What are "intrinsic hypothalamic fibers"? (From an article on neuronal mechanisms of sexual desire)
&quot;Intrinsic hypothalamic fibers&quot; are fibers instrinsic to the hypothalamus; so yes, these are fibers either projecting from somewhere in the hypothalamus to somewhere else in the hypothalamus, or locally in the hypothalamus (the hypothalamus itself is a pretty complex structure containing many distinct nuclei). The paper you reference specifically mentions the medial preoptic area, mPOA, as a target:

Infusions of mu opioid receptor agonists to the mPOA inhibit sexual behavior in male rats

and

infusions of a mu antagonist to the mPOA, but not NAcc, block sexually conditioned place preference

&quot;Fibers&quot; does not necessarily refer to myelinated axons, just axons of any sort.