Cardiac muscles are primarily controlled by the autonomic nervous system and the heart's own specialized conduction system. This dual control allows for both involuntary regulation and inherent rhythmicity.
Autonomic Nervous System Control
The autonomic nervous system exerts its influence through two branches: the sympathetic and parasympathetic nervous systems.
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Sympathetic Nervous System: When activated (e.g., during exercise or stress), the sympathetic nervous system releases norepinephrine. This neurotransmitter increases heart rate and the force of contraction, preparing the body for "fight or flight."
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Parasympathetic Nervous System: Conversely, the parasympathetic nervous system, primarily via the vagus nerve, releases acetylcholine. This neurotransmitter slows down heart rate, promoting a state of rest and digestion.
Intrinsic Control: The Cardiac Conduction System
The heart possesses its own internal pacemaker and conduction system, enabling it to beat independently.
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Sinoatrial (SA) Node: Often called the "pacemaker" of the heart, the SA node is a cluster of autorhythmic cells located in the right atrium. These cells spontaneously depolarize, generating electrical impulses that initiate each heartbeat.
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Atrioventricular (AV) Node: The electrical impulse from the SA node travels to the AV node, also located in the right atrium. The AV node briefly delays the impulse, allowing the atria to fully contract before the ventricles.
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Bundle of His and Purkinje Fibers: From the AV node, the impulse travels down the Bundle of His and then branches into the Purkinje fibers, which spread throughout the ventricles. This rapid conduction ensures coordinated ventricular contraction.
Hormonal Influence
Besides the nervous system, hormones can also affect cardiac muscle function. For example, epinephrine (adrenaline) released by the adrenal glands can increase heart rate and contractility, similar to the effects of sympathetic stimulation.
Summary
In summary, cardiac muscle control is a complex interplay between the autonomic nervous system (sympathetic and parasympathetic branches), the heart's intrinsic conduction system (SA node, AV node, Bundle of His, Purkinje fibers), and hormonal influences. This multifaceted regulation ensures that the heart can adapt its function to meet the body's changing needs.
