Synaptic currents and Potentials Channels in the post synaptic membranes of skeletal muscle have been studied extensively because they are large. The reversal potential in mammals is about -15 mV. A reversal potential is the voltage at which there is no net current flow into or out of the cell even though the channels are open. It is a function of the relative permeability of the channel to the ions that pass through it. It is done with a voltage clamp machine that holds the voltage of a cell steady while measuring the current flow into or out of the cell Fig 6-18. It is not the neurotransmitter that determines whether or not a cell will be stimulated or inhibited, but rather the type of channel that the receptor molecule is attached to.
Skeletal muscle is stimulated by ACh because the receptor is of a type called nicotinic .
Some smooth muscle, as in the pyloric sphincter or even cardiac muscle, is inhibited by ACh. Fig 6-23 p 188. Pre-synaptic inhibition occurs when a bouton ends upon an excitatory bouton. Pre-synaptic inhibition is like using a scalpel instead of a chain saw as only one input to a cell is reduced in its effectiveness and not necessarily the whole cell. We know a lot about this particular receptor because it can be obtained in quantity from fish electric organs.