Physiology of Neuromuscular Junction

Alpha bungarotoxin has another use. Because it binds to tightly to the AChR, it provides a means of seeing each muscle fiber’s postsynaptic site. This is accomplished by tagging bungarotoxin with a small organic fluorescent dye so that the alpha bungarotoxin (red in the figure) clearly delineates the postsynaptic site. Over the last several years, the ability to visualize nerves has become much easier thanks to the Nobel Prize–winning discovery (2003 chemistry prize) of the green fluorescent protein (GFP). This gene for a jellyfish protein that gives certain jellyfish their green bioluminescent glow has been modified so that it can be inserted into the genome of mammals, especially mice. The transgenic mice are engineered by molecular biologists to express the GFP in neurons selectively. In this way, a transgenic mouse can express GFP in its motor nerves, while the AChRs are labeled with red alpha bungarotoxin. The two distinct colors in the nerve (green) and the muscle membrane (red) show the remarkably precise alignment of the nerve’s release sites and the muscle’s AChRs. The fluorescent protein expression also allows visualization and identification of all the muscle fibers innervated by a single axon. The motor axon and all the muscle fibers it innervates are called a motor unit, and this is a critical aspect of neuromuscular function. Because an action potential impulse heading out an axon in a peripheral nerve will enter all the branches of the axon, the motor unit is the unitary muscle contraction from a single axon.

Motor units are recruited in a fixed order when muscles are used. Typically, the weakest motor units that cause the smallest muscle twitches are recruited first. If these are insufficient for the task, additional motor units are recruited so that each gives rise to progressively larger amounts of muscle tension. In this way, there is fine control of small muscle contractions and less control as the force of muscle contraction is increased. All the muscle fibers within a single motor unit have very similar contraction properties because they have the same subtype of the contraction protein myosin.

The first motor units recruited comprise muscle fibers having “slow” fatigue-resistant myosin that causes slow contractions. The last motor units to be recruited activate muscle fibers that have fast contractions, thanks to fast myosin, but are highly fatigable. It is possible to see positions of all muscle fibers in each of the motor units in one muscle. Such descriptions reveal “connectomes,” which are complete maps of all the positions of all the motor axons and their connections within a muscle.

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