Medical Terminology Daily (MTD) is a blog sponsored by Clinical Anatomy Associates, Inc. as a service to the medical community. We post anatomical, medical or surgical terms, their meaning and usage, as well as biographical notes on anatomists, surgeons, and researchers through the ages. Be warned that some of the images used depict human anatomical specimens.

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A Moment in History

Larsen

William J. Larsen, PhD
(1942-2000)

An American scientist, Dr. Larsen was a gifted scientist, consistently producing research at the forefront of cell, developmental, and reproductive biology. Early in his career he published a landmark paper that conclusively established mitochondrial fission as the mechanism of mitochondrial biogenesis. He went on to become the first to demonstrate the endocytosis of gap junctions. Moreover, his work on the hormonal regulation of gap junction formation and growth culminated in an authoritative review article in Tissue and Cell, “Structural Diversity of Gap Junctions (1988)”, which became a citation classic.

Throughout his 25 year teaching career, his sixty-seven peer reviewed publications—not to mention numerous invited reviews, abstracts, and book chapters—covered a wide range of research areas including adrenal cortical tumor cells, human ovarian carcinomas, preterm labor, cumulus expansion, oocyte maturation, ovulation, folliculogenesis, and in-vitro fertilization.

In addition to his many contributions to basic research, Dr. Larsen loved to teach and was much appreciated by his students. His exceptional ability was reflected in the four teaching awards he received as a professor at the University of Cincinnati.

Notably, he was the author of Human Embryology, a textbook for medical students that was the first to incorporate modern experimental research into a subject that had traditionally been taught in a strictly descriptive style. On its initial publication in 1998 it was hailed as, “a magnificent book…” by the European Medical Journal. With the release of the fourth edition in 2008, the book was renamed “Larsen’s Human Embryology” in recognition of Dr. Larsen's place as the originator of this revolutionary text. This book is today in it's 6th Edition.

His stellar scientific career would be enough for most people, but Dr. Larsen pursued his numerous and varied interests with such extraordinary passion, energy, and skill that he seemed to have more hours in a day than the ordinary person. He was fascinated with the American Southwest and studied and collected traditional arts and crafts of the Hopi, Zuni, and Navajo peoples. He was a woodworker who built three harpsichords and a fortepiano for his wife, and, with his two children, over 100 pieces of gallery-quality furniture. In addition, he loved to regale his friends, colleagues, and students with jokes and stories, and to share his love for gourmet cooking.

The William J. Larsen Distinguished Lecture Series

An annual lecture series was created for the Department of Cancer & Cell Biology at the University of Cincinnati to honor Dr. Larsen's research which was at the forefront of cell developmental and reproductive biology. This series recognizes forward-thinking research scientists in the field of developmental biology and asks that they share their research and findings with students and faculty of the University of Cincinnati, College of Medicine.

Personal note: I had the opportunity to meet and attend Dr. Larsen’s embryology lectures as he and I worked in the Anatomy, Embryology, and Histology program at the University of Cincinnati Medical College. Unfortunately, I never had the opportunity to have Dr. Larsen sign my personal copy of his book. He is sorely missed, Dr. Miranda

Sources:

1. "The William J. Larsen Distinguished Lecture Series" University of Cincinnati, College of Medicine.
2. https://www.larsenbooks.com
3. 2022 Larsen Lecture Series brochure (download here)
4. Dr. Larsen's family personal communications


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Conduction system of the heart

Cardiomyocyte-based conduction system of the heart
Conduction system of the heart

[UPDATED] The conduction system of the heart a binary system that includes a cardiomyocyte-based component which acts as an automatic base, and an autonomic nervous system component which acts as a modulator.

The classic description of the conduction system of the heart emphasizes only the cardiomyocyte-based component and refers to a group of specialized cardiac muscle structures that serve as pacemakers and distributors of the electrical stimuli that make the heart beat coordinatedly. It is important to stress the fact that this primary "conduction system of the heart" is not formed by nerves but rather by specialized cardiac muscle cells.

Components of the cardiomyocyte-based conduction system of the heart:

• SA node: The sinoatrial (SA) node is a small nodule of cardiac muscle tissue, somewhat horseshoe-shaped that is found at the junction of the superior vena cava and the right atrium. It receives blood supply from the SA node artery, a branch of the right coronary artery. Later research indicates that the pacemaker function of the SA node includes areas of the lateral wall of the right atrium which are involved in different heart rate speeds.

• AV node: The atrioventricular (AV) node is found at the junction of atria and ventricles in an area known as the "Triangle of Koch". Its function is to delay the electrical impulse passing from the atria to the ventricles by 1/10th of a second, enabling the sequential pumping action of the heart. The eponymic name for the AV node is "node of Aschoff-Tawara", and it receives its blood supply by way of the AV node artery, a branch that usually arises from the right coronary artery

• AV bundle: Also known as the "Bundle of His", this thick bundle of specialized myocardial cells is found in the interventricular septum. It divides into the right and left bundle branches

• Bundle branches: Sometimes known as the "crura" of the bundle of His, these two divisions of the AV bundle help distribute the electrical stimuli to the ventricular walls. The right bundle branch has an extension that crosses the lumen of the right ventricle, from the base of the anterior papillary muscle to the interventricular septum, forming a cord of tissue known as the "moderator band" or "septomarginal trabecula"

• Purkinje Fibers: These thin fibers are the terminal end of the conduction system of the heart and finish the distribution of the electrical stimuli to all parts of the ventricular walls

Although the structural components of the conduction system of the heart were known, it was Dr. Sunao Tawara (1873-1952) who discovered the AV node and described the connections between the components of what he called the "Reitzleitungssytem" (conduction system) of the heart.

A separate article on the secondary conduction system of the heart will be published shortly

Click on the image for a larger version. Image modified from the original: "3D Human Anatomy: Regional Edition DVD-ROM." Courtesy of Primal Pictures.