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.
Causes of Spinal Muscular Atrophy. Most types of spinal muscular atrophy are caused by a mutation in the survival motor neuron gene, SMN1. Other types of spinal muscular atrophy are caused by different genes.