Cardiac muscle is capable of which of the following?
A. ?Tetanus
B. ?Wave summation
C. ?Autorhythmicity
D. ?All of the above
Phase 2 ?of the cardiac action potential, when the calcium channels are open and the potassium channels are closed, is called
A. ?threshold.
B. ?relative refractory period.
C. ?plateau phase.
D. ?absolute refractory period.
The effect of the parasympathetic nervous system on the heart is to
A. ?decrease the heart rate.
B. ?increase the heart rate.
The Correct Answer and Explanation is :
Correct Answers:
- C. Autorhythmicity
- C. Plateau phase
- A. Decrease the heart rate
Explanation:
1. Cardiac Muscle Capabilities
Cardiac muscle is capable of autorhythmicity, meaning it has the ability to generate its own rhythmic electrical impulses without needing external stimuli. This is due to the presence of pacemaker cells in the sinoatrial (SA) node, which spontaneously depolarize and initiate the heartbeat.
However, tetanus and wave summation are not possible in cardiac muscle. Tetanus is a sustained contraction caused by rapid, repeated stimulation without relaxation, which occurs in skeletal muscle but not in cardiac muscle. The heart’s long absolute refractory period prevents tetanus, ensuring the heart muscle fully relaxes between beats. Similarly, wave summation, where successive stimuli increase muscle tension, does not occur in cardiac muscle due to its unique action potential properties.
Thus, the correct answer is C. Autorhythmicity.
2. Phase 2 of the Cardiac Action Potential
Phase 2 of the cardiac action potential is known as the plateau phase. During this phase, calcium channels remain open, allowing Ca²⁺ influx, while potassium channels are closed, preventing repolarization. This sustained depolarization is crucial because it extends the contraction time, ensuring efficient blood ejection from the heart.
This phase also contributes to the long absolute refractory period, which prevents premature contractions and allows the heart to function as an effective pump.
Thus, the correct answer is C. Plateau phase.
3. Effect of the Parasympathetic Nervous System
The parasympathetic nervous system (PNS), primarily through the vagus nerve (cranial nerve X), releases acetylcholine (ACh), which acts on muscarinic receptors in the heart. This leads to hyperpolarization of the pacemaker cells, slowing down depolarization and reducing the heart rate.
In contrast, the sympathetic nervous system (SNS) releases norepinephrine, which increases heart rate by enhancing depolarization.
Since the PNS slows down the heart rate, the correct answer is A. Decrease the heart rate.