ISSN:
1432-0878
Keywords:
Motor endplate
;
Rana esculenta
;
Ultrastructure and function
;
Freeze-etching electron microscopy
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Medicine
Notes:
Summary The frog motor endplate in its simplest form consists of an elongated, slender nerve ending embedded in a gutter-like depression of the sarcolemma. This nerve terminal contains the usual synaptic organelles. It is covered by a thin coating of Schwann cell cytoplasm which embraces the terminal with thin finger-like processes from both sides, thereby sub-dividing it into 300–1000 regularly spaced compartments. The individual synaptic compartments correspond to the strings of varicosities or grape-like configurations of motor nerve terminals in endplates of other species and in the cerebral neuropil of vertebrates. Each compartment contains one or more bar-like densities of the presynaptic membrane, ‘active zones’, which are associated with the attachment sites between synaptic vesicles and plasmalemma. Active zones have a regular transverse arrangement and occur at specific loci opposite the junctional folds. The attachment sites for synaptic vesicles are at the edges of the bars which are bilaterally delineated by a double row of 10 nm particles attached to the A-face. The structural appearance of vesicle attachment sites in freeze-etch replicas corresponds to that of micropinocytosis. The active zones are often fragmented and the frequency of their association with vesicle attachment sites is highly variable. The junctional folds are characterized by “specific sites” in which intramembranous particle aggregations occur at relatively high packing density (7500/μm2). These sites are located opposite the active zones at the juxtaneural lips, a location where one would expect ACh-sensitive receptors on the postsynaptic membrane.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00223024
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