The
The Correct Answer is {"dropdown-group-1":"C"}
A. SA node: The sinoatrial node is the primary pacemaker of the heart. It is richly innervated by both sympathetic and parasympathetic fibers, allowing autonomic modulation of heart rate. Its sympathetic and parasympathetic balance is critical for initiating and regulating cardiac rhythm rather than forceful contraction.
B. Atria: The atrial myocardium receives autonomic innervation from both sympathetic and parasympathetic fibers. Parasympathetic input via the vagus nerve predominates in controlling heart rate and atrial conduction, whereas sympathetic input modulates contractility modestly. Atrial innervation is less dominated by sympathetic fibers compared with the ventricles.
C. Ventricles: Ventricular myocardium is more richly innervated by sympathetic fibers than parasympathetic fibers. Sympathetic stimulation increases ventricular contractility (positive inotropy) and conduction velocity, which enhances stroke volume and cardiac output during stress or exercise. Parasympathetic influence on ventricles is minimal.
D. AV node: The atrioventricular node is innervated by both sympathetic and parasympathetic fibers. Parasympathetic input slows conduction through the AV node, prolonging the PR interval, while sympathetic stimulation increases conduction velocity. However, its innervation is not predominantly sympathetic compared with ventricular myocardium.
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Correct Answer is {"dropdown-group-1":"D"}
Explanation
A. Distension: Cardiac muscle tissue is capable of stretching to accommodate varying volumes of blood, a property shared with smooth muscle (in blood vessels and hollow organs) and skeletal muscle to some extent. Distension alone does not distinguish cardiac muscle from other muscle types.
B. Relaxation: Relaxation is a fundamental property of all muscle types, including skeletal, smooth, and cardiac muscle. After contraction, all these muscles return to their resting length to allow subsequent filling or preparation for the next contraction.
C. Contracting: Contraction is a universal property of all muscle types. Skeletal, smooth, and cardiac muscles all contract via actin–myosin cross-bridge cycling in response to appropriate stimuli.
D. Autorhythmicity: Cardiac muscle is unique in its ability to generate spontaneous action potentials without external nervous stimulation. Specialized pacemaker cells within the sinoatrial node initiate rhythmic depolarization, allowing the heart to maintain a coordinated and continuous heartbeat. This intrinsic automaticity is distinct from skeletal and smooth muscle, which require neural or hormonal stimuli to contract.
Correct Answer is A
Explanation
A. Atrial cells have a resting potential of -80 mV, while ventricular cells have a resting potential of -90 mV: Cardiac contractile cells maintain a negative resting membrane potential due to selective permeability to potassium ions and the activity of the Na+/K+ ATPase pump. Atrial myocytes have a resting potential around -80 mV, while ventricular myocytes are slightly more negative at approximately -90 mV.
B. Both atrial and ventricular cells have a resting potential of -70 mV: A resting potential of -70 mV is characteristic of neurons, not cardiac contractile cells. Cardiac myocytes require a more negative resting potential to maintain proper excitability and ensure effective action potential generation for coordinated contraction.
C. Atrial cells have a resting potential of -90 mV, while ventricular cells have a resting potential of -80 mV: This reverses the actual values of atrial and ventricular cells. Ventricular cells are more polarized than atrial cells at rest due to higher potassium conductance and greater expression of inward-rectifier potassium channels.
D. Both atrial and ventricular cells have a resting potential of -60 mV: A resting potential of -60 mV is typical of pacemaker (autorhythmic) cells, such as those in the sinoatrial node, rather than contractile myocytes. Contractile cells require a more negative resting potential to maintain a stable resting state before depolarization.
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