Neuromuscular Junctions of Smooth Muscle Physiologic Anatomy of Smooth Muscle
Neuromuscular Junctions.
Neuromuscular junctions of the highly structured type found on skeletal
muscle fibers do not occur in smooth muscle. Instead, the autonomic nerve fibers that innervate
smooth mus le generally branch diffusely on top of a sheet of muscle fibers, as shown in Figure 8-4.
Outside Contraction Relaxation Figure 8-3 Intracellular calcium ion (Ca++) concentration
increases when Ca++ enters the cell through calcium channels in the cellmembrane or the
sarcoplasmic reticulum (SR). The Ca++ binds to
calmodulin to form a Ca++-calmodulin complex, which then activates myosin light chain
kinase (MLCK). The MLCK phosphorylates the myosin light chain (MLC) leading to
contraction of the smooth muscle. When Ca++ concentration decreases, due to pumping of
Ca++ out of the cell, the process is reversed and myosin phosphatase removes the
phosphate from MLC, leading to relaxation. Varicosities Gap junctions Visceral Multi-unit
Figure 8-4 Innervation of smooth muscle. Chapter 8 Excitation and Contraction of mooth
Muscle 95.
