Why does stimulation of the adrenal medulla and stimulation of the sympathetic nervous system produce similar responses?

The sympathetic system gives rise to the same effects as the hormones of the adrenal medulla (the so-called fight or flight response), but does so more rapidly.  Present evidence suggests that this nervous mechanism is far more important than the endocrine system in preparing an animal for emergency situations, as indeed it should be considering how much faster a pathway it is.  The reason that the sympathetic system and the hormones of the adrenal medulla produce similar effects is clear: the neurotransmitter at the sympathetic post-ganglionic neuron’s junction with the target organ is norepinephrine (or in a few cases, epinephrine).  The effect on the target organs is identical because the transmitter of the sympathetic postganglionic system and the hormones of the adrenal medulla are the same, epinephrine and norepinephrine.

The reason the adrenal medulla releases the same substances as the postganglionic sympathetic system lies in a fascinating functional and evolutionary relationship between them.  As you have seen, autonomic pathways normally include two motor neurons, a preganglionic neuron and a postganglionic neuron.  There is a single exception: the sympathetic pathway to the adrenal medulla has only one motor neuron, the preganglionic neuron (see Figure 49.8 on page 1066 of your textbook).  It turns out that in the embryo, the adrenal medulla forms from a cluster of highly modified postganglionic neurons in this sympathetic pathway, and is specialized for neurosecretion rather than conduction.  So the adrenal medulla is in effect a sympathetic ganglion in which the postganglionic neurons have lost their axons and become secretory cells!  Evolution has converted what were probably once postganglionic neurons of the sympathetic system into an endocrine gland specialized for high-quantity secretion of the very substances that nearly all postganglionic neurons in the sympathetic system produce.

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