A pharmacological compound selectively and completely blocks gap junctions in cardiac muscle cells. What is the most likely consequence for heart function?
AThe heart stops beating because it loses the ability to generate action potentials
BHeart rate increases because the SA node fires faster without inhibitory coupling from other cells
CVentricular fibrillation occurs because action potentials can no longer propagate rapidly across the ventricular mass, destroying coordinated contraction
DContraction becomes weaker but coordinated because the mechanical connections (desmosomes) remain intact
Gap junctions are the channels through which action potentials propagate from one cardiomyocyte to the next. They make the heart a functional syncytium — electrically, it behaves as one large cell. Block these channels and each cell must wait for extracellular signals that are too slow to coordinate simultaneous activation. Different regions of the ventricle fire at different times, producing uncoordinated twitches (fibrillation) rather than a unified squeeze. Option A is wrong because the SA node can still fire and cells can still generate their own action potentials — the problem is propagation, not generation.
Question 2 Multiple Choice
A transplanted heart — with all nervous connections to the body severed — continues to beat. What anatomical and physiological feature of cardiac muscle explains this?
AThe transplanted heart generates action potentials via motor neurons embedded within the myocardium that survived the surgery
BSkeletal muscle fibers in the chest wall contract rhythmically and mechanically stimulate the transplanted heart
CSpecialized pacemaker cells in the SA node are autorhythmic — they depolarize spontaneously without requiring a nerve signal, so the heart initiates its own beats intrinsically
DThe heart beats because blood pressure from the pulmonary circulation mechanically stretches the atrial walls, triggering contraction
Cardiac muscle's autorhythmicity is the key feature. SA node pacemaker cells have unstable resting potentials that slowly drift toward threshold and fire spontaneously via a combination of funny current (If), L-type calcium channels, and T-type calcium channels. This intrinsic automaticity means the heart does not need a nerve signal to start each beat — it starts itself. The nervous system modulates rate (faster via sympathetic, slower via parasympathetic) but does not initiate the beat. A transplanted heart denervated of all autonomic input will beat at its intrinsic rate (~100 bpm), confirming autorhythmicity.
Question 3 True / False
Intercalated disks contain two distinct structures: desmosomes, which provide mechanical coupling between adjacent cardiomyocytes, and gap junctions, which provide electrical coupling.
TTrue
FFalse
Answer: True
Intercalated disks are specialized junctions at the ends of cardiomyocytes that serve two complementary functions. Desmosomes (and fascia adherens) anchor neighboring cells together mechanically, ensuring that the contractile force generated by one cell is transmitted to the next rather than simply deforming the cell membrane. Gap junctions (made of connexin proteins) create direct cytoplasmic connections that allow ions — and therefore action potentials — to flow from cell to cell. Both structures are required for effective coordinated pumping: mechanical integrity without electrical coupling would produce uncoordinated force, and electrical coupling without mechanical integrity would cause cells to pull apart under contraction.
Question 4 True / False
The heart requires continuous nerve signals from the autonomic nervous system to initiate each heartbeat — without these signals, the cardiac muscle would not contract.
TTrue
FFalse
Answer: False
This is a common misconception that conflates initiation with modulation. The SA node pacemaker cells are autorhythmic — they spontaneously depolarize and fire without any nerve input. The autonomic nervous system modulates rate (sympathetic speeds it up; parasympathetic slows it down) but does not initiate the beat. This is demonstrated by cardiac transplantation: a denervated transplanted heart beats at its intrinsic rate (~100 bpm, without the resting parasympathetic brake) and can sustain life. Contrast this with skeletal muscle, which is neurogenic — it contracts only when activated by a motor neuron.
Question 5 Short Answer
Explain why the heart is described as a 'functional syncytium,' and what structural feature of cardiomyocytes makes this possible.
Think about your answer, then reveal below.
Model answer: A syncytium is a structure that behaves as a single coordinated unit despite being composed of many individual cells. The heart is described as a functional syncytium because an action potential initiated at the SA node propagates almost simultaneously throughout the atria and then the ventricles, causing near-simultaneous activation of the entire muscle mass. This coordination is made possible by gap junctions in the intercalated disks — protein channels (made of connexins) that connect the cytoplasm of adjacent cardiomyocytes and allow ions to flow directly between them. Because ions carry charge, this ionic flow spreads depolarization across cell boundaries nearly as fast as it would travel within a single cell. The result is that all ventricular cells contract together, producing a powerful unified squeeze rather than a series of small uncoordinated twitches.
The functional syncytium concept explains why cardiac rhythm disturbances (arrhythmias) are so physiologically devastating: they represent a breakdown in this coordinated activation. Ventricular fibrillation is essentially the failure of the syncytial mechanism — cells fire asynchronously, producing no net pumping despite ongoing electrical and mechanical activity.