![]() These fibers descend down both sides of the septum as the right and left bundle branches and conduct the action potential over the ventricles about 0.2 s after the appearance of the P wave. The signal rapidly spreads through the AV bundle reaching the top of the septum. The electrocardiogram (ECG) records the action of this electrical conduction system and contraction of the myocardium (Figure 1). The sinoatrial (SA) node and atrioventricular (AV) node are the two internal pacemakers that are primarily responsible for initiating the heartbeat. Autorhythmic cells function as pacemakers and provide a conduction pathway for pacemaker potentials. These cells continue to initiate heartbeats after surgeons sever all efferent cardiac nerves and remove a heart from the chest cavity for transplantation. The heart contains autorhythmic cells that spontaneously generate the pacemaker potentials that initiate cardiac contractions. ![]() During diastole, BP is lowest when the left ventricle relaxes. Systolic BP is measured during this phase. During systole, blood pressure (BP) peaks as contraction by the left ventricle ejects blood from the heart. The cardiac cycle consists of systole (ventricular contraction) and diastole (ventricular relaxation). ![]() When the left ventricle contracts, blood is ejected through the aorta to the arterial system ( Marieb and Hoehn, 2013 Tortora and Derrickson, 2014). Oxygenated blood is transported through the pulmonary veins to the left atrium and enters the left ventricle. Deoxygenated blood enters the right atrium, flows into the right ventricle, and is pumped to the lungs via the pulmonary arteries, where wastes are removed and oxygen is replaced. The ventricles comprise most of the heart's volume, lie below the atria, and pump blood from the heart into the lungs and arteries. The atria are upper receiving chambers for returning venous blood. The muscular heart consists of two atria and two ventricles. The heart is about the size of a closed fist, weighs between 250 and 350 g, and beats approximately 100,000 times a day and 2.5 billion times during an average lifetime. Future research should expand understanding of how the heart and its intrinsic nervous system influence the brain. The authors conclude that a coherent heart is not a metronome because its rhythms are characterized by both complexity and stability over longer time scales. In its final section, this article integrates Porges' polyvagal theory, Thayer and colleagues' neurovisceral integration model, Lehrer et al.'s resonance frequency model, and the Institute of HeartMath's coherence model. Additionally, it reviews the most common time and frequency domain measurements as well as standardized data collection protocols. It also considers new perspectives on the putative underlying physiological mechanisms and properties of the ultra-low-frequency (ULF), very-low-frequency (VLF), low-frequency (LF), and high-frequency (HF) bands. This article also discusses the intrinsic cardiac nervous system and the heart-brain connection, through which afferent information can influence activity in the subcortical and frontocortical areas, and motor cortex. This article reviews sympathetic and parasympathetic influences on the heart, and examines the interpretation of HRV and the association between reduced HRV, risk of disease and mortality, and the loss of regulatory capacity. The cardiovascular regulation center in the medulla integrates sensory information and input from higher brain centers, and afferent cardiovascular system inputs to adjust heart rate and blood pressure via sympathetic and parasympathetic efferent pathways. This article briefly reviews neural regulation of the heart, and its basic anatomy, the cardiac cycle, and the sinoatrial and atrioventricular pacemakers. Heart rate variability (HRV), the change in the time intervals between adjacent heartbeats, is an emergent property of interdependent regulatory systems that operate on different time scales to adapt to challenges and achieve optimal performance.
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