Problem 6 - Biology: The Dynamic Science

Question

Keeping the mammalian cardiac cycle balanced is/are: a. an \(\mathrm{AV}\) node between the right atria and right ventricle, which signals the Purkinje fibers. b. pacemaker cells, which compose the AV node and signal the SA node. c. an insulating layer that isolates the SA node from the right atrium. d. ion channels in pacemaker cells, which close to depolarize their plasma membranes. e. neurogenic stimuli from the nervous system.

Step-by-Step Solution

Answer

The correct answer to what keeps the mammalian cardiac cycle balanced is e. Neurogenic stimuli from the nervous system.

Step 1: Understanding the Option (a)

The AV node, situated between right atria and right ventricle, is fundamental in the cardiac cycle. However, it doesn't signal the Purkinje fibers directly; instead it transmits signals to the bundle of His that further contacts the Purkinje fibers. Hence, this option is not entirely accurate.

Step 2: Understanding the Option (b)

Pacemaker cells form the SA node, not the AV node. The SA node is known as the natural pacemaker because it initiates the electrical signals for heart contraction. Also, pacemaker cells signal from the SA node to the AV node, not the other way around. So, this option is incorrect.

Step 3: Understanding the Option (c)

There is no insulating layer isolating the SA node from the right atrium. The SA node is actually located in the right atrium, so this statement is false.

Step 4: Understanding the Option (d)

Pacemaker cells do possess ion channels which open and close to help generate electrical signals. However, these channels open, not close, to depolarize their plasma membranes. So this option is incorrect.

Step 5: Understanding the Option (e)

The cardiac cycle is indeed affected by neurogenic stimuli from the nervous system. The autonomic nervous system regulates the heart rate and force of contraction through sympathetic and parasympathetic innervation. Hence, this option is correct.