Schematic representation of neurotransmitters and receptor subtypes involved in yawning

Important neural pathways involved in the regulation of yawning include: (1) two groups of oxytocinergic neurons projecting from the paraventricular nucleus of the hypothalamus (PVN) to the CA1 region of the hippocampus, or the pons, medulla or spinal cord; (2) ACTH-/α-MSH-activated neurons projecting from the PVN to the hippocampus by activation of cholinergic septo-hippocampal neurons; (3) direct activation of cholinergic septo-hippocampal/hippocampal neurons; and (4) a serotonergic-cholinergic pathway. Neurotransmitter systems and receptors that stimulate or enhance yawning are denoted by a plus sign, whereas those that inhibit yawning are denoted by a minus sign. Abbreviations: ACh - acetylcholine; ACTH - adrenocorticotrophic hormone; Adr - adrenergic; GABA - gamma amino butyric acid; 5-HT - serontonin; MSH - alpha-melanocyte-stimulating hormone; NMDA - N-methyl-D-aspartate; NO - nitric oxide; OTXR - oxytocin receptor; PVN - paraventricular nucleus of the hypothalamus. (Source: Teive HA, Munhoz RP, Camargo CHF, Walusinski O. Yawning in neurology: a review. Arq Neuropsiquiatr 2018;76(7):473-80. Creative Commons Attribution [CC BY] License. The original source of the image and the source explanation for the components illustrated was from: Collins GT, Eguibar JR. Neuropharmacology of yawning. Front Neurol Neurosci 2010;28:90-106.)