The locus coeruleus of the rat was studied with the Golgi method and electron microscope in an attempt to demonstrate morphological substrates for the functional significance of these monoamine neurons.
The locus coeruleus is mainly composed of densely packed medium-sized, spindle-shaped nerve cells with frequent soma spines, while in the rostral part of the nucleus the medium-sized cells are less densely located and mingled with small oval cells. Most of the axons originating from the medium-sized nerve cells take a rostro-ventral course usually giving off an initial collateral. It is interesting that some of the axons demonstrate a curious recurrent way or run caudalwards to the medulla oblongata after a short rostro-ventral course. Afferent fibers are composed of thick and fine fibers, which make complicated neuropil around the nurons. The thick afferents enter the nucleus from a ventral to dorsal direction and take an undulating tortuous course among the neurons, revealing axonal swelling and giving off many terminal boutons.
The cytoplasm of the medium-sized nerve cells demonstrates marked aggregates of rough surfaced ER and prominent development of Golgi complexes, and contains a large number of lysosomal dense bodies. Large granular vesicles as seen in the terminal bags are only small in number in the cytoplasm. Cytoplasmic protrusions are characteristic for the medium-sized nerve cells, being moderate in number, varying in shape and length (0.4-1.7μ) and constituting mostly the postsynaptic sites. Axo-somatic synapses are frequent which make contact with axon terminals similar to axo-soma spine synapses. The nuropil contains different kinds of axo-dendritic and axo-dendritic spine synapses. Occasionally one can observe thickened preterminal axons or axon terminals contacting several dendrites or dendritic spines. Astroglial cells fill up the interneuronal spaces and demonstrate large oval mitochondria with homogeneous content and occasional lipofuscin granules.