Which neurons are considered first order neurons in the anorexigenic pathway?

In the context of the anorexigenic pathway, first-order neurons refer to those that directly respond to signals related to energy balance and appetite regulation before they relay this information to second-order neurons. The neurons categorized as POMC (Pro-opiomelanocortin) and CART (Cocaine- and Amphetamine-Regulated Transcript) are integral components of the appetite regulation system.

POMC/CART neurons are located in the arcuate nucleus of the hypothalamus and play a crucial role in promoting satiety. Upon activation by signals such as leptin, these neurons release neuropeptides that inhibit appetite and promote energy expenditure. Thus, they act as first-order neurons by receiving peripheral signals related to hunger and satiety and initiating the necessary physiological responses.

In contrast, NYP/AGRP neurons, while also found in the arcuate nucleus, are part of the orexigenic pathway, which stimulates feeding, rather than suppressing it. Leptin receptors and insulin-sensitive neurons play roles in the overall signaling mechanisms but are not classified as first-order neurons due to their position within the broader network of appetite regulation. They may influence the activity of POMC/CART neurons or other pathways but do not directly respond to energy