Final answer:
The autonomic nervous system activates target organs by releasing either norepinephrine, which binds to adrenergic receptors, or acetylcholine (ACh), which binds to nicotinic or muscarinic receptors, leading to 'fight or flight' or 'rest and digest' responses respectively.
Step-by-step explanation:
In the autonomic nervous system, activation of receptors on target organs involves complex interactions between neurotransmitters and their respective receptors. When the sympathetic nervous system is activated, it causes the release of norepinephrine, which binds to adrenergic receptors on the target organ, initiating the 'fight or flight' response. In contrast, activation of the parasympathetic nervous system causes the release of acetylcholine (ACh), which binds to muscarinic receptors on the target organ, promoting the 'rest and digest' response.
All preganglionic neurons, both in the sympathetic and parasympathetic systems, release ACh, which then binds to nicotinic receptors on the ganglionic neurons. Subsequently, postganglionic parasympathetic fibers release ACh to act on muscarinic receptors, whereas most postganglionic sympathetic fibers release norepinephrine to act on adrenergic receptors. An exception is the sympathetic fibers that project to sweat glands and to blood vessels associated with skeletal muscles; these release ACh.
The neurotransmitters released by the autonomic nervous system, whether ACh or norepinephrine, determine the activation of different types of receptors, leading to specific physiological responses in the target organs, such as affecting heart rate, gastrointestinal activity, or glandular secretion. These systems and their corresponding neurotransmitters and receptors are antagonistic, often exerting opposing effects to maintain homeostasis within the body.