Spinal samples of tibialis anterior (TA), paraspinal (PS), intercostal

Spinal muscular
atrophy (SMA) is a motor neuron disease that typically results in death and is characterized
by muscle weakness. The cause of SMA is not completely understood, but studies
have been done to try and determine it. Firstly, experimenters obtained samples
of tibialis anterior (TA), paraspinal (PS), intercostal (IC), and diaphragm
muscles from mice that had SMA and wildtype mice. After observing the
neuromuscular junction of these muscles at different stages of the mouse’s life,
experimenters found that though SMA caused muscle weakness, muscles did not
experience denervation nor any of the characteristics commonly associated with
denervation. There was, however, an increase in the denervation of
neuromuscular junctions in the PS and TA muscles, but this was not identified
as the cause for SMA. When end plate currents were observed, then, it was found
that the endplate current amplitude was smaller in mice with SMA but miniature
endplate currents were no different in either group of mice, suggesting that
the number of synaptic vesicles was decreased in SMA mice. Experimenters subsequently
concluded that, “reduced probability of release contributed to the reduction of
NMJ current amplitude in SMA,” (Kong 845). In later stages of the experiment,
researchers used an antibody to determine that neurofilaments were accumulating
in presynaptic terminals of neuromuscular junctions, but this was only a characteristic
in proximal muscles. Lastly, as is known, acetylcholine receptors in adults
typically contain an epsilon subunit as opposed to a gamma subunit in embryos. When
SMA mice muscles were observed, experimenters found a gamma subunit present in
the muscles even in the later stages of the mice’s lives. This indicates that
because the gamma subunit does not develop into an epsilon subunit, the neuromuscular
postsynaptic terminals are not in the proper state to fully develop and instead
are much smaller, leading to a decreased acetylcholine action amplitude.