Essay topic 5.
5. The neuromuscular junction as a site of disease process
Proteins involved in formation of the neuromuscular junction
The neuromuscular junction is a specialized junction, where a motor nerve forms its synaptic terminal with a muscle fibre, one of many fibres that make up a whole muscle. The mature neuromuscular junction is composed of three types of cells - a motor nerve terminal, a muscle fibre and a Schwann cell covering the junction. All three of these cells are highly differentiated and specialized for their functions (Kandel 2000, p. 1089).
The events that occur during the formation of the neuromuscular junction have been extensively studied and are the most comprehensively understood of any nerve-to-target cell contact.
Prior to formation of the synapse all three components of the neuromuscular junction develop and acquire identities independently. The muscle cells are derived from the mesoderm and migrate from the dermomyotomal portion of the somite.
Motoneurons migrate from the ventricular zone of the neural tube to a ventral-lateral location before axons grow out of the spinal cord. Schwann cells are glial cells, which insulate the axons outside of the spinal cord, they are derived from neural crest cells and associate with axons from the somite onwards to the peripheral target.
At the time of the first axons reaching the developing muscle, the muscle fibres are myoblasts that have just fused to form multinucleated myotubes, there is no evidence to suggest that motor neurons prefer certain site on the developing myotube or that there is a predetermined site for the formation of the synapse, on the contrary, synapse formation can occur on most, if not all of the myotube surface (Lichtman et al in Zigmond et al 2000, pp. 547-8). Acetylcholine receptors (AChR) are found uniformly dispersed over the surface...