Dendrite bundles in medulla and raphe

The medulla is a central brain structure that contains the raphe neurons, which are some of the oldest neurons in the nervous system. Global state functions like arousal and sleep are attributed to neurons in the medulla. As shown in the figure below, dendrite bundles are the major feature of the rabbit medulla, thus dendrodendritic activities are presumably important for global arousal functions.

From raphe neurons

The dendrites of raphe neurons form a midline bundle, a defining feature of the serotonergic system.

From raphe neurons

The dendrites of each neuron in the bundle extend for several hundred micrometers. Dendrites from a single neuron were noted ascending and descending in the bundle. Some dendrites leave the bundle at right angles and extend towards the reticular formation, making contact with perikarya and dendrites of other raphe neurons and reticular formation cells.



Dendritic communication networks serve to connect raphe dendrites of the bundle to other serotonergic dendrites, and to the dendrites of reticular formation cells. Large multipolar reticular formation neurons send numerous dendrites towards and away from the midline 5-HT bundle. The close relationship between raphe neurons and reticular formation neurons has led some authors to conclude that the raphe nuclei should be considered as part of the reticular formation on the basis of cytoarchitecture. These neurons are characterized by a few long, straight dendrites with long spines, and possess a small soma in comparison to the length and thickness of the dendrites.

One difference between the raphe and reticular formation neurons is that most of the raphe dendrites stay in the plane of the midline, and the raphe neurons seem to have a special relationship to the blood and cerebrospinal fluid (CSF).

The figure below is a drawing of a medium-sized neuron in raphe obscuris, one of several nuclei that compose the raphe. One ascending dendrite ends in the third ventricle, which is a brain cavity filled with CSF. The raphe neuron dendrites are laden with amine-rich vesicles than can be released into the blood stream and CSF, thus reaching wide territories of the body. Alternatively CSF-borne substances may be uptaken by dendrite bundles and transported into raphe soma.

Dye-coupling experiments with raphe neurons (figure below, rat) suggest that raphe neuron dendrites contain gap junctions. Dendrodendritic connections in raphe obscurus and raphe pallidus have been identified with electron micrograph experiments.

From raphe neurons

Electrical communication in the raphe and medulla may be important for the coordination of wakefulness and sleep. The receptive fields for many arousal processes overlap extensively in the midline dendrite bundle.

References

Cummings J. P. and D. L. Felten. (1979). A raphe dendrite bundle in the rabbit medulla. J. Comp. Neurol. 183, 1-23.

Felten D. L. and J. P. Cummings. (1979). The raphe nuclei of the rabbit brain stem. J. Comp. Neurol. 187, 199-243.