Final answer:
The posterior cricoarytenoid muscle is the only muscle responsible for abducting the vocal cords, allowing the glottis to open for respiration.
Step-by-step explanation:
The only muscle responsible for abducting the vocal cords is the posterior cricoarytenoid muscle. This muscle attaches to the cricoid and arytenoid cartilages of the larynx, and its contraction moves the vocal folds away from the midline, thereby opening the glottis for breathing. This action is critical as it enables air passage during respiration and prevents choking. Conversely, when sounds are generated, other muscles bring the vocal cords together for vibration and phonation. The size and tension of the vocal cords affect the pitch of the sound produced, with longer and thicker vocal cords typically producing deeper sounds, which explains why men generally have lower-pitched voices than women.