Which plexus is responsible for controlling the relaxation and contraction of the gastrointestinal (GI) tract?

The myenteric plexus, also known as Auerbach's plexus, plays a crucial role in controlling the contraction and relaxation of the smooth muscle layers in the gastrointestinal (GI) tract. This plexus is located between the longitudinal and circular muscle layers and primarily regulates peristalsis, the wave-like muscle contractions that facilitate the movement of food through the digestive system.

In conjunction with the myenteric plexus, the submucosal plexus, or Meissner's plexus, is also important as it manages the local secretion of digestive enzymes and regulates blood flow within the gut. However, it does not directly control muscular contractions to the same extent as the myenteric plexus.

Therefore, both the myenteric and submucosal plexuses work together to ensure the overall function of the GI tract, with the myenteric plexus being the primary driver of muscular processes, while the submucosal plexus addresses secretion and nutrient absorption. This collaborative interaction is why the inclusion of both plexuses as responsible for the overall control of contraction and relaxation in the GI tract is the correct choice.