ISSN:
1573-7381
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary A general account is given of synaptic organization in the rat LGd, with special emphasis on the intrinsic neurons, and the vesicle-containing P-boutons to which they give rise. The account is based on ultrastructural studies of normal adult rats, and of experimental animals surviving from less than one day to more than four weeks following unilateral eye removal or ablation of the visual and peri-visual cortex. P-boutons originate as narrow stalks or as blunt outpushings from presynaptic dendritic shafts. Commonly a single appendage comprises a series of P-boutons interconnected by narrow intervaricose portions. There is a very close correspondence in size and appearance between such appendages characterized by electron microscopy and the complex dendritic appendages seen in Golgi impregnations. P-boutons represent the principal synaptic territory of the intrinsic neuron. They arepresynaptic to relay cell dendrites (or dendritic appendages), and to other P-boutons (or, less commonly, to presynaptic dendrites). They arepostsynaptic to other P-boutons (or, less commonly, to presynaptic dendrites), to axon terminals containing ‘flattened’ vesicles (F-boutons), and to large intraglomerular axon terminals containing spherical synaptic vesicles (R-boutons). Degeneration of R-boutons after enucleation indicates that most are the terminals of retinal afferents. The P-boutons are intermediate elements in various serial synapses, the most characteristic of which is the intraglomerular triplet (or triad) synapse. At triplet synapses, a P-bouton and a relay cell dendrite onto which the P-bouton synapses, both receive at least one synaptic contact from an R-bouton. The possible role of triplet synapses in rapid feed-forward inhibition is considered. P-boutons and presynaptic dendrites also establish reciprocal synaptic relationship with one another, although closely spaced reciprocal synapses (c. 0.25 μm apart) are rare. The F-bouton population originates at least in part from small myelinated axons. The F-boutons contain closely packed, cylindrical synaptic vesicles, establish Gray type 2 contacts and are never postsynaptic. They are greatly outnumbered by P-boutons within the glomeruli, but synapse extensively in the extraglomerular neuropil onto both intrinsic and relay cell somas and dendrites, and onto the initial axons of relay cells. The possibility that some F-boutons originate from brain stem afferents is considered.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF01097191
Permalink
