. Here are references:
 * SCIATIC NERVE CONDUCTION VELOCITY AND LOCOMOTORY PATTERNS IN FROG, UROMASTIX AND RABBIT; Locating the target nerve and injectate spread in rabbit sciatic nerve block; Ultrastructural study of rabbit sciatic nerve regeneration; Nerve conduction and microanatomy in the rabbit sciatic nerve; Magnetic resonance microneurography of rabbit sciatic nerve; The toe-spreading reflex of the rabbit; NERVE STIMULATOR–GUIDED SCIATIC-FEMORAL BLOCK IN PET RABBITS; Effects of muscle relaxants on EEG, ABR and EMG in rabbits; Experimental EMG-anatomopathological study of rabbits; Sympathetic influences on the activity of the cerebral cortex (experiments on rabbits). Blooteuth (talk) 11:50, 9 September 2017 (UTC)

Thank you. Sure I got mixed up and I certainly meant "sciatic nerve." Thank you @Blooteuth. That's a wealth of information. --AboutFace 22 (talk) 13:07, 9 September 2017 (UTC)

Interaction between types of muscles
What kind of interaction can appear between types of muscles like the bulbospongiosus muscle and anal sphincter muscles and colon muscles in ejaculation? (Thanks.)--82.137.9.100 (talk) 12:02, 9 September 2017 (UTC)


 * I do not think muscle interacts with muscle, unless you are referring to the muscle fibers myosin and actin in the process of muscle contraction. Rather, it is the axon terminal of the motor neuron that interacts with skeletal muscle cells (such as anal sphincter and bulbospongiosus) at the neuromuscular junction. While ejaculation does involve the autonomic and somatic nervous systems, I don't think colonic smooth muscles are involved in ejaculation. 50.4.236.254 (talk) 16:17, 10 September 2017 (UTC)

Genetics questions.
1. What is the mechanism that determines how many possible children you can have with different dna or genes. I know the final answer, it comes to around 69,000. That is, a the probability that the 2nd child will be genetically identical to the 1st is 1 in 69,000, which is also how many different possible combinations of genes or dna in children you can have altogether.

2. What's the mechanism that says, although you are exactly 50/50 of your parents, you are not necessarily 25/25/25/25 of your 4 grandparents. So that 50/50 of your parents is not a 50/50 of their parents. If I recall, the ultimate answer is "chromosome layering" but can someone get more specific on that, thanks. 12.130.157.65 (talk) 13:10, 9 September 2017 (UTC).


 * The probability that two children will have identical DNA is virtually zero unless they are identical twins, and even then they may differ very slightly. The reason is genetic recombination, which mixes together the gene-pairs of the male and female parents in a very random way. Each egg and sperm cell sees a different pattern of recombination from every other. Genetic recombination is also the answer to your second question. Looie496 (talk) 13:46, 9 September 2017 (UTC)


 * 1) There are many mechanisms. At the basic level of selecting 1 of each pair of chromosomes to go into each sperm (or egg), there are 23 chromosomes, giving 2^23 different possible chromosome combinations for each sperm and another 2^23 for each egg, or 2^46 ~= 7*10^13 different chromosome combinations for the children of a given 2 parents.  (If I've done the combinatorics correctly.)--Wikimedes (talk) 02:45, 10 September 2017 (UTC)


 * Once you get the DNA from your parents, for the most part, there is no "memory" of which DNA came from which parent. Therefore, there is no way to guarantee you will pass exactly half of that DNA from each parent on to your children.  A thought experiment may help.  Take two bowls of random marbles, and pour half of each into a third bowl.  Now pour half of those out.  What guarantee do you have that half came from each of the original two bowls ? StuRat (talk) 02:56, 10 September 2017 (UTC)


 * Incorrect, genomic imprinting. Fgf10 (talk) 08:38, 10 September 2017 (UTC)


 * According to that article: "For the vast majority of autosomal genes, expression occurs from both alleles simultaneously. In mammals, however, a small proportion (<1%) of genes are imprinted, meaning that gene expression occurs from only one allele." So, modify my example to show one marble in each bowl was labelled to show which parent it came from.  This doesn't change the ability to guarantee grandparents each contribute DNA equally to their grandkids.  They would ALL need to be labelled to do that. StuRat (talk) 15:27, 10 September 2017 (UTC)


 * Your statement "Once you get the DNA from your parents there is no "memory" of which DNA came from which parent." is incorrect. Nobody said there is no equal contribution, but there is clearly a mechanism for determining the origin of each chromosome (a 'memory'). Also, this is not just a scientific curiosity, it is the basis for a number of diseases, as stated in the article. Fgf10 (talk) 16:43, 10 September 2017 (UTC)


 * OK, I've added "for the most part" to my original statement. StuRat (talk) 18:09, 10 September 2017 (UTC)


 * Also of interest may be Population viability analysis and Minimum viable population — Paleo  Neonate  – 03:00, 10 September 2017 (UTC)


 * The number seems to be a lot higher than that. It would be 246 if the chromosome were a faithful copy of a prior chromosome, which is (almost always) true during Mitosis. However, gametes are produced by a two-stage process that includes Meiosis, Where the genes within each chromosome pair are swapped around at the level of (approximately) the individual gene. So a gamete selects (very roughly) half of each of 20,000 or more genes, not half of each pair of 23 chromosomes, so the chance of identical gametes is more like 2-20,000 and the chance of identical non-twins is 2-40,000. -Arch dude (talk) 05:13, 10 September 2017 (UTC)
 * You're dramatically over-estimating the number of chromosomal crossovers per meiosis. It's at least two, close to three orders of magnitude lower than the number of protein-coding genes. See here. "Genes within each chromosome pair are swapped around at the level of (approximately) the individual gene" just isn't true. Adrian J. Hunter(talk•contribs) 06:03, 10 September 2017 (UTC)
 * This is true for each crossover, but there are millions of sperm per male, and each crossover event is (sort of) independent. Therefore, we cannot simply say that there are 23*2 separate units to choose from (i.e., 246 different sperm possibilities.) Instead, the total number of possible unique sperm genomes from a single father is closer to the number I gave. I will admit that the 20,000 number is a guess about the total number of possible crossover points. -Arch dude (talk) 17:17, 10 September 2017 (UTC)
 * There is about one crossover per chromosome, and there are very roughly 1024 possible crossover points per chromosome. If we grossly oversimplify and assume equal probabilities, we get 102423 possible outcomes, which is "only" 2230 for each gamete or 2460 per child, so Adrian is correct and my initial oversimplification is wrong. But this number is still unimaginably large: more than the number of electron widths needed to span the diameter of the visible universe. -Arch dude (talk) 21:35, 10 September 2017 (UTC)
 * Great analogy . Adrian J. Hunter(talk•contribs) 02:31, 11 September 2017 (UTC)
 * Note that cells contain mitochondria, which are transmitted only through the mother. 2A00:23C1:3180:B601:FC37:C608:7C91:52D6 (talk) 11:57, 10 September 2017 (UTC)


 * True, but not relevant to the question. you will have identical mitochondiral DNA with your mother and with all your siblings. This does not affect the number of possible chromosomal combinations -Arch dude (talk) 17:17, 10 September 2017 (UTC)


 * Depends on how you interpret question 2. If we are including mitochondrial DNA, then you are likely to have slightly more than 1/4 of your total DNA from your maternal grandmother, and slightly less than 1/4 from your paternal grandfather. StuRat (talk) 21:48, 10 September 2017 (UTC)
 * Your paternal grandfather, paternal grandmother, or maternal grandfather (you don't get your mitochondrial DNA from any of those three, apart from rare exceptions). The human mitochondrial genome only has 13 protein-coding genes, though: less than 0.1% of the total.  I'd think the effect of the mitochondrial genome would be dwarfed by random variation in the proportion of grandparent DNA inherited due to random cross-over locations. Sex chromosomes would make a bigger difference, especially for males, who get their X chromosomal DNA from their maternal grandparents only (again, excluding rare exceptions). Adrian J. Hunter(talk•contribs) 02:31, 11 September 2017 (UTC)
 * If you are male, the DNA on your Y chromosome came from your paternal grandfather. This has a much bigger statistical effect than the mitochondrial DNA. -Arch dude (talk) 02:49, 11 September 2017 (UTC)
 * But only maybe one order of magnitude if that, our Y chromosome says over 200 genes, at least 72 protein coding. So I suspect Adrian J. Hunter point extends to this as well, in pure statistical terms it's unlikely either of these are particularly relevant, random variation is likely to be a bigger factor. Nil Einne (talk) 07:08, 11 September 2017 (UTC)
 * Also I forgot to mention but the Pseudoautosomal regions means it's not the entire Y chromosome which is inherited "intact". Nil Einne (talk) 07:13, 11 September 2017 (UTC)
 * The Y chromosome is about 3000 times as long as the mitochondrial DNA. (The genes in the mtDNA are relatively small and very densely packed.) The pseudoautosomal region only makes up about 5% of the Y chromosome. Looie496 (talk) 15:16, 11 September 2017 (UTC)
 * True but are we considering variation of inheritance of genes or variation of inheritance of BPs? The OP's question was fairly ambigious but the earlier responses seem to be referring to variation at a gene level although admittedly not StuRat's comment. (The PAR point was more of a reminder that we should not think the entire Y chromosome as inherited intact rather than it being significant in terms of statistical analysis. I was thinking of making this clearer but unfortunately did not.) Of course once we start to consider the matter more, we get into issues like whether you can actually determine which parent the genetic material (allelles or BPs) came from (which touches on StuRat's point but in a more important way), how you count things like Trinucleotide repeat disorder (admittedly it seems very unlikely this will make a difference to the statistics), and what you do with mutations (e.g. do you count the gene as having coming from the parent it came from, even though it has a mutation with known effects bearing in mind the mutation probably arose in the parent and does it count as a different offspring when the child clearly has a unique phenotype arising from that genetic mutation); issues already touched on by you and others and how these affect any attempt to actually come up with answers other than a lot more than 69k or a lot of mechanisms. (Although I think the basic answer to the OP's 2nd question is probably chromosal crossover and independent assortment.) Nil Einne (talk) 17:42, 11 September 2017 (UTC)

Feynman Lectures. Exercises. Exercise 14-6 JPG
. .

According to the Ch. 14-5 and Eq. (14.6), the potential of a point charge $$\phi(x) = + \tfrac{Q}{4\pi\epsilon_o x}$$. I assume that inside outer shell difference of its potentials = 0, so only inner shell is counted. Then difference of the potentials will be $$\phi(R) - \phi(r) = \tfrac{Q}{4\pi\epsilon_o R} - \tfrac{Q}{4\pi\epsilon_o r}$$ ,where R = radius of outer shell; r = radius of inner shell.

$$\phi(R) - \phi(r) = \tfrac{Q(r-R)}{4\pi\epsilon_o Rr} $$

The difference and so the capacitance are negative. Is it ok? Answer gives C = 4πεoAB/(B - A).Username160611000000 (talk) 13:30, 9 September 2017 (UTC)
 * The sign of potential difference obviously depends on what you subtract from what. Ruslik_ Zero 18:06, 9 September 2017 (UTC)
 * The capacitance was negative only because you chose the inner shell (with higher potential) as reference, but Feynman's expression "potential difference between [the shells]" does not stipulate that. If you allow A = radius of inner shell and B= radius of outer shell the textbook answer applies. Blooteuth (talk) 19:05, 9 September 2017 (UTC)