From the Research
The cardiac accelerator fibers are located in the sympathetic nervous system, specifically in the upper thoracic segments (T1-T5) of the spinal cord, as described in the study by 1. These fibers originate from the sympathetic chain ganglia and travel to the heart via the cardiac nerves. They primarily innervate the sinoatrial (SA) node, atrioventricular (AV) node, and the ventricular myocardium. When stimulated, these fibers release norepinephrine, which binds to beta-1 adrenergic receptors in the heart. This binding triggers a cascade of cellular events that increases heart rate (chronotropy), contractility (inotropy), and conduction velocity (dromotropy). The cardiac accelerator fibers are essential for the "fight or flight" response, allowing the heart to rapidly increase its output during times of stress or physical exertion. Their function is counterbalanced by the parasympathetic vagus nerve, which slows heart rate. This dual innervation allows for precise control of cardiac function to meet the body's changing metabolic demands.
Some key points to consider:
- The study by 1 provides an overview of the organization of the cardiac sympathetic nerves, from the preganglionic origin to the postganglionic innervations.
- The cardiac nerves differentially supply the nodal and myocardial tissue of the heart and are dependent on activity generated in spinal neurones in the upper thoracic cord.
- Networks of spinal interneurones determine the pattern of activity, and groups of spinal neurones selectively target specific regions of the heart.
- The other studies provided, such as 2, 3, 4, and 5, discuss the effects of beta-blockers, such as metoprolol and carvedilol, on heart failure and arrhythmias, but do not directly address the location of the cardiac accelerator fibers.
- Therefore, the study by 1 is the most relevant to the question of the location of the cardiac accelerator fibers.