Neuromuscular junctions in the diaphragm muscle of an adult mouse.
Dr. Bruce Patton
[Cover art, Nature Neuroscience, June, 2001.]
 Nerve
terminals were labeled in vivo by "green fluorescent protein" (GFP, here shown
blue), which is expressed in the motor nerves by transgenic methods. The postsynaptic
surfaces of the muscle are labeled with rhodaminated alpha-bungarotoxin (shown
yellow), a snake venom toxin which binds to (and blocks) muscle acetylcholine
receptors. The majority of the muscle, and the glial cells which cover the nerve,
are unlabeled (black). A single confocal optical section is shown, reflected about
the midline of the image.
Several aspects of synaptic specialization are apparent. Nerve terminals (blue) are branched and varicose, unlike thin
pre-terminal axons (not shown). The distribution of terminal varicosities (boutons) is mirrored in the muscle
fiber surface by form-fitting postsynaptic indentations in the muscle membrane, which also contain high concentrations
of ACh receptors (yellow). Striations visible in the distribution of the ACh receptors mark the location of
synaptic folds, deep invaginations which fenestrate postsynaptic surfaces at NMJs in mouse and man (and even
snake). In return, "active zones" in the nerve terminal membrane are located precisely opposite to the postsynaptic
folds (not shown). Active zones are special molecular complexes that regulate the timing and location of neurotransmitter
release by controlling synaptic vesicle fusion. Thus, the machinery for neurotransmitter release and detection
are maintained in exquisite registration. Several laminin isoforms appear to collaborate in promoting these
features of coincident pre- and postsynaptic differentiation. |
