Summary Learning Objectives Physiology Basic Concepts Part 1 (PBC1)
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Course
Physiology Basic concepts
Institution
Universiteit Leiden (UL)
This document is a summary of all lectures, complemented with summaries of several prescribed readings from the books from Boron and Levick. Concepts discussed in workgroups and self-study assignments are also included.
Part 1: Cardiovascular & Respiratory System
Theme A: Cardiac electrophysiology
SA 1.1: Cardiac anatomy, electrical aspects
To be able to recognise the various structures that constitute the tissues of the cardiac conduction
system
● A cardiac muscle fiber consists of myofibrils that in turn consist of cylindrically shaped
mononucleated cardiomyocytes. The cardiomyocytes are arranged end-to-end through
anchoring points formed by intercalated disks. Cardiac muscle fibers also branch as opposed to
skeletal muscle fibers.
● The nucleus in cardiomyocytes are located centrally as opposed to the peripheral localization in
skeletal muscle cells.
● Cardiac muscle cells contain 1 T-Tubule per sarcomere (as opposed to multiple in skeletal
muscles). The sarcoplasmic network adjacent to the T-tubule is not an expanded cisternae that
forms a triad, rather an SR anastomosis network that forms a diad with the T-tubule invag.
● Between the myofibrils run densely packed mitochondria along the entire length of the
sarcomere. Furthermore, myofibrils separate to bypass the nucleus which leaves a juxtanuclear
cytoplasmic space where mitochondria can also be located (see * in image)
Skeletal Muscle Cells Cardiac Muscle Cell
Peripheral nucleus under plasma membrane Central nucleus, sometimes bi-nucleate,
juxtanuclear cytoplasm
Single multinucleated protoplasmic unit (fiber) End-to-end alignment formed by junctions
Branching of muscle fibers
Intercalated disks
, ● Intercalated disks (ID) consists of a transverse component that runs perpendicular to the
sarcomere and a lateral component that runs parallel to the sarcomere.
● The lateral and transverse components contain characteristic cell-to-cell junctions.
● The fascia adherens is the main constituent of the transverse component and holds the muscle
cells at their ends to form the fibers. Moreover, they provide sarcoplasmic membrane anchoring
for the thin filaments in the terminal sarcomere.
● The macula adherens (desmosomes) are present in both the lateral and transverse components
and support the fascia adherens; they prevent cells from pulling apart during repetitive
contraction.
● Gap junctions are the main constituent of the lateral component and provide ionic continuity
between adjacent cells. These junctions allow syncytial coupling while maintaining cellular
integrity and individuality; the lateral positioning serves to protect against contractile forces.
● A syncytium provides chemical, electrical and therefore mechanical coupling of cardiomyocytes.
Integration of anatomy with electrophysiology of the heart, that can be recognised by the surface ECG
● A heartbeat and cardiomyocyte contraction is locally initiated, regulated, and coordinated by
specialized cardiac conductor cells which are already present in embryonic cardiac muscle.
● These cells are organized into nodes (SA/NA and AV) and purkinje fibers.
● Purkinje fibers cells have their myofibrils/nuclei mostly peripherally located as oppose to
centrally in normal cardiac muscle cells
● The sinoatrial (SA) node of the heart, located in the lateral wall of the superior vena cava,
spontaneously generates pulses.
● This spontaneous signal then travels through the internodal myocardium. Subsequently, the
atrium contracts which corresponds to the P-wave in the ECG.
● The AV node is located in the triangle of Koch which is bordered by the tricuspid valve, the
coronary sinus and the continuation of the valves
,● The AV node delays the electrical impulse from the SA node for 100-150 ms to allow for
ventricular filling which corresponds to the PR segment or AV-time. The AV node can also take
over pulse generation but then at a lower frequency (pathological).
● The SA and AV node are autonomically regulated by the sympathetic and parasympathetic nerve
fibers that terminate in the nodes.
● After the AV node, the signal travels through the AV/His/Common bundle which is the only part
that can cross the heart to ventricles through the non-conducting cardiac skeleton. The cardiac
skeleton is the non-conducting tissue surrounding the valves, ventricles and atria.
● The His bundle then splits into the right and left bundle branch. At the end of the right and left
branches, the branches split into the purkinje fibers.
, ● When the ventricles contract it produces the QRS complex. Finally, the T-wave is the recovery
phase in which the myocardium recovers and returns to baseline i.e. repolarization.
Study basic cardiac anatomy and orientation of the heart in the thorax
● The heart and great vessels are approximately in the middle of the thorax, surrounded laterally
and posteriorly by the lungs and anteriorly by the sternum and the central part of the thoracic
cage.
● The anterior or sternocostal surface is primarily formed by the RV.
● The diaphragmatic surface (inferior) is primarily formed by the LV and partly RV.
● The right pulmonary surface is mainly formed by the RA
● The left pulmonary surface is primarily formed by the LV and LA
Study the morphological structure and functional aspects of the atria and ventricles, in relation to
functional aspects
● The right side of the heart is more trabeculated, whereas the left side is more smooth.
● The left ventricle is also much thicker than the right ventricle which corresponds to higher
pressure in the left side; however this is different in some diseases.
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