Orexin (hypocretin), is a neuropeptide produced by a subset of neurons in the lateral hypothalamus. From the lateral hypothalamus, the orexin-containing neurons project their fibres extensively to other brain structures, and the spinal cord constituting the central orexinergic system. Generally, the term ‘’orexinergic system’’ usually refers to the orexin peptides and their receptors, as well as to the orexin neurons and their projections to different parts of the central nervous system. The extensive networks of orexin axonal fibres and their terminals allow these neuropeptidergic neurons to exert great influence on their target regions. The hypothalamic neurons containing the orexin neuropeptides have been implicated in diverse functions, especially related to the control of a variety of homeostatic functions including feeding behaviour, arousal, wakefulness stability and energy expenditure. The broad range of functions regulated by the orexinergic system has led to its description as ‘’physiological integrator’’. In the last two decades, the orexinergic system has been a topic of great interest to the scientific community with many reports in the public domain. From the documentations, variations exist in the neuroanatomical profile of the orexinergic neuron soma, fibres and their receptors from animal to animal. Hence, this review highlights the distinct variabilities in the morphophysiological aspects of the orexinergic system in the vertebrate animals, mammals and non-mammals, its presence in other brain-related structures, including its involvement in ageing and neurodegenerative diseases. The presence of the neuropeptide in the cerebrospinal fluid and peripheral tissues, as well as its alteration in different animal models and conditions are also reviewed.

Orexin（hypocretin）是一种由下丘脑外侧神经元子集产生的神经肽。从下丘脑外侧，含食欲素的神经元将它们的纤维广泛投射到其他脑结构和构成中央食欲素能系统的脊髓。通常，术语“食欲素能系统”通常指食欲素肽及其受体，以及食欲素神经元及其对中枢神经系统不同部分的投射。食欲素轴突纤维及其末端的广泛网络使这些神经肽能神经元对其目标区域产生巨大影响。含有食欲素神经肽的下丘脑神经元与多种功能有关，特别是与控制多种稳态功能有关，包括摄食行为、唤醒、觉醒稳定性和能量消耗。食欲能系统调节的广泛功能导致其被描述为“生理整合者”。在过去的二十年里，食欲能系统一直是科学界非常感兴趣的话题，在公共领域有许多报道。从文献中可以看出，动物之间的食欲能神经元胞体、纤维及其受体的神经解剖学特征存在差异。因此，本综述强调了脊椎动物、哺乳动物和非哺乳动物的食欲能系统形态生理学方面的明显差异，它在其他大脑相关结构中的存在，包括它与衰老和神经退行性疾病的关系。脑脊液和外周组织中存在神经肽，