Cholinergic modulation of sensory information

Early studies on the effect of acetylcholine (ACh) on cortical neurons indicated that ACh is primarily excitatory. In contrast to glutamate, however, it was recognized that ACh has two puzzling characteristics: the delayed nature of its excitation and the fact that only 20%–30% of the cortical neurons appeared to respond to ACh. It is now recognized that the former is largely, due to the closure of potassium channels by ACh via muscarinic receptors, which results in a slow depolarization, while the latter is the result of the fact that these early experiments used a change in spontaneous firing as the major criterion of a response. However, if ACh is applied together with a second input, ACh has a modulatory effect on the response to this second input in the vast majority of cortical cells, a phenomenon known as heterosynaptic facilitation. By using this more subtle modulation, ACh could regulate sensory processing in the cortex. In addition, this type of heterosynaptic facilitation may be relevant to the phenomenon of associative learning and memory. This chapter deals with the immediate and long-lasting effects of ACh action separately. For simplicity, it refers only to ACh, although many of the studies used other muscarinic cholinergic agonists, and concentrates on the neocortex.