The habenula is an epithalamic structure through which descending connections pass from the telencephalon to the brainstem, putting it in a key location to provide feedback control over the brainstem monoaminergic projections ascending to the telencephalon.
Whereas the laryngeal innervation by the vocal motor nucleus, the nucleus ambiguus, also follows the common mammalian scheme, certain brainstem areas feeding into the nucleus ambiguus play a peculiar role in bats and operate in parallel to the descending connections from the periaqueductal gray to the final common vocal motor pathway commonly described in other mammals.
Note for convenience, we refer to r as the connection length, where negative values correspond to ascending connections and positive values correspond to descending connections.
After tuning, for a chain of ten oscillators, we have 18 intersegmental connection functions (H_{r}) for (r=-9,ldots,-1,1,ldots,9) representing both ascending and descending connections.
Note in this case, for connections of length 1 and 2, ascending strengths are stronger than descending strengths, but the curves cross transversely (at approximately coupling length 3), after which descending connections become stronger than ascending connections.
Similarly, Fig. 9B shows connection strengths, again as a function of the connection length, for both ascending and descending connections where (A_{a}=0.006), (A_{d}=0.0004), (lambda_{a}=0.75), and (lambda_{d}=4).
Ayali et al. experimentally showed differences in CPG output between blocking short ascending and descending connections, which further supports the idea of coupling asymmetry in the lamprey CPG [43].
Crucial for the normal function of arousal control, sleep regulation, homeostatic maintenance and possibly pain processing [29], the HCRTs may be involved in CH pathology by way of a polymorphism of the HCRT-2 receptor gene [32 35] or an otherwise related mechanism, possibly involving descending connections from the hypothalamus to brainstem circuits involved in trigeminal nociception (Fig. 1).
The descending connections from the brain allow for more sophisticated control.
Do descending connections in the motor system share these properties?
