Systems and methods are provided for use by an implantable medical device capable of automatically adjusting the sensitivity with which electrical cardiac signals are sensed within a patient, i.e. a device equipped with Automatic Sensitivity Control (ASC.) In a first example, ASC parameters are automatically adjusted by the device itself based on parameters derived from both R-waves and T-waves and further based on a detected noise floor. In a second example, a profile representative of the shape of cardiac signals is generated by the device. ASC parameters are then adjusted based on the profile. In various embodiments, histograms are used to determine sizes and shapes of the R-waves and T-waves via statistical prevalence techniques. The histograms are also employed to derive the aforementioned profile.