Below, is an image of ionotropic vs. metabotropic action. The figure illustrates the distinction between the 2, which, at a basic level, boils down to their molecular response upon binding neurotransmitter.
As a review of ionotropic vs. metabotropic:
Image taken from: http://hebb.mit.edu/courses/8.515/lecture1/sld037.htm
a. ionotropic - ion flows through | b. metabotropic - secondary messenger
There are importance differences between receptor categories regarding (1) speed of effect & (2) duration of effect.
speed of effect:
- ionotoropic binds NT, opens channel, an immediate flow of ions, inducing an EPSP or IPSP.
- metabotropic binds NT, a cascade of secondary messenger systems occur. This can have several diverging effects, which include: (1) opening another channel via an internal binding site, (2) increasing or decreasing transcription, (3) protein modifications, including phosphorylation… (4) and many more downstream effects.
duration of effect:
- ionotropic kinetics are usually a very quick response.
- metabotropic kinetics are usually delayed, also depending on the receptor and its end effect. This is due to the signal cascade, rather than a channel directly opening.
To further discuss these receptors, take for example: (1) cardiac muscle vs. (2) skeletal muscle. In cardiac muscle, the ACh receptors are muscarinic metabotropic receptors. In skeletal muscle, ACh receptors are nicotinic ionotropic receptors.
One question is why muscarinic and nicotinic? Its a legacy name, based on the drug researchers found which bound these receptors.
Functionally, the cardiac muscles slows down when ACh is applied, but there is a slight delay (compared to ionotropic receptor response). Conversely, the skeletal muscles twitch when ACh is applied, and the effect is typically quicker than metabotropic. This is because in the cardiac muscle, several secondary steps must occur, whereas in skeletal muscles, the ionotropic receptors open up and ions flow in. This elicits some immediate change to the cellular conditions, and thus the twitch.
This also provides an opportunity to make another vital point. An important distinction, is that the receptor type, rather than the neurotransmitter, determines the end effect. This can be an excitatory postsynaptic potential (EPSP) or an inhibitory postsynaptic potential (IPSP) from an ionotropic or metabotropic receptor. So despite the receptors binding the same NT, ACh, these receptors have different effects!