When a stressor begins, epinephrine and norepinephrine are released from the adrenal medulla and norepinephrine is released from the sympathetic nerve terminals. The physiological responses in mammals and nonmammals, however, appear to be essentially identical ( Young and Landsberg, 2001). These cells are called chromaffin and are homologous to cells in the adrenal medulla of mammals. In these species, cells that release epinephrine and norepinephrine are embedded in the wall of the kidneys. Most nonmammalian species, however, lack a well-defined adrenal medulla. Another major source of norepinephrine is nerve terminals of the sympathetic nervous system. Epinephrine and norepinephrine are released from two different cell populations in the adrenal medulla. The adrenal medulla is essentially a modified sympathetic ganglion where each secretory cell is a neuron without an axon ( Young and Landsberg, 2001). In mammals, epinephrine is primarily produced in the adrenal medulla – the center portion of the adrenal gland. The location of epinephrine release depends in part on the species. Michael Romero, in Encyclopedia of Animal Behavior (Second Edition), 2019 Anatomy
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