Given a scientific passage and an answer, generate a question for the given answer.

Passage: Neurons that innervate the muscles of the respiratory system are responsible for controlling and regulating pulmonary ventilation. The major brain centers involved in pulmonary ventilation are the medulla oblongata and the pontine respiratory group (Figure 22.20). Answer: neurons
What basic structures of the nervous system innervate the muscles of the respiratory system and are responsible for controlling and regulating pulmonary ventilation?

Passage: Solids have a fixed volume and a fixed shape. Liquids have a fixed volume but take the shape of their container. Gases take both the volume and the shape of their container. Answer: gases
What takes both the shape and the volume of their container?

Passage: A protein is an organic compound made up of small molecules called amino acids . There are 20 different amino acids commonly found in the proteins of living organisms. Small proteins may contain just a few hundred amino acids, whereas large proteins may contain thousands of amino acids. The largest known proteins are titins, found in muscle, which are composed from over 27,000 amino acids. Answer: titins
What are the largest known proteins called?