The orexin-A/hypocretin-1 and orexin-B/hypocretin-2 are neuropeptides synthesized by a cluster of neurons in the lateral hypothalamus. Orexins play an significant role in regulating arousal and attention. The term "arousal" refers to a person's general state of alertness or excitation, both physiological and psychological. Arousal levels ranges on a continuum from drowsiness through alertness to excitement. Orexin plays an important role in the regulation of feeding behaviour. The orexin stimulates food intake and reduces the metabolic rate, increasing and maintaining the body's energy reserves. Orexins play also a fundamental role in the regulation of the sleep-wake cycle by increasing arousal levels and maintaining wakefulness. Its deficit can cause narcolepsy. Key to the diagnosis of narcolepsy is the combination of a common symptom, excessive daytime sleepiness, and an uncommon symptom, cataplexy, i.e. a sudden and transient episode of muscle weakness.
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