Smooth muscle contraction closely resembles the cell motility exhib-
ited in other cell types. It also occurs through the action of actin and
myosin, which are arranged in a lattice-like pattern. Troponin is notpresent
in smooth muscle.
NERVOUS SYSTEMMyelin,which insulates neuronal projections and permits rapid (saltatory)
conduction, is produced by oligodendroglia (oligodendrocytes)in the
central nervous system (CNS) and by Schwann cellsin the peripheral
nervous system (PNS). Microgliaare brain macrophages. Astrocytes
have a complicated role in physical and metabolic support of neurons.
Astrocytes induce and maintain the blood-brain-barrier,but they do not
constitute the barrier functionof the blood-brain barrier which is estab-
lished by endothelial tight junctions (zonula occludens).Neurons con-
duct electrochemical impulses and move neurotransmitters to their
synaptic termini by axoplasmic transport.Transneuronal transmission is
accomplished by calcium-regulated release of synaptic vesicles.Neurons
also synapse with muscle cells. A typical contact between a myelinated
neuron and skeletal muscle (neuromuscular junction)is shown below.
High-Yield Facts 23Neuromuscular junction.Axonal terminals (telodendria) rest in shallow depressions
(primary clefts) on the surface of the striated muscle fiber. Secondary clefts increase
the surface area for interaction with a neurotransmitter (acetylcholine). Muscle cell
nuclei and mitochondria are abundant near the junction.