I have been reading a lot about neurotransmitters and hormones but what's the difference between them both or are they the same? It's been confusing for a while now. Also, why do some neurotransmitters act as hormones?
At heart, the distinction between neurotransmitters and hormones is how they are transmitted - not necessarily a difference in the chemicals themselves.
Neurotransmitters are sent over synapses, between neurons. They may be small molecules (e.g. dopamine) or larger molecules such as proteins (e.g. substance P). Conversely, hormones are sent through the circulatory system but could also be small molecules (e.g. adrenaline) or large molecules (e.g. growth hormone).
Looking at the structures of these chemicals, we can see that both neurotransmitters and hormones have an enormous variability in the chemicals they involve. Furthermore, dopamine looks much closer to adrenaline than it does to substance P. But dopamine is primarily a neurotransmitter and adrenaline is typically thought of as a hormone!
There may also be overlap between roles of chemicals as neurotransmitters or hormones. In fact, adrenaline may act as both a neurotransmitter and a hormone. So, to answer your question of "why do some neurotransmitters act as hormones?" - the distinction is in the context of how the chemical is sent through the body. There's variability in which chemicals are used for either purpose, and no rule saying a chemical can't be sent via multiple mechanisms.
Furthermore, neurotransmission also involves spillover of neurotransmitter, which in theory could find its way into the circulatory system. So there could be overlap between neurotransmission and hormone signaling. And lastly, as @Remi.b points out, the particular mechanisms of neurotransmission and hormone signaling have implications on the dynamics of the signaling.
Neurotransmitters, also known as chemical messengers, are endogenous chemicals that enable neurotransmission.
A hormone [..] is any member of a class of signaling molecules produced by glands in multicellular organisms that are transported by the circulatory system to target distant organs to regulate physiology and behaviour
There is even a section called Comparison with neurotransmitters in the wikipedia > hormone page
There are various clear distinctions between hormones and neurotransmitters:
- A hormone can perform functions over a larger spatial and temporal scale than can a neurotransmitter.
- Hormonal signals can travel virtually anywhere in the circulatory system, whereas neural signals are restricted to pre-existing nerve tracts
- Assuming the travel distance is equivalent, neural signals can be transmitted much more quickly (in the range of milliseconds) than can hormonal signals (in the range of seconds, minutes, or hours). Neural signals can be sent at speeds up to 100 meters per second.
- Neural signalling is an all-or-nothing (digital) action, whereas hormonal signalling is an action that can be continuously variable as dependent upon hormone concentration.