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artery branches off the aorta and quickly divides into the anterior interventricular artery and circumflex artery.

A. Sympathetic stimulation releases norepinephrine (NE), which binds to beta-1 receptors, increasing heart rate: Sympathetic nerve fibers release norepinephrine, which binds to beta-1 adrenergic receptors on the sinoatrial node and ventricular myocardium. This increases the rate of depolarization in pacemaker cells, enhancing heart rate (positive chronotropy), and also increases contractility (positive inotropy), preparing the heart for increased cardiac output.

B. Parasympathetic stimulation releases norepinephrine (NE), which binds to beta-1 receptors, increasing heart rate: Parasympathetic fibers do not release norepinephrine; they release acetylcholine. Norepinephrine is specific to sympathetic innervation. Therefore, this description incorrectly attributes sympathetic neurotransmission to parasympathetic activity.

C. Sympathetic stimulation releases norepinephrine (NE), which binds to muscarinic receptors, slowing heart rate: Muscarinic receptors are activated by acetylcholine released from parasympathetic fibers, not by norepinephrine. Sympathetic stimulation increases heart rate, rather than slowing it, through beta-1 receptor activation.

D. Parasympathetic stimulation releases acetylcholine (ACh), which binds to the beta-1 receptors, increasing heart rate: Parasympathetic stimulation releases acetylcholine, but it binds to muscarinic receptors on pacemaker cells, not beta-1 adrenergic receptors. Activation of muscarinic receptors slows the heart rate (negative chronotropy) rather than increasing it.

A. Z-line to Z-line: A sarcomere, the functional contractile unit of a cardiomyocyte, is defined as the region between two adjacent Z-lines (or Z-discs). The Z-lines anchor the thin filaments (actin) and mark the lateral boundaries of the sarcomere. During contraction, the sarcomere shortens as actin and myosin filaments slide past each other, generating tension and ultimately producing myocardial contraction.

B. A-line to A-line: The A-band corresponds to the length of the thick filaments (myosin) within a sarcomere and does not represent the full functional unit. It includes both overlapping regions with thin filaments and the central H-zone, but A-band boundaries do not define sarcomere length.

C. I-band to I-band: The I-band contains only thin filaments and is bisected by the Z-line. It shortens during contraction, but its boundaries alone do not encompass the entire sarcomere, making it an incomplete reference for the sarcomere’s limits.

D. M-line to M-line: The M-line lies at the center of the sarcomere, anchoring thick filaments. While it is important for structural integrity, using M-line to M-line does not define the full sarcomere; it represents only the midpoint rather than the full functional contractile unit.