*Central Nicotinic and Muscarinic Receptors in Health and Disease DOI: http://dx.doi.org/10.5772/intechopen.112447*

Thus, extensive trajectories throughout the brain, such as cortical connections mediating decision making and planning; projections to the hippocampus and amygdala influencing attention, memory, fear, and stress responses; mesolimbic pathways affecting response to reward; hypothalamic system controlling homeostatic responses such as thermoregulation, food intake, and sleep, all utilize ACh as a neurotransmitter. Furthermore, cholinergic system plays an important role in facilitating synaptic plasticity and neuronal development [1]. For these reasons, the cholinergic systems, particularly, the basal forebrain complex, PPN and LDTN have been extensively studied in relation to age-related progressive neurodegenerative diseases such as Alzheimer's disease (AD) and PD [1].

#### **Figure 1.**

*Simplified schematic diagram depicting central cholinergic system as well as its interaction with the peripheral organs. The afferent and efferent connections of the vagus nerve with the heart, lung, gastrointestinal (GI) tract, kidney and the gonadal system are highlighted. Moreover, the mediators of acetylcholine (ACh) actions (i.e., muscarinic and nicotinic receptors) and their structural composition (i.e., G-protein coupled receptor vs. ligandgated ion channels) are distinctly shown in the insert.*

It is also of relevance to note that the cholinergic projections, in general, have a modulatory, rather than strictly excitatory or inhibitory effects on other neuronal systems. As mentioned earlier, ACh action is mediated by both nicotinic and muscarinic receptors, which interact at both pre- and post-synaptic junctions. A major emphasis of this chapter is to provide an up-to-date understanding of this complex interaction (**Figure 1**).
