The Structures And Component Of Atrial Chamber

Europace (2007) 9, vi3–vi9 doi:10.1093/europace/eum200

The structure and components of the atrial chambers
Robert H. Anderson* and Andrew C. Cook
Cardiac Unit, Institute of Child Health, University College, 30 Guilford Street, London WC1 N 1EH, UK

KEYWORDS
Attitudinally appropriate nomenclature; Atrial appendages; Sinus node; Conduction tissues

We discuss the implications of accurateknowledge of the human atrial chambers for those seeking to model atrial structure, and correlate the muscular activity with electrical signals. We stress first the importance of describing atrial components in attitudinally appropriate fashion, a feature sadly ignored by generations of morphologists. When considered relative to the body, the right atrium is positioned anteriorly relative to itsalleged left-sided counterpart. We then described how each atrium possesses a venous component, an appendage, a vestibule, these parts being supported by the body of the atrium, and how the two chambers are separated by the septum. We extend this information by describing the detailed structure of each atrium, and then emphasise that it is only the floor of the oval fossa, and its antero-inferiorrim, that are true septal structures. The so-called ‘septum secundum’ is the superior interatrial fold. Emphasis is then given to the muscular connections between the atriums, these unions obviously underscoring the potential for interatrial conduction. We then continue by discussing the structure of the atrial walls, which vary markedly in their thickness. It is the alignment of the myocytes withinthese walls that determines the velocity of conduction through them. In this setting, we also discuss the morphological features that distinguish between working myocytes and the myocytes of the conduction system, stressing the importance of rules established almost 100 years ago.

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Introduction
It is intuitivethat those seeking to model atrial structure so as to correlate muscular activity with electrical signals should seek to use the most accurate anatomic information available. Although new diagnostic techniques are now permitting the shape of the atrial cavities to be reconstructed with exquisite accuracy, and are revealing remarkably variable arrangements of structure such as the pulmonary veins,1as far as we are aware there have been relatively few attempts to incorporate such information into models, with one of the best examples using a data set that is now several years old.2 If the greatest value is to be gained from the information now becoming available from the new diagnostic modalities, it is necessary also to understand the arrangement of the myocytes aggregated together to formthe atrial walls, since this arrangement is far from isotropic.3,4 At the same time, it is necessary to know how the myocardial walls of the two atrial chambers are joined together, and to appreciate the location of the sinus node, the initiator of atrial activation.5 A review of the anatomy of the atrial chambers, therefore, can put all of this information into an appropriate morphologicalcontext, and also cast light on ongoing disputes as to the ‘specialized’ nature of the muscular sleeves of the pulmonary veins. We seek here to provide such an overview, but we begin by emphasizing the importance of attitudinally appropriate

descriptions, since unless atrial structures are described and modelled as they are positioned within the body,6 it will never be possible to make accuratecorrelations with electrocardiographic tracings.

Attitudinally appropriate nomenclature
It is a time-honoured convention that all structures within the human body are described using the anatomical position, in which the subject stands upright and faces the observer. Structures positioned towards the head are then described as being superior, whereas structures closer to the feet are said to be...