[HAPS-L] Neuromuscular junctions and motor end plates

[Ken Saladin <ksaladin@xxxxxxxxxx>]

Generally a skeletal muscle fiber is innervated by only one motor neuron, but does that motor neuron form one NMJ, or several, with the muscle fiber? The nerve fiber loses its myelin sheath, branches several times, and each branch ends in a synaptic knob that establishes its own contact with the sarcolemma. But is this entire complex one NMJ, or is each point of synaptic contact a separate NMJ?

Kandel's Principles of Neural Science (4th ed. p. 188) illustrates the neuron and muscle fiber and draw a box around the entire region, illustrating 9 or 10 synaptic contacts, and calls this entire region a single neuromuscular junction. All the branches are shown as ending on a single motor end-plate and the implication is that this entire end-plate region, with all its synaptic contacts, is one NMJ.

Haines's Fundamental Neuroscience uses "neuromuscular junctions" as a parenthetical synonym for "motor end plates" and later says (2nd ed., p. 374) "This specialized type of synapse is called a neuromuscular junction or motor end plate." His illustration shows the branching nerve fiber ending at several points on the muscle fiber, and circles only two of the synaptic knobs and labels that region "motor end-plate." Ross, Kaye, and Pawlina's Histology and Junqueiro & Carneiro's Basic Histology also treats NMJ (or myoneural junction) as synonymous with motor end plate, as does the Stedman medical dictionary. The Dorland dictionary defines motor end plate as the expanded tip of the axon, not as the entire junction or as part of the muscle fiber!  I had always taken "motor end-plate" to mean only the depressed region of the sarcolemma that receives the nerve endings; all the A&P books I looked at just now (Martini, Marieb, Tortora, Seeley), and mine, use the term in that way -- b
ut it seems we are all in disagreement with the neuro textbooks and medical dictionaries.

I ask because someone recently asked me how I construe this terminology, and as far as I can see, even the "authorities" are pretty sloppy in their use of  it (especially "motor end plate"). What should we be telling our students and writing in our textbooks?

Ken