Some morphological features of the myoneural junctions in certain normal muscles of the rat

Comparison of motor endings of the cat's muscle spindle stained for NADH-tetrazolium reductase and cholinesterase

Cat muscle spindles were examined histochemically in serial transverse sections of tenuissimus muscles stained for ATPase, NADH-TR and ChE alternating sequentially. Motor nerve terminals on nuclear bag1, bag2 and nuclear chain intrafusal muscle fibers were identified in periodic sections stained for ChE. Intrafusal fiber regions that carried ChE-active areas were then examined in staining for NADH-TR. The motor endings on the three types of intrafusal fiber differed in their apparent histochemical content of both ChE and NADH-TR. The observations suggest that functional differences may exist among motor nerve terminals on the various intrafusal fiber types.

Patterns of motor innervation in the pectoral muscle of adult Xenopus laevis: evidence for possible synaptic remodelling

1. An anatomical and electrophysiological study was performed on the pectoral muscle of Xenopus laevis. 2. Silver-impregnated preparations revealed immature endings, collateral and terminal sprouting and signs of synaptic regression. 3. Twenty percent of the fibres received a dual innervation from two different nerves. The synapses of 25% of these fibres are formed in close vicinity. 4. Some of the singly innervated and most of the dually innervated end-plates generated only subthreshold electrical activity. Synaptic efficacy in dually innervated muscle fibres with closely spaced or distant endings was, on the average, one third and two thirds, respectively, of that obtained in singly innervated fibres. 5. Fibres with subthreshold electrical activity displayed normal ACh sensitivity. 6. The existence of non-transmitting synapses, of dually innervated end-plate sites and of morphological signs of the sprouting of new endings and the degeneration of old ones suggests that synaptic remodelling may occur in normal adult muscles.

Sprouting and regression of the nerve at the frog neuromuscular junction in normal conditions and after prolonged paralysis with curare

A light microscopical, histochemical and electron microscopical investigation of the frog neuromuscular junction has been performed on muscles from animals in different functional states of activity. The combined staining of axon terminals and cholinesterase (ChE) allows a precise description of the nerve terminal arborization and its synaptic contacts. Most terminal arborizations form long continuous contacts with the muscle cell. Distinquishable from these are nerve branches (usually of small diameter)d or distal endings of branches with one or several small and isolated contacts. It is assumed that these are sprouts with newly-formed synaptic sites. Other sprouts end without apparent synaptic contact. At the uttrastructural levet, nerve sprouts end without apparent synaptic contact. At the uttrastructural levet, nerve sprouts growing into empty, well-differentiated synaptic gutters or inducing the formation of new synaptic sites were observed. In other sites, ChE is apparently located at postsynaptic gutters with no nerve present. Similarly, in the electron microscope, well-differentiated synaptic gutters lacking any nerve or Schwann cell elements were observed. In addition, synaptic gutters only partially occupied by the nerve were frequently seen. These features have been interpreted as signs of regression of the nerve terminals. Nerve regression and sprouting were found in animals chronically paralysed with curare over several weeks as well as in untreated frogs (winter and summer frogs, laboratory frogs, fed and unfed). When quantitatively evaluating the occurence of presumed features of nerve sprouting and nerve regression, differences were found between different experimental groups. From this it is concluded that, in addition to developmental changes, the degree of nerve sprouting and regression is controlled by external factors such as muscle activity and seasonal variations. Signs of sprouting and nerve regression can be simultaneously present in a single synapse. It appears that the frog neuromuscular synapse is not a static structure, but is in a state of permenent remodelling.

Structure of developing frog neuromuscular junctions

Developing neuromuscular junctions in the cutaneous pectoris muscle from tadpoles and and postmetamorphic frogs were studied in the light microscope. Presynaptic nerve terminals and postsynaptic acetylcholinesterase (AChE) activity simultaneously demonstrated at these developing junctions using the NBT-AChE method. The earliest nerve contacts studied were small enlargements at the ends of unmyelinated axons. As development progressed, single nerve contacts, often with growth cones, grew in length, generally parallel to the long axis of the myotube. Further endplate maturation was characterized by individual terminal processes developing secondary and tertiary branches, ultimately leading to highly complex terminal arborizations. The initial synaptic contact at developing neuromuscular junctions was made by a single axon, but with further development these same synaptic sites became multiply innervated. The occurrence of multiple innervation was a transient phenomena; the multiple synaptic inputs were eliminated during further maturation. The time course of synapse elimination was protracted, with some multiple innervation even persisting in relatively large adult frogs. The first nerve contacts were generally devoid of observable endplate AChE activity. Early appearance of AChE activity was sometimes graded and in some cases portions of the nerve terminal processes were associated with AChE while other regions of the same terminal arborization were not.

Structural differences of pale and strongly stained motor end-plates of the rat diaphragm

According to the staining intensities for AChE, the motor end-plates of the rat diaphragm can be classified into strong (S) and pale (P) types. About 34% of the total end-plates of the rat diaphragm are of S type and 50% of P type. The P end-plates differ from S end-plates in two aspects. First, the secondary subneural clefts of the S end-plates are well developed. They are numerous, long, closely packed and often branched. On the other hand, the secondary subneural clefts of the P end-plates are short, sparse and usually unbranched. Secondly, there seems to be a variation in AChE activity in the P end-plates. Focal negative AChE areas are found in the subneural apparatus of some P end-plates. It is concluded that the less well developed secondary subneural clefts and focal areas of negative AChE activity contribute to the paler staining of the P end-plates.