I was speaking with a substitute teacher of mine, and we were discussing whether neurotransmitters are part of the endocrine system or not. My class just spent an entire semester on the topic of the endocrine and nervous systems. From my knowledge, they are not part of the same system, because they function very differently and they are not activated by the same triggers. Does anyone have any info on this subject? I am interested what the case is an why.
Synaptic signaling can be seen as a type of paracrine signaling, and is hence not an example of an endocrine system.
Khan Academy has a nice accessible overview on this confusing topic:
Signalling between nearby cells that are near one another van occur through the release of chemical messengers that diffuse through the intercellular space. This relatively short-distance communication is called paracrine signaling. Notable paracrine signaling occurs during development, where growth-hormone gradients regulate the differentiation of the various tissues.
A unique type of paracrine signaling occurs between neuronal cells, namely synaptic signaling.
Autocrine signaling refers to cell signaling to itself. Like paracrine signaling, autocrine signaling is important during development, but also in tumor metastasis.
Communication over longer distances often occurs through the circulatory system is called endocrine signaling. Often it occurs through the release of hormones into the bloodstream. Hormones are produced and released by specialized cells and target cells in distant parts of the body. Endocrine glands include the thyroid, hypothalamus, and the pituitary, among others. For example, the pituitary in the brain releases growth hormone that affects growth and other functions throughout the body.
Cells can also interact via direct contact. For instance, gap junctions in animals and plasmodesmata in plants allow for a direct communication between cells by forming tiny channels, allowing small compounds and ions to cross the cell's membrane to a neighboring cell. It occurs for instance in the brain.
Neurotransmitters are secreted by the nerve cells and act directly on other nerve cells, muscle cells or glands (like adrenal medulla) via synapses, so they travel "short distance" (nanometers).
Endocrine glands secrete substances (mainly hormones) into the blood by which they travel "long distance" to the target organs.
Neurotransmitters are quick (seconds) and endocrine substances are slow messengers (minutes).
Norepinephrine (noradrenaline) acts both as a neurotransmitter (between the nerve cells) and hormone (secreted from the adrenal medulla into the blood). Several other substances, such as dopamine, can also act both as neurotransmitters and hormones, but they are still a part of two different systems - the quick one that acts via the nerves and the slow one that acts via the blood.
As mentioned in comments, nervous and endocrine system are connected, so one can merge two systems into a larger neuroendocrine system, a part of which is, for example, hypothalamic–pituitary–adrenal axis that enables stress response.