The activity of the heart depends on intrinsic properties of cardiac muscle and on neural factors. Thus, a. vagus nerve stimulation of the heart reduces heart rate, b. sympathetic nerve stimulation of the heart decreases time available for ventricular filling, c. sympathetic stimulation of the heart increases its force of contraction, d. all of the above.

Verified step by step guidance

1. Step 1: Understand the role of the vagus nerve in heart function. The vagus nerve is part of the parasympathetic nervous system and typically acts to decrease heart rate.
2. Step 2: Consider the effects of sympathetic nerve stimulation. The sympathetic nervous system generally increases heart rate and force of contraction, which can reduce the time available for ventricular filling.
3. Step 3: Analyze the statement about sympathetic stimulation increasing the force of contraction. This is consistent with the role of the sympathetic nervous system in preparing the body for 'fight or flight' responses.
4. Step 4: Evaluate each option (a, b, c) to see if they are true based on the understanding of parasympathetic and sympathetic influences on the heart.
5. Step 5: Determine if all the statements (a, b, c) are correct, which would support choosing option d, 'all of the above.'

Key Concepts

Intrinsic Properties of Cardiac Muscle

Cardiac muscle has unique intrinsic properties, such as automaticity and rhythmicity, allowing it to generate electrical impulses independently. This means that the heart can contract and pump blood without external neural input, although it is influenced by the autonomic nervous system.

Autonomic Nervous System

The autonomic nervous system (ANS) regulates involuntary bodily functions, including heart rate and contraction strength. It has two main branches: the sympathetic nervous system, which increases heart rate and contractility, and the parasympathetic nervous system, primarily through the vagus nerve, which decreases heart rate.

Ventricular Filling Time

Ventricular filling time refers to the duration during which the ventricles fill with blood before contraction. Sympathetic stimulation can shorten this time, potentially affecting cardiac output and efficiency, as the heart may not fill adequately if the rate of contraction is too high.