Final answer:
Neurons that release signaling molecules into the circulatory system are essentially releasing hormones. These molecules, which can be neurotransmitters within the synapse, are classified as hormones when they enter the bloodstream and have a systemic effect.
Step-by-step explanation:
Neurons that secrete neurotransmitters directly into the circulatory system are releasing hormones. This is because while neurotransmitters act locally at the synapse between neurons, signaling molecules like norepinephrine can function as hormones when they are released into the bloodstream, such as from the adrenal gland.
Consider the autonomic nervous system, which uses chemicals for its signaling processes. These chemicals can either be neurotransmitters or hormones based on the location of their release. Neurotransmitters are released at synapses and have select target interactions, while hormones are released into the bloodstream and have more widespread effects throughout the body.
For instance, acetylcholine is a neurotransmitter at the neuromuscular junction, facilitating communication between neurons and muscle cells. However, when the adrenal medulla releases epinephrine and norepinephrine directly into the circulation, rather than just across a synaptic gap, these molecules act as hormones.