A good review of yawning is provided by Askenasy (1989), and the points that follow are derived from this source.
Several causes of yawning are discussed in the review:
psychological - this includes boredom,
drowsiness,
fatigue, separation, imitation, and autosuggestion
hormonal - ACTH and MSH
neurological - various brain pathologies including coma, encephalitis,
hypoxia, and neoplasms
psychiatric - for example schizophrenia, psychosis,
depression and withdrawal
general pathologies - ear disorders, gastric and biliary disorders, and motion sickness
drug overdose - for example naloxone, valproate, imipramine, clomipramine,
seratonin, and pentobarbital
...
Yawning occurs in many species from reptiles to birds and mammals under different conditions (Argiolas and Melis, 1998). Although the physiology of yawning is not clearly understood, research has revealed that:
Yawning is under the control of a range of
neurotransmitters and neuropeptides within the brain:
dopamine, excitatory amino acids, acetylcholine,
serotonin, nitric oxide, adrenocorticotropic hormone-related peptides, and oxytocin.
Opioid peptides inhibit yawning.
The paraventricular nucleus of the hypothalamus is involved in the control of yawning.
When activated by dopamine, excitatory amino acids and oxytocin, neurons in the paraventricular nucleus facilitate yawning by releasing oxytocin at distant sites in the brain, ie: the hippocampus, pons and/or the medulla oblongata.
Nitric oxide is also involved in yawning mediated by acetylcholine, serotonine, and adrenocorticotropic hormone (ACTH) and related peptides
Other neurotransmitters, i.e. gamma-aminobutyric acid (GABA) and noradrenaline, and neuropeptides, i.e. neurotensin and luteinizing hormone-releasing hormone (LH-RH), also influence yawning.
References
Aloe, F. (1994) "Yawning."
Arq Neuropsiquiatr, 52(
2), 273-276.
Argiolas, A., and Melis, M.R. (1998) "The neuropharmacology of yawning."
European Journal of Pharmacology, 343(
1), 1-16.
Askenasy, J.J.M. (1989) "Is yawning an arousal defense reflex?"
The Journal of Psychology, 123(
6), 609-621.
from:
Minerva - Nervous System