The figure shows a recording from a patch-clamp experiment in which the electrical current passing across a patch of membrane is measured as a function of time. 

The membrane patch was plucked from the plasma membrane of a muscle cell by the technique shown in Figure 12–25 and contains molecules of the acetylcholine receptor, which is a ligand-gated cation channel that is opened by the binding of acetylcholine to the extracellular face of the channel. To obtain a recording, acetylcholine was added to the solution inside the microelectrode. 

(A) Describe what you can learn about the channels from this recording. 

(B) How would the recording differ if acetylcholine were (i) omitted or (ii) added to the solution outside the microelectrode only?

2 pA 10 msec TLT LITL