A 28-year-old male graduate student reports debilitating anxiety limited to public speaking situations. He experiences tachycardia, tremor, diaphoresis, and a shaky voice during class presentations, but has no anxiety in other social situations and no generalized worry. He does not meet criteria for generalized anxiety disorder or social anxiety disorder. The PMHNP recommends propranolol 20 mg taken 30-60 minutes before presentations. Which of the following best describes propranolol's mechanism in this clinical scenario?