Friday, March 26, 2010

Erwin Neher

I subscribe to a monthly magazine, The Scientist, which was founded by Eugene Garfield (who also was a founder of the Science Citation Index). It provides print and online coverage about biomedical research, technology and business. I am not sure what I did to deserve it, but I get a paper copy delivered to my office for free, and I can tell you for certain that the magazine is worth the price. Seriously, it is a valuable resource, and the articles are general enough that I can follow them without having to consult my physiology and biochemistry textbooks. The online site contains many of the articles for free, and also has career information for young scientists. I recommend it.

The March 2010 issue of The Scientist contains a profile of Nobel Prize winner Erwin Neher, the developer of the patch clamp technique. Russ Hobbie and I discuss patch-clamp recording in Chapter 9 of the 4th edition of Intermediate Physics for Medicine and Biology.
“The next big advance was patch-clamp recording [Neher and Sakmann (1976)]. Micropipettes were sealed against a cell membrane that had been cleaned of connective tissue by treatment with enzymes. A very-high-resistance seal resulted [(2-3) × 107 Ohm] that allowed one to see the opening and closing of individual channels. For this work Erwin Neher and Bert Sakmann received the Nobel Prize in Physiology or Medicine in 1991. Around 1980, Neher’s group found a way to make even higher-resistance (1010-1011 Ohm) seals that reduced the noise even further and allowed patches of membrane to be torn from the cell while adhering to the pipette [Hamill et al. (1981)]…”
The profile in The Scientist provides some insight into how this research began.
“[Neher’s former postdoc Fred] Sigworth remembers it well. ‘I came into lab that Monday morning and Erwin said, with a twinkle in his eye, ‘I think I know how you’re going to see sodium channels,’’ he says. These channels—essential to neural communication—had proven elusive because they produce such small currents and remain open for such a short time. But thanks to the team’s new ‘patch-clamp’ technique—and in particular, the formation of an incredibly tight seal, or ‘gigaseal,’ between the pipette tip and the cell membrane—‘seeing sodium channels suddenly became really easy,’ says Sigworth, who, along with Neher, published these observations (and the first description of the tight-seal patch-clamp technique) in Nature in 1980.”
I always enjoy reading about the quirks and odd twists of fate that often accompany scientific advance. The profile in The Scientist provides an entertaining anecdote.
“You also needed to suck. ‘You had to apply a little bit of suction in order to pull some membrane into the orifice of the pipette,’ says Neher. ‘If you did it the right way, it worked.’ At least for Neher. ‘There was a weird period where we could no longer get gigaseals,’ recalls [Owen] Hamill [a postdoc at the time]. ‘Then Bert suggested you have to blow before you suck.’ Gently blowing a solution through the pipette as it approaches the surface of the cell keeps the tip from picking up debris during the descent. Between the blowing and the sucking, Hamill says, ‘our effeiciency went up to 99 percent.”
I fond it interesting that Neher’s undergraduate degree was in physics, and it was only after he arrived at the University of Wisconsin on a Fulbright Scholarship that he began studying biophysics. In his Nobel autobiography he describes his early motivation for studying biological physics.
“At the age of 10, I entered the 'Maristenkolleg' at Mindelheim [...] the local 'Gymnasium' is operated by a catholic congregation, the 'Maristenschulbrüder'. The big advantage of this school was that our teachers - both those belonging to the congregation and others - were very dedicated and were open not only to the subject matter but also to personal issues. During my years at the Gymnasium (1954 to 1963) I found out that, next to my interest in living things, I also could immerse myself in technical and analytical problems. In fact, pretty soon, physics and mathematics became my favourite subjects. At the same time, however, new concepts unifying these two areas had seeped into the literature, which was accessible to me. I eagerly read about cybernetics, which was a fashionable word at that time, and studied everything in my reach on the 'Hodgkin-Huxley theory' of nerve excitation. By the time of my Abitur - the examination providing access to university - it was clear to me that I should become a 'biophysicist'. My plan was to study physics, and later on add biology.”
Neher provides a classic example of how a strong background in physics can lead to advances in biology and medicine, a major theme underlying Intermediate Physics for Medicine and Biology.

Friday, March 19, 2010

How Should We Teach Physics to Future Life Scientists and Physicians?

The American Physical Society publishes a monthly newspaper, the APS News, and the back page of each issue contains an editorial that goes under the name—you guessed it— “The Back Page.” Readers of the 4th edition of Intermediate Physics for Medicine and Biology will want to read The Back Page in the March 2010 issue, subtitled “Physics for Future Physicians and Life Scientists: A Moment of Opportunity.” This excellent editorial--written by Catherine Crouch, Robert Hilborn, Suzanne Amador Kane, Timothy McKay, and Mark Reeves—champions many of the ideas that underlie our textbook. The editorial begins
“How should we teach physics to future life scientists and physicians? The physics community has an exciting and timely opportunity to reshape introductory physics courses for this audience. A June 2009 report from the American Association of Medical Colleges (AAMC) and the Howard Hughes Medical Institute (HHMI), as well as the National Research Council’s Bio2010 report, clearly acknowledge the critical role physics plays in the contemporary life sciences. They also issue a persuasive call to enhance our courses to serve these students more effectively by demonstrating the foundational role of physics for understanding biological phenomena and by making it an explicit goal to develop in students the sophisticated scientific skills characteristic of our discipline. This call for change provides an opportunity for the physics community to play a major role in educating future physicians and future life science researchers.

A number of physics educators have already reshaped their courses to better address the needs of life science and premedical students, and more are actively doing so. Here we describe what these reports call for, their import for the physics community, and some key features of these reshaped courses. Our commentary is based on the discussions at an October 2009 conference (www.gwu.edu/~ipls), at which physics faculty engaged in teaching introductory physics for the life sciences (IPLS), met with life scientists and representatives of NSF, APS, AAPT, and AAMC, to take stock of these calls for change and possible responses from the physics community. Similar discussion on IPLS also took place at the 2009 APS April Meeting, the 2009 AAPT Summer Meeting, and the February 2010 APS/AAPT Joint Meeting.”
One key distinction between our textbook and the work described in The Back Page editorial is that our book is aimed toward an intermediate level, while the IPLS movement is aimed at the introductory level. Like it or not, premedical students have a difficult time fitting additional physics courses into their undergraduate curriculum. I know that here at Oakland University, I have been able to entice only a handful of premed students to take my PHY 325 (Biological Physics) and PHY 326 (Medical Physics) classes, despite my best efforts to attract them and despite OU’s large number of students hoping to attend medical school (these classes have our two-semester introductory physics sequence as a prerequisite). So, I think there is merit in revising the introductory physics class, which premedical students are required to take, if your goal is to influence premedical education. As The Back Page editorial states, “the challenge is to offer courses that cultivate general quantitative and scientific reasoning skills, together with a firm grounding in basic physical principles and the ability to apply those principles to living systems, all without increasing the number of courses needed to prepare for medical school.” The Back Page editorial also cites the "joint AAMC-HHMI committee […] report, Scientific Foundations for Future Physicians (SFFP). This report calls for removing specific course requirements for medical school admission and focusing instead on a set of scientific and mathematical ‘competencies.’ Physics plays a significant role […]"

How do you fit all the biomedical applications of physics into an already full introductory class? The Back Page editorial gives some suggestions. For instance, “an extended discussion of kinematics and projectile motion could be replaced by more study of fluids and continuum mechanics [… and] topics such as diffusion and open systems could replace the current focus on heat engines and equilibrium thermal situations.” I agree, especially with adding fluid dynamics (Chapter 1 in our book) and diffusion (Chapter 4), which I believe are absolutely essential for understanding biology. I have my own suggestions. Although Newton's universal law of gravity, Kepler’s laws of planetary motion, and the behavior of orbiting satellites are fascinating and beautiful topics, a premed student may benefit more from the study of osmosis (Chapter 5) and sound (Chapter 13, including ultrasound). Electricity and magnetism remains a cornerstone of introductory physics (usually in a second semester of a two-semester sequence), but the emphasis could be different. For instance, Faraday’s law of induction can be illustrated using magnetic stimulation of the brain, Ampere’s law by the magnetic field around a nerve axon, and the dipole approximation by the electrocardiogram. In a previous post to this blog, I discussed how Intermediate Physics for Medicine and Biology addresses many of these issues. Russ Hobbie will be giving an invited paper about medical physics and premed students at the July 2010 meeting of the American Association of Physics Teachers. When he gives the talk it will be posted on the book website.

One way to shift the focus of an introductory physics class toward the life sciences is to create new homework problems that use elementary physics to illustrate biological applications. In the 4th edition of Introductory Physics for Medicine and Biology, Russ Hobbie and I constructed many interesting homework problems about biomedical topics. While some of these may be too advanced for an introductory class, others may (with some modification) be very useful. Indeed, teaching a traditional introductory physics class but using a well-crafted set of homework problems may go a long ways toward achieving the goals set out by The Back Page editorial.

Let me finish this blog entry by quoting the eloquent final paragraph of The Back Page editorial. Notice that the editorial ends with the same central question that began it. It is the question that motivated Russ Hobbie to write the first edition of Intermediate Physics for Medicine and Biology (published in 1978) and it is the key question that Russ and I struggled with when working on the 4th edition.
“The physics community faces a challenging opportunity as it addresses the issues surrounding IPLS courses. A sizable community we serve has articulated a clear set of skills and competencies that students should master as a result of their physics education. We have for a number of decades incorporated engineering examples into our physics classes. The SFFP report asks us to respond to another important constituency. Are we ready to develop courses that will teach our students how to apply basic physical principles to the life sciences? The challenges of making significant changes in IPLS courses are daunting if we each individually try to take on the task. But with a community-wide effort, we should be able to meet this challenge. The physics community is already moving to develop and implement changes in IPLS courses, and the motivations for change are strong. The life science and medical school communities stress that a working knowledge of physical principles is essential to success in all areas of life science including the practice of medicine. Thus we see significant teaching and learning opportunities as we work to answer the question that opened our discussion: how should we teach physics to future physicians and life scientists?”

Friday, March 12, 2010

The Strangest Man

I recently read The Strangest Man: The Hidden Life of Paul Dirac, Mystic of the Atom, by Graham Farmelo, a fascinating biography of the Nobel Prize winning physicist Paul Adrien Maurice Dirac. One thing I did not find in the book was biological or medical physics. Nevertheless, Russ Hobbie and I mention Dirac in Chapter 11 of the 4th edition of Intermediate Physics for Medicine and Biology, in connection with the Dirac delta function.
“The δ function can be thought of as a rectangle of width a and height 1/a in the limit [as a goes to zero…]. The δ function is not like the usual function in mathematics because of its infinite discontinuity at the origin. It is one of a class of ‘generalized functions’ whose properties have been rigorously developed by mathematicians since they were first used by the physicist P. A. M. Dirac.”
Dirac won his Nobel Prize for contributions to quantum mechanics. I bought a copy of his famous textbook The Principles of Quantum Mechanics when I was an undergraduate at the University of Kansas. Farmelo describes it as “never out of print, it remains the most insightful and stylish introduction to quantum mechanics and is still a powerful source of inspiration for the most able young theoretical physicists. Of all the textbooks they use, none presents the theory with such elegance and with such relentless logic.”

One of Dirac’s greatest contributions was the prediction of positive electrons, or positrons, a type of antimatter. His prediction arose from the relativistic wave equation for the electron, now called the Dirac equation. One interesting feature of the Dirac equation is that it implies negative energy states. The only time these negative states are observable is when an electron is missing from one of the states: a hole. Farmelo writes
“The bizarre upshot of the theory is that the entire universe is pervaded by an infinite number of negative-energy electrons – what might be thought of as a ‘sea’. Dirac argued that this sea has a constant density everywhere, so that experimenters can observe only departures from this perfect uniformity. […] Only a disturbance in Dirac’s sea—a bursting bubble, for example—would be observable. He envisaged just this when he foresaw that there would be some vacant states in the sea of negative-energy electrons, causing tiny departures from the otherwise perfect uniformity. Dirac called these unoccupied states ‘holes’. […] Each hole has positive energy and positive charge—the properties of the proton, the only other subatomic particle known at that time [1929]. So Dirac made the simplest possible assumption by suggesting that a hole is a proton.”
We now know that these holes are not protons but are positrons, discovered experimentally in 1932 by Carl Anderson. Positrons are vital for understanding how x-rays interact with matter, as Russ and I describe in Section 15.6 of Intermediate Physics for Medicine and Biology
“A photon with energy above 1.02 MeV can produce a particle-antiparticle pair: a negative electron and a positive electron or positron. […] Since the rest energy (mc2) of an electron or positron is 0.51 MeV, pair production is energetically impossible for photons below 2mc2 = 1.02 MeV.

One can show, using o = pc for the photon, that momentum is not conserved by the positron and electron if Eq. 15.23 [conservation of energy] is satisfied. However, pair production always takes place in the Coulomb field of another particle (usually a nucleus) that recoils to conserve momentum.”
In Sec. 17.14, Russ and I describe the crucial role positrons play in medical imaging.
“If a positron emitter is used as the radionuclide, the positron comes to rest and annihilates an electron, emitting two annihilation photons back to back. In positron emission tomography (PET) these are detected in coincidence. This simplifies the attenuation correction, because the total attenuation for both photons is the same for all points of emission along each gamma ray through the body (see Problem 54). Positron emitters are short-lived, and it is necessary to have a cyclotron for producing them in or near the hospital. This is proving to be less of a problem than initially imagined. Commercial cyclotron facilities deliver isotopes to a number of nearby hospitals. Patterson and Mosley (2005) found that 97% of the people in the United States live within 75 miles of a clinical PET facility.”
Another famous prediction of Dirac’s was magnetic monopoles. Russ and I only mention monopoles in passing in Section 8.8.1: “Since there are no known magnetic charges (monopoles), we must consider the effect of magnetic fields on current loops or magnetic dipoles.” Dirac predicted that magnetic monopoles could in fact exist. Farmelo tells the story:
“In Cambridge, during the spring of 1931, Dirac happened upon a rich new seam of ideas that would crystallize into one of his most famous contributions to science. […] As usual, Dirac appears to have said nothing of this to anyone, even to his close friends. In the early months of 1931, a quiet time for his fellow theoreticians, he was working on the most promising new theory he had conceived for years. The theory broke new ground in magnetism. For centuries, it had been a commonplace of science that magnetic poles come only in pairs, labeled north and south: if one pole is spotted, then the opposite one will be close by. Dirac had found that quantum theory is compatible with the existence of single magnetic poles. During a talk at the Kapitza Club, he dubbed them magnons, but the name never caught on in this context; the particles became known as magnetic monopoles.”
Physicists have searched for magnetic monopoles, and once they even thought they found one. In 1982, physicist Blas Cabrera observed a signal consistent with the experimental signature of a monopole (Physical Review Letters, Volume 48, Pages 1378-1381), but it now appears to have been an artifact, as the result has never been reproduced. I have my own remote (indeed, very remote) connection with this experiment (and thus to Dirac). Cabrera’s PhD advisor, William Fairbank, was John Wikswo’s PhD advisor, and Wikswo was in turn my PhD advisor. Thus, academically speaking, I am one of Cabrera’s scientific nephews.

Dirac was known for saying little and behaving rather oddly (the title of the book is, after all, “The Strangest Man”), and Farmelo suggests a possible reason why: Dirac may have been autistic.
“[Dirac] always attributed his extreme taciturnity and stunted emotions to his father’s disciplinarian regime; but there is another, quite different explanation, namely that he was autistic. Two of Dirac’s younger colleagues confided in me that they had concluded this, each of them making their disclosure in sotto voce, as if they were imparting a shameful secret. Both refused to be quoted. […] There is not nearly enough detail in her [Dirac’s mother’s] comments or in reports of Dirac’s behaviour in school to justify a diagnosis that he was then autistic. His behavior as an a adult, however, had all the characteristics that almost every autistic person has to some degree—reticence, passivity, aloofness, literal-mindedness, rigid patterns of activity, physical ineptitude, self-centredness and, above all, a narrow range of interests and a marked inability to empathise with other human beings.”
Whatever the cause of Dirac’s unusual behavior, he was a great physicist. Farmelo sums up Dirac’s enduring legacy at the end of his book.
“There is no doubt that Dirac was a great scientist, one of the few who deserves a place just below Einstein in the pantheon of modern physicists. Along with Heisenberg, Jordan, Pauli, Schrodinger and Born, Dirac was one of the group of theoreticians who discovered quantum mechanics. Yet his contribution was special. In his heyday, between 1925 and 1933, he brought a uniquely clear vision to the development of a new branch of science: the book of nature often seemed to be open in front of him.”

Friday, March 5, 2010

Magnetic Measurements of Peripheral Nerve Function Using a Neuromagnetic Current Probe

Section 8.9 in the 4th Edition of Intermediate Physics for Medicine and Biology describes how a toroidal probe can be used to measure the magnetic field of a nerve.
“If the signal [a weak biomagnetic field] is strong enough, it can be detected with conventional coils and signal-averaging techniques that are described in Chapter 11. Barach et al. (1985) used a small detector through which a single axon was threaded. The detector consisted of a toroidal magnetic core wound with many turns of fine wire (Fig. 8.26). Current passing through the hole in the toroid generated a magnetic field that was concentrated in the ferromagnetic material of the toroid. When the field changed, a measurable voltage was induced in the surrounding coil.”
My friend Ranjith Wijesinghe, of the Department of Physics at Ball State University, recently published the definitive review of this research in the journal Experimental Biology and Medicine, titled Magnetic Measurements of Peripheral Nerve Function Using a Neuromagnetic Current Probe (Volume 235, Pages 159-169).
“The progress made during the last three decades in mathematical modeling and technology development for the recording of magnetic fields associated with cellular current flow in biological tissues has provided a means of examining action currents more accurately than that of using traditional electrical recordings. It is well known to the biomedical research community that the room-temperature miniature toroidal pickup coil called the neuromagnetic current probe can be employed to measure biologically generated magnetic fields in nerve and muscle fibers. In contrast to the magnetic resonance imaging technique, which relies on the interaction between an externally applied magnetic field and the magnetic properties of individual atomic nuclei, this device, along with its room-temperature, low-noise amplifier, can detect currents in the nano-Ampere range. The recorded magnetic signals using neuromagnetic current probes are relatively insensitive to muscle movement since these probes are not directly connected to the tissue, and distortions of the recorded data due to changes in the electrochemical interface between the probes and the tissue are minimal. Contrary to the methods used in electric recordings, these probes can be employed to measure action currents of tissues while they are lying in their own natural settings or in saline baths, thereby reducing the risk associated with elevating and drying the tissue in the air during experiments. This review primarily describes the investigations performed on peripheral nerves using the neuromagnetic current probe. Since there are relatively few publications on these topics, a comprehensive review of the field is given. First, magnetic field measurements of isolated nerve axons and muscle fibers are described. One of the important applications of the neuromagnetic current probe to the intraoperative assessment of damaged and reconstructed nerve bundles is summarized. The magnetic signals of crushed nerve axons and the determination of the conduction velocity distribution of nerve bundles are also reviewed. Finally, the capabilities and limitations of the probe and the magnetic recordings are discussed.”
Ranjith and I were both involved in this research when we were graduate students working in John Wikswo’s laboratory at Vanderbilt University. I remember the painstaking process of making those toroids; just winding the wire onto the ferrite core was a challenge. Wikswo built this marvelous contraption that held the core at one spot under a dissection microscope but at the same time allowed the core to be rotated around multiple axes (he's very good at that sort of thing). When Russ Hobbie and I wrote about “many turns of fine wire” we were not exaggerating. The insulated copper wire was often as thin as 40-gauge (80 microns diameter), which is only slightly thicker than a human hair. With wire that thin, the slightest kink causes a break. We sometimes wound up to 100 turns on one toroid. It was best to wind the toroid when no one else was around (I preferred early morning): if someone startled you when you were winding, the result was usually a broken wire, requiring you to start over. We potted the toroid and its winding in epoxy, which itself was a job requiring several steps. We machined a mold out of Teflon, carefully positioned the toroid in the mold, soldered the ends of those tiny wires to a coaxial cable, and then injected epoxy into the mold under vacuum to avoid bubbles. If all went as planned, you ended up with a “neuromagnetic current probe” to use in your experiments. Often, all didn’t go as planned.

Ranjith’s review describes the work of several of my colleagues from graduate school days. Frans Gielen (who now works at the Medtronic Bakken Research Centre in Maastricht, the Netherlands) was a post doc who used toroids to record action currents in skeletal muscle. Ranjith studied compound action currents in the frog sciatic nerve for his PhD dissertation. My research was mostly on crayfish axons. Jan van Egeraat was the first to measure action currents in a single muscle fiber, did experiments on squid giant axons, and studied how the magnetic signal changed near a region of the nerve that was damaged. Jan obtained his PhD from Vanderbilt a few years after I did, and then tragically died of cancer just as his career was taking off. I recall that when my wife Shirley and I moved from Tennessee to Maryland to start my job at the National Institutes of Health, Jan and his wife drove the rented truck with all our belongings while we drove our car with our 1-month old baby. They got a free trip to Washington DC out of the deal, and we got a free truck driver. John Barach was a Vanderbilt professor who originally studied plasma physics, but changed to biological physics later in his career when collaborating with Wikswo. I always have admired Barach’s ability to describe complex phenomena in a very physical and intuitive way (see, for instance, Problem 13 in Chapter 8 of our textbook). Of course, we were all led by Wikswo, whose energy and drive are legendary, and whose grant writing skills kept us all in business. For his work developing the Neuromagnetic Current Probe, Wikswo earned an IR-100 Award in 1984, presented by the R&D Magazine to recognize the 100 most technologically significant new products and processes of the year.