Initiation of voiding.
D000818:Animals; D002648:Child; D004285:Dogs; D005260:Female; D006801:Humans; D008297:Male; D009130:Muscle, Smooth / Q000033:anatomy & histology; D011312:Pressure; D011859:Radiography; D014521:Urethra / Q000033:anatomy & histology; D001743:Urinary Bladder / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000502:physiology*; D014554:Urination