Final answer:
Postganglionic sympathetic fibers release norepinephrine, which binds to adrenergic receptors, affecting the physiology of target organs. Some fibers, like those to sweat glands, release ACh instead. The adrenal medulla can also release norepinephrine and epinephrine into the bloodstream as hormones.
Step-by-step explanation:
Postganglionic sympathetic fibers primarily release the neurotransmitter norepinephrine, which binds to adrenergic receptors on target cells as part of the sympathetic nervous system response. There are exceptions to this, such as the fibers that project to sweat glands and blood vessels associated with skeletal muscles; these release acetylcholine (ACh) instead.
Additionally, a specialized preganglionic connection to the adrenal medulla allows for the release of norepinephrine and epinephrine directly into the bloodstream. This complex system, with both neurotransmitters and hormonal signals, ensures widespread and efficient control over various physiological processes.