Friday, July 27, 2012

Frank Netter, Medical Illustrator

When I started graduate school at Vanderbilt University, I had a strong background in physics but was weak in biology and medicine. One of the sources I used to learn some anatomy was the eight-volume CIBA Collection of Medical Illustrations by Frank Netter. I dearly loved browsing through his illustrations. Because of my interest in cardiac electrophysiology, I was particularly fond of Volume 5 about the heart. Some of his illustrations can be seen online here, here, here, here, and here.

At http://www.netterimages.com you can learn much about Netter and his work. Some of Netter’s books have recently been updated and reissued. A video about this reissue includes an interview with Netter, showing him at work on his drawings. Netter’s Atlas of Neuroscience was updated by David Felten, the Associate Dean for Research at the Oakland University William Beaumont School of Medicine. I often see students walking around the OU campus carrying Netter’s Atlas of Human Anatomy. You can even buy Netter flash cards.

The Society of Illustrators Hall of Fame contains an entry about Netter.
“Frank H. Netter (1906–1991) was born in New York and grew up during the Golden Age of Illustration. He studied at the National Academy of Design, and later at the Art Students League. But his mother wanted him to be a doctor, and when she died suddenly, he resolved to give up art, and study medicine as she had wished. He graduated from City College of New York, BS 1927, and New York University Medical College, MD 1931. But the demand for his pictures far exceeded the demand for his surgery.”
More about Netter’s life is described in his New York Times obituary. Also, see the article Frank H Netter, Medicine's Michelangelo: An Editorial Perspective, by Rita Washko (Science Editor, Volume 29, Pages 16-18, 2006).

Readers of the 4th edition of Intermediate Physics for Medicine and Biology who need to brush up on the anatomy should take a look at Netter’s books.

Friday, July 20, 2012

A Mechanism for Anisotropic Reentry in Electrically Active Tissue

1992 was a good year for me. My wife and I, who had been married for seven years, had two young daughters, and we had just bought a house in Kensington, Maryland. While working at the National Institutes of Health I published eight papers in 1992, mostly about magnetic stimulation of nerves. My favorite paper from that year, however, was one about the heart: “A Mechanism for Anisotropic Reentry in Electrically Active Tissue” (Journal of Cardiovascular Electrophysiology, Volume 3, Pages 558-566). The lead author was Joshua Saypol, an engineering undergraduate at Brown University who would come home to Maryland each summer and work at NIH. Josh was a big, strong fellow, and handy to have around when we had heavy things to move. But he was also smart and hard-working, and we ended up publishing three papers together. The cardiac paper was the last of these, and the least cited (indeed, according to the Web of Science the paper has not been cited by anyone other than me for the last ten years). You can get the gist of it from the abstract.
Introduction: Numerical simulations of wavefront propagation were performed using a two-dimensional sheet of tissue with different anisotropy ratios in the intracellular and extracellular spaces.
Methods and Results: The tissue was represented by the bidomain model, and the active properties of the membrane were described by the Hodgkin-Huxley equations. Two successive stimuli, delivered through a single point electrode, resulted in the formation of a reentrant wavefront when the second stimulus was delivered during the vulnerable period of the first wavefront.
Conclusion: The mechanism for the development of reentry was that the bidomain tissue responded to point cathodal stimulation by depolarizing the tissue under the electrode in the direction perpendicular to the fiber axis, and hyperpolarizing the tissue in the direction parallel to the fiber axis. Such a distribution of depolarization and hyperpolarization modifies the refractory period of the action potential differently in each direction, resulting in block in the direction perpendicular to the fiber axis and leading to reentry and the formation of stable, rotating wavefronts.”
The paper arises from two previous lines of research. First is the calculation of the transmembrane potential induced by a point stimulus, performed by Nestor Sepulveda, John Wikswo and me (“Current Injection Into a Two Dimensional Bidomain,” Biophysical Journal, Volume 55, Pages 987-999, 1989), which I discussed in a previous blog entry. We found that cardiac tissue is depolarized (positive transmembrane potential) under a cathode, but hyperpolarized (negative transmembrane potential) a millimeter or two from the cathode in each direction along the cardiac fibers (at what are nowadays called “virtual anodes”). That paper used a passive steady-state membrane, but in a subsequent paper I derived an algorithm to solve the bidomain equations including time dependence and an active model for the membrane kinetics (“Action Potential Propagation in a Thick Strand of Cardiac Muscle,” Circulation Research, Volume 68, Pages 162-173, 1991). Having this algorithm, I decided to investigate what effect the virtual anodes had on propagation following an extracellular stimulus. Both of these papers were based on the bidomain model, which is a mathematical model of the electrical properties of cardiac tissue that Russ Hobbie and I describe in Chapter 7 of the 4th edition of Intermediate Physics for Medicine and Biology.

Another reason I like the paper with Josh so much is that we collaborated with Arthur Winfree while writing it. I’ve written about our work with Winfree previously in this blog. Art had made a prediction about the induction of “reentry” (a cardiac arrhythmia) following a premature stimulus (a stimulus applied to tissue near the end of its refractory period). It provided the ideal hypothesis to test with our model. I remember vividly Josh coming to me with plots of the transmembrane potential showing the first signs of reentry, and how even after our discussions with Art we didn’t quite believe what we were seeing. Winfree helped Josh and I publish a preliminary letter about our calculation in the International Journal of Bifurcation and Chaos (Volume 1, Pages 927-928, 1991), which was just starting up. It has been almost ten years now since Art passed away, and I still miss him.

Astute readers (and I’m sure ALL readers of this blog are astute) will notice something odd in the abstract quoted above: “the active properties of the membrane were described by the Hodgkin-Huxley equations” (my italics). What are the Hodgkin-Huxley equations--which describe a squid nerve axon action potential (see Chapter 6 of Intermediate Physics for Medicine and Biology)--doing in a paper about cardiac tissue? This is a legitimate criticism, and one the reviewers raised, but surprisingly it didn’t prove fatal for publication of the article (although I was asked to change the title to the generic "...Electrically Active Tissue," and you won't find the word "cardiac" in the abstract). Basically, I used the Hodgkin-Huxley model because I didn’t know any better at the time. (In the paper, we claimed that “we used the Hodgkin-Huxley model instead of a myocardial membrane model because of a limitation of computer resources.”, and that might be part of the reason too.) Models more appropriate for cardiac tissue (such as the Beeler-Reuter model that I used in later publications) were more complicated, and I wasn’t familiar with them. Besides, Josh and I were most interested in generic properties of reentry induction that would probably not be too sensitive to the membrane model (or so we told ourselves). I wonder now why we didn’t use the generic FitzHugh-Nagumo model, but we didn’t. Never again would reviewers let me get away with using the Hodgkin-Huxley model for cardiac tissue (and rightly so), and I suspect it is one of the reasons the paper is rarely cited anymore.

Nevertheless, the paper did make an important contribution to our understanding of the induction of reentry, which is why I like it so much. It was the first paper to present the idea of regions of hyperpolarization shortening the refractory period, and thereby creating a region of excitable tissue through which wave fronts can propagate. We state this clearly in the discussion
“The crucial point is that a premature stimulus causes an unusually-shaped transmembrane potential distribution that produces a directionally-dependent change of the refractory period, thereby creating a necessary condition for conduction block in one direction”
We go into more detail in the results
“The depolarization wavefront is followed by a front of refractoriness. During the refractory period, the sodium channel inactivation gate (the h gate) opens slowly, while the potassium channel [activation gate] (the n gate) closes slowly; the tissue remains refractory until these two gates have recovered sufficiently. If a hyperpolarizing current is applied to the tissue during the refractory period, it will cause the h gate to open and the n gate to close more quickly than they normally would, thereby shortening the refractory period. Thus, when the tissue is stimulated, the refractory period is shortened in the area of hyperpolarization along the x axis [parallel to the fibers]. In the area along the y axis [perpendicular to the fibers] that is depolarized by the stimulus, on the other hand, the h and n gates move away from their resting values, and therefore the refractory period is lengthened. If the second stimulus is timed just right, it can take the tissue along the x direction out of the refractory period, while along the y direction the tissue remains unexcitable. Thus, the action potential elicited by the large depolarization directly below the electrode can propagate only in the x direction.”
This idea influenced subsequent work by myself (“Nonsustained Reentry Following Successive Stimulation of Cardiac tissue Through a Unipolar Electrode,” Journal of Cardiovascular Electrophysiology, Volume 8, Pages 768-778, 1997) and others (Efimov et al., “Virtual Electrode-Induced Phase Singularity: A Basic Mechanism of Defibrillation Failure,” Circulation Research, Volume 82, Pages 918-925, 1998), and now, twenty years later, lies at the heart of the concept of “virtual electrodes” and their role during defibrillation (see, for instance: Efimov, Gray, and Roth, “Virtual Electrodes and Deexcitation: New Insights into Fibrillation Induction and Defibrillation,” Journal of Cardiovascular Electrophysiology, Volume 11, Pages 339-353, 2000). After I left NIH, Marc Lin, Wikswo and I confirmed experimentally this mechanism of reentry induction (“Quatrefoil Reentry in Myocardium: An Optical Imaging Study of the Induction Mechanisms,” Journal of Cardiovascular Electrophysiology, Volume 10, Pages 574-586, 1999).

The acknowledgments section of the paper brings back many memories.
"Acknowledgments: We thank Art Winfree for his many ideas and suggestions, Peter Basser for his careful reading of the manuscript and Barry Bowman for his editorial assistance. The calculations were performed on the NIH Convex C240 computer. We thank the staff of the NIH computer center for their support."
First, of course, we mentioned Art’s contributions. Peter Basser, the inventor of MRI Diffusion Tensor Imaging, was a friend of mine at NIH, and we used to read each others papers before submission to a journal. Barry Bowman also worked at NIH. He was a former high school English teacher, and I would always give him drafts of my papers for polishing. Much of what I know about writing English well I learned from him. I suspect my current laptop computer can calculate faster than the Convex C240 supercomputer could, but it was fairly powerful for the time. In 1992, Josh and I did our programming in FORTRAN. Some things never change.

Friday, July 13, 2012

Magnetic Characterization of Isolated Candidate Vertebrate Magnetoreceptor Cells

Big news this week in the field of magnetoreception. A paper titled “Magnetic Characterization of Isolated Candidate Vertebrate Magnetoreceptor Cells” by Stephan Eder and his colleagues was published online (“early edition”) in the Proceedings of the National Academy of Sciences. One commentator went so far as to suggest that these magnetoreceptors are "the biological equivalent of the elusive Higgs boson" (an exaggeration, but a catchy quote with a grain of truth). The abstract to the paper is given below.
Over the past 50 y, behavioral experiments have produced a large body of evidence for the existence of a magnetic sense in a wide range of animals. However, the underlying sensory physiology remains poorly understood due to the elusiveness of the magnetosensory structures. Here we present an effective method for isolating and characterizing potential magnetite-based magnetoreceptor cells. In essence, a rotating magnetic field is employed to visually identify, within a dissociated tissue preparation, cells that contain magnetic material by their rotational behavior. As a tissue of choice, we selected trout olfactory epithelium that has been previously suggested to host candidate magnetoreceptor cells. We were able to reproducibly detect magnetic cells and to determine their magnetic dipole moment. The obtained values (4 to 100 fA m2) greatly exceed previous estimates (0.5 fA m2). The magnetism of the cells is due to a μm-sized intracellular structure of iron-rich crystals, most likely single-domain magnetite. In confocal reflectance imaging, these produce bright reflective spots close to the cell membrane. The magnetic inclusions are found to be firmly coupled to the cell membrane, enabling a direct transduction of mechanical stress produced by magnetic torque acting on the cellular dipole in situ. Our results show that the magnetically identified cells clearly meet the physical requirements for a magnetoreceptor capable of rapidly detecting small changes in the external magnetic field. This would also explain interference of ac powerline magnetic fields with magnetoreception, as reported in cattle.
The PNAS published a highlight about the article.
Identification of cells that sense Earth's magnetic field
Researchers have isolated magnetic cells thought to underlie certain animals' ability to navigate by Earth's magnetic field. Behavioral studies have long provided evidence for the existence of a magnetic sense, but the identity of the specialized cells that comprise this internal compass has remained elusive. Stephan Eder and colleagues isolated the putative magnetic field-sensing cells that line the trout's nasal cavity, and which contain iron-rich deposits of the magnetic material called magnetite. The authors placed a suspension of trout nasal tissue under a light microscope, and identified magnetic cells by their rotational motion in the presence of a slowly rotating external magnetic field. After siphoning off the rotating cells to characterize them in greater detail, the authors discovered that each cell contained reflective, iron-rich magnetic particles that were anchored to the cell membrane. The authors also determined that the cells are about 100 times more sensitive to magnetic fields than previously estimated. The findings suggest that the cells are capable of detecting magnetic north as well as small changes in the external magnetic field, and could form the basis of an accurate magnetic sensory system, according to the authors.
Russ Hobbie and I discuss the role of magnetic materials in biology in our chapter about biomagnetism in the 4th edition of Intermediate Physics for Medicine and Biology. We included in our book a photograph of magnetosomes (intracellular magnetite particles) in magnetotactic bacteria. In the photo, the magnetosomes are each about 0.05 μm on a side, and about 20 particles form a line roughly 1 μm long. Eder et al., on the other hand, find magnetic inclusions that are more spherical, and roughly 1-2 μm across. A trout cell has a really big magnetic moment when it contains such a large inclusion, but less than one cell in a thousand responds to the magnetic field and therefore presumably contains one. For a magnetotactic bacterium to have the same magnetic moment, it would need to be packed solid with magnetite.

I find the PNAS paper to be fascinating, and the method to detect individual cells using a rotating magnetic field is clever. However, in my opinion the last sentence of the abstract is a bit speculative, given that typical residential 60 Hz magnetic fields are 10,000 times smaller than the 2 mT fields used by Eder et al., and the frequency is almost 200 times higher. Granted the large magnetic moment makes the idea of powerline field detection intriguing, but that hypothesis is far from proven and I remain skeptical.

One of the coauthors on the PNAS paper is Joseph Kirschvink, whose work we discuss extensively in Section 9.10, “Possible Effects of Weak External Electric and Magnetic Fields.” Kirschvink is the Nico and Marilyn Van Wingen Professor of Geobiology at Caltech. He has developed several fascinating and controversial hypotheses, such as the Snowball Earth concept and the idea that a meteor found in 1984 contains evidence of life on Mars (he collaborated with my PhD advisor, John Wikswo, to make magnetic field measurements on that meteor). Kirschvink received the William Gilbert Award from the American Geophysical Union in 2011 for his work on geomagnetism. In the citation for this award, Benjamin Weiss of MIT wrote that "Joe represents everything we are looking for in a William Gilbert awardee. He is an 'ideas man,' a gadfly, working at the edge of the crowd while the crowd chases after him!" Kirschvink has also won Caltech's Feynman Prize for Excellence in Teaching.

The PNAS paper has triggered an avalanche of press reports, including those in Science News,  the International Science Times, Science Daily, Phys.org, Live Science, and Discover Magazine.

Friday, July 6, 2012

Women in Medical Physics

Last week in this blog, I discussed the medical physicist Rosalyn Yalow, who was the second female to win the Nobel Prize in Physiology or Medicine (The first was biochemist Gerty Cori), and who developed, with Solomon Berson, the radioimmunoassay technique. Her story reminds us of the important contributions of females to medical physics. I am particularly interested in this topic because Oakland University recently was awarded an ADVANCE grant from the National Science Foundation, with the goal of increasing the participation and advancement of women in academic science and engineering careers. I am on the leadership team of this project, and we are working hard to improve the environment for female STEM (science, technology, engineering, and math) faculty.

Of course the real reason I support increasing opportunities for women in the sciences is that I am certain many of the readers of the 4th edition of Intermediate Physics for Medicine and Biology are female. Medical physics provides several role models for women. For instance, Aminollah Sabzevari published an article in the Science Creative Quarterly titled Women in Medical Physics. Sabzevari begins
“Traditionally, physics has been a male-dominated occupation. However, throughout history there have been exceptional women who have risen above society’s restrictions and contributed greatly to the advancement of physics. Women have played an important role in the creation, advancement and application of medical physics. As a frontier science, medical physics is less likely to be bound by society’s norms and less subject to the inherent glass ceiling limiting female participation. Women such as Marie Curie, Harriet Brooks, and Rosalind Franklin helped break through that ceiling, and their contributions are worth observing.”
Another notable female medical physicist is Edith Hinkley Quimby, who established the first measurements of safe levels of radiation. The American Association of Physicists in Medicine named the Edith H. Quimby Lifetime Achievement Award in her honor.

On a related note (through having little to do with medical or biological physics), I recently read a fascinating biography of Sophie Germain (1776-1831), who did fundamental work in number theory and elasticity.

Finally, in my mind the greatest female physicist of all time (yes, greater than Marie Curie) is Lise Meitner, who first discovered nuclear fission. A great place to learn more about her life and work is Richard Rhodes’ masterpiece The Making of the Atomic Bomb.

One characteristic these women have in common is that they overcame great obstacles in order to become scientists. Their tenacity and determination inspires us all.