Cross references: Amphioxus Motor Nerves Lamprey Muscles
Lamprey Fast-Slow Twitch
A study of the morphology of the large reticulospinal neurons of the lamprey ammocoete by intracellular injection of Procion yellow.
"47 reticulospinal neurons of the lamprey brain-stem were stained by intracellular injection of Procion yellow, so that their morphology and dendro-architecture could be determined. The vagal, bulbar, Mauthner, isthmic and mesencephalic groups could be distinguished by the shape and size of their somata and the distribution of their dendrites. However, they shared the following features: the somata of all these cells were relatively large (up to 80 micrometer largest diameter); the dendrites were extensively distributed in the brain-stem in an open-field arrangement; the initial segment expanded abruptly at the soma junction but tapered gradually to form a narrow axon, which joined the medial longitudinal fasciculus in the brain-stem."
"The vagal, bulbar, Mauthner, isthmic and mesencephalic groups could be distinguished by the shape and size of their somata and the distribution of their dendrites." This is the first time I've encountered any mention of identifiable "groups" of reticulospinal neurons.
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Development and organization of the descending serotonergic brainstem-spinal projections in the sea lamprey (PubMed)
"... two serotonergic populations of the isthmic and vagal reticular regions present reticulospinal neurons from the beginning of the larval period.
Of the three serotonergic subpopulations recognized in the isthmic reticular group, only two – the medial and ventral subpopulations – project to the spinal cord, withmost of the projecting cells in the caudal part of the medial isthmic subpopulation.
The vagal reticular serotonergic nucleus situated in the caudal rhombencephalon also presents cells with descending projections. The early development of the brainstem serotonergic projections to the spinal cord appears to be a conserved trait in all vertebrates studied.
Although a serotonergic hindbrain–spinal projection system appears to have been present before the divergence of agnathans and gnathostomes, no serotonergic cells were observed in the raphe region in lamprey. Moreover, proportionally more rostral hindbrain serotonergic cells contribute to the spinal serotonergic projections in the sea lamprey than in jawed vertebrates."
Ontogeny of 5-HT Neurons in the
Brainstem of the Lamprey
"... three main 5-HT-ir neuron groups. The rostralmost brainstem group was located near the posterior commissure, the second group at the isthmus, and the third group in the bulbar area. For each of those groups, the distribution of the 5-HT-ir neurons was very similar in the three developmental stages examined, suggesting that the 5-HT system is relatively mature early in larval animals."
Organization of the lamprey striatum - transmitters and projections. - 1997