Describe the three types of muscle contractions seen in the esophagus.

1. Primary muscle contraction is smooth, coordinated contraction of the muscle that is initiated by swallowing.
2. Secondary muscle contraction clears material that is not cleared by the primary stripping wave.
3. Tertiary muscle contraction is irregular, uncoordinated, and not propulsive; the incidence of this abnormal type of muscle contraction increases with age.