What distinguishes the contraction stimulus of cardiac contractile cells from that of skeletal muscle fibers?
Skeletal muscle fibers are activated by somatic motor neurons, while cardiac cells generate action potentials via pacemaker activity.
Cardiac contractile cells require anaerobic metabolism, whereas skeletal fibers depend solely on aerobic metabolism.
Cardiac cells require direct stimulation from motor neurons, while skeletal fibers generate their own action potentials.
Skeletal muscle fibers have a long refractory period, unlike cardiac cells.
The Correct Answer is A
A. Skeletal muscle fibers are activated by somatic motor neurons, while cardiac cells generate action potentials via pacemaker activity: Skeletal muscle fibers are innervated by somatic motor neurons, and each action potential originates from an external neural stimulus at the neuromuscular junction. Cardiac contractile cells, in contrast, can depolarize spontaneously due to pacemaker cells in the sinoatrial node, generating intrinsic action potentials that propagate through gap junctions without direct neural input.
B. Cardiac contractile cells require anaerobic metabolism, whereas skeletal fibers depend solely on aerobic metabolism: Both cardiac and skeletal muscle fibers primarily rely on aerobic metabolism to meet energy demands. Cardiac muscle has a high density of mitochondria for continuous aerobic ATP production, whereas skeletal muscle can use both aerobic and anaerobic pathways depending on activity intensity.
C. Cardiac cells require direct stimulation from motor neurons, while skeletal fibers generate their own action potentials: Cardiac contractile cells do not require direct neural stimulation; they depolarize via pacemaker activity and conduct impulses through the myocardium. Skeletal fibers, on the other hand, rely entirely on motor neuron input to initiate contraction and cannot generate spontaneous action potentials.
D. Skeletal muscle fibers have a long refractory period, unlike cardiac cells: The refractory period of cardiac contractile cells is much longer than that of skeletal muscle fibers. This prolonged refractory period prevents tetanic contractions in the heart, allowing sufficient time for filling between beats.
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Correct Answer is {"dropdown-group-1":"A"}
Explanation
Correct answer: Left coronary
The left coronary artery arises from the left side of the ascending aorta, just above the aortic valve, and quickly branches to supply the heart muscle. Its two main branches are the anterior interventricular artery (also called the left anterior descending artery), which runs along the interventricular groove supplying the anterior walls of both ventricles, and the circumflex artery, which curves around the left side of the heart to supply the lateral and posterior walls of the left ventricle and left atrium. This anatomical arrangement ensures that oxygen-rich blood from the aorta is delivered efficiently to the majority of the left heart myocardium.
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.
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