The adrenal medulla develops from neural tissue and secretes two hormones, epinephrine and norepinephrine. These two hormones are secreted in response to stimulation by sympathetic nerve, particularly during stressful situations. A lack of hormones from the adrenal medulla produces no significant effects.
While the sinoatrial node typically regulates the heartbeat during normal sinus rhythm, PACs occur when another region of the atria depolarizes before the sinoatrial node and thus triggers a premature heartbeat.
In situ SCC may be caused by: 1  Immunosuppression from drugs such as ciclosporin or azathioprine. 2  In situ squamous cell carcinoma is most often found on sun exposed sites of fair skinned individuals. 3  Ionising radiation: in situ SCC was common on the hands of radiologists early in the 20th century.