Final answer:
The UES is tonically active to maintain a level of contraction that prevents air from entering the esophagus during breathing and safeguards the airway from food and gastric content aspiration.
Step-by-step explanation:
The Upper Esophageal Sphincter (UES) displays tonic activity to maintain a consistent level of contraction when not actively swallowing. This continuous muscle activity is an example of tonic activity in a neuron, which refers to a slight, continuous level of activity even while at rest. In the case of the UES, this tonic contraction helps to prevent air from entering the esophagus during respiration and also acts as a barrier to protect the airway from potential aspiration of food and gastric contents.