How does the DRG control the "on-off" switch for breathing?

The correct answer reflects the role of the Dorsal Respiratory Group (DRG) in the regulation of breathing through its influence on the inspiratory muscles. During the "on" state, the DRG sends signals to the diaphragm and intercostal muscles, which contracts these muscles to facilitate inhalation. This contraction creates a negative pressure in the thoracic cavity that allows air to enter the lungs.

Conversely, during the "off" state, the DRG stops sending excitatory signals to these muscles, leading to their relaxation. This cessation of contraction allows the lungs to deflate passively, expelling air during expiration. This cycle of contraction and relaxation is vital for the rhythmic nature of breathing.

The other choices do not accurately describe the function of the DRG in the context of breathing regulation. Blood pressure alteration is not directly responsible for switching breathing on or off; conducting electrical signals through the spinal cord is a broader representation that does not specify the respiratory function; and blocking signals from the brain would inhibit breathing rather than regulate it. Understanding the specific mechanics of how the DRG operates to control muscle contractions during respiration is crucial for grasping the fundamentals of respiratory physiology.