The two main divisions of the nervous system are the Central Nervous System and the Peripheral nervous system.
The Central Nervous System (CNS) is responsible for the said actions. During the alarm buzzing off, the temporal lobe (primary function: hearing, speech, memory) processes the sound from the alarm and interprets it. This lobe also associates the memory of needing to prepare for the class upon hearing the alarm. Afterward, the hypothalamus (which regulates sleep) receives the sensory information, sending arousal signals to the cerebral cortex. We stay awake due to the activation of the cortex and the neurons in the arousal areas. On the other hand, salivation is an involuntary response, whereas the organ responsible for it is the medulla. The secretion of saliva is a reflex caused by a stimulus traveling through the salivary nuclei found in the medulla oblongata. In this scenario, the smell of the brewing coffee acted as a stimulus, which triggered the salivation.
In contrast, the Peripheral Nervous system is responsible for yawning, particularly the Paraventricular nucleus of the hypothalamus. It consists of numerous chemical messengers, including dopamine, glycine, oxytocin, and adrenocorticotropic hormone (ACTH), which induces yawning. For instance, the ACTH is prominent every night and before waking up, causing humans to yawn and stretch. One way is when the PVN’s neurons are stimulated, activating the cells from the brain stem or the hippocampus, resulting in yawning. However, although there is still no specific scientific explanation, tiredness and boredom also cause us to yawn. To explain further, yawning and stretching instinctively upon waking up is a process called pandiculation. This process involves the nervous system, which is a natural way of awakening our muscles after waking up or being sedentary momentarily. Pandiculation also prepares us to move and avoid muscle tension.