General anesthesia is defined as a drug-induced reversible condition characterized by unconsciousness, amnesia, analgesia, immobility, and maintenance of physiological stability. Monitoring brain and behavioral states under general anesthesia is challenging, and the brain states of patients under general anesthesia are often tracked using physiologic signs and, at times, electroencephalography (EEG)-based markers of unconsciousness.

1. During the maintenance of general anesthesia, anesthesia providers often use changes in the physiologic measures of heart rate, blood pressure, and movement to track anesthetic states and guide the administration of anesthetic and analgesic medications.
2. The changes in heart rate and blood pressure that anesthetized patients show in response to a nociceptive stimulus can be explained in terms of the nociceptive-medullary-autonomic (NMA) circuit, which comprises the spinoreticular tract, the brainstem arousal circuits, and the sympathetic and parasympathetic efferent pathways.
   1. The ascending nociceptive (pain) pathway begins with A-delta and C-fibers whose free nerve endings bring nociceptive (painful) information from the periphery to the spinal cord.
   2. In the spinal cord, these fibers synapse in the dorsal horn on projection neurons that travel through the anterolateral fasciculus and synapse at multiple sites in the brainstem, including the nucleus of the tractus solitarius in the medulla.
   3. The autonomic response to a painful stimulus is initiated within the nucleus of the tractus solitarius, which mediates sympathetic output through the rostral ventral lateral medulla and the caudal ventral lateral medulla to the heart and peripheral blood vessels through projections to the thoracolumbar sympathetic ganglia. The parasympathetic output from the nucleus of the tractus solitarius is mediated through the nucleus ambiguus, which projects through the vagus nerve to the sinoatrial node of the heart.
3. A potentially nociceptive operative stimulus initiates an increase in sympathetic output and a decrease in parasympathetic output through the NMA circuit that rapidly results in an increase in heart rate and blood pressure. As such, these signs can be a rapid indicator of an inadequate level of analgesia.
4. Apart from physiologic signs, intraoperative EEG can be used to track altered levels of consciousness induced by general anesthesia and sedation.