This pathophysiology diagram illustrates the 'remote preconditioning reflex arc,' a complex neural and humoral pathway involved in remote ischemic preconditioning (RIPre). The diagram depicts a stimulus applied to an arm via a blood pressure cuff, which triggers afferent somatic nerves that project to the brain. Efferent signals are shown descending from the central nervous system via the vagus nerve (n. vagus) and sympathetic nerves to the heart and other visceral organs, specifically the intestine. Key cardiovascular features include the involvement of intrinsic cardiac ganglia and a potential rise in acetylcholine (ACh) concentration in the myocardium. The diagram highlights several scientific uncertainties using question marks, specifically regarding the role of sympathetic nerves, whether abdominal vagal fibers are afferent or efferent, and the origin of humoral factors like GLP-1 and nitrite. Humoral pathways are shown as dashed lines connecting systemic factors to the brain and heart, indicating a parallel mechanism of cardioprotection alongside the primary neural reflex arc.

A pathophysiology diagram illustrating the neural pathways involved in cluster headaches within a sagittal view of the human head and brain. The diagram highlights the trigeminal-autonomic reflex arc. Key neural structures depicted include the hypothalamus (HT), which serves as a central hub with connections to the thalamus (T), suprachiasmatic nucleus (SN), superior salivatory nucleus (SSN), and the trigeminocervical complex (TCC). Sensory pathways are shown in purple and blue, originating from the dural structures and trigeminal nerve, synapsing in the TCC and projecting to the cortex. The parasympathetic pathway (pink) involves the SSN and sphenopalatine ganglion (SPG), leading to autonomic symptoms. The sympathetic pathway (yellow) is traced from the intermediolateral (IML) tract of the spinal cord through the superior cervical ganglion (SCG) and the plexus surrounding the internal carotid artery (ICA) in the cavernous sinus. Red dashed lines indicate the hypothalamic modulation of the SSN and TCC, which are critical in the pathogenesis of trigeminal autonomic cephalgias.

This historical anatomical illustration depicts René Descartes' early 17th-century mechanistic theory of the reflex arc. The diagram shows a human figure kneeling by a fire, illustrating the physiological response to a noxious stimulus. Key elements are labeled to represent a functional pathway: (a) indicates the external stimulus (fire); (b) represents the peripheral sensory site on the skin of the foot; (c) shows a 'fine thread' or nerve fiber extending from the foot, up the leg and back, to the head. The path includes nodes representing the spinal column or ganglia. Within the brain, (d) and (e) represent mechanical valves that govern the release of 'animal spirits' from a central cavity (f). This anatomical diagram serves as a foundational concept in the history of neuroscience, demonstrating an early understanding of the stimulus-response pathway, where sensory input travels to the central nervous system to trigger a motor reflex. It is a critical visual resource for teaching the evolution of neurophysiology and the history of medicine.