Innervation of muscles and joints

10 Innervation of muscles and joints




In gross anatomy, the nerves to skeletal muscles are branches of mixed peripheral nerves. The branches enter the muscles about one-third of the way along their length, at motor points (Figure 10.1). Motor points have been identified for all major muscle groups for the purpose of functional electrical stimulation by physical therapists, in order to increase muscle power.



Only 60% of the axons in the nerve to a given muscle are motor to the muscle fibers that make up the bulk of the muscle. The rest are sensory in nature, although the largest sensory receptors—the neuromuscular spindles—have a motor supply of their own.



Motor Innervation of Skeletal Muscle


The nerve of supply branches within the muscle belly, forming a plexus from which groups of axons emerge to supply the muscle fibers (Box 10.1 and Figure 10.1). The axons supply single motor end plates placed about halfway along the muscle fibers (Figure 10.2A).




A motor unit comprises a motor neuron in the spinal cord or brainstem together with the squad of muscle fibers it innervates. In large muscles (e.g. the flexors of the hip or knee), each motor unit contains 1200 muscle fibers or more. In small muscles (e.g. the intrinsic muscles of the hand), each unit contains 12 muscle fibers or less. Small units contribute to the finely graded contractions used for delicate manipulations.


There are three different types of skeletal muscle fiber.






Motor end plates


At the myoneural junction, the axon divides into a handful of branchlets that groove the surface of the muscle fiber (Figure 10.2B). The underlying sarcolemma is thrown into junctional folds. The basement membrane of the muscle fiber traverses the synaptic cleft and lines the folds. The underlying sarcoplasm shows an accumulation of nuclei, mitochondria, and ribosomes known as a sole plate.


Each axonal branchlet forms an elongated terminal bouton containing thousands of synaptic vesicles loaded with acetylcholine (ACh). Synaptic transmission takes place at active zones facing the crests of the junctional folds (Figure 10.2C). Vesicular ACh is extruded at great speed by exocytosis into the synaptic cleft. The ACh diffuses through the basement membrane to bind with ACh receptors in the sarcolemma.


Activation of the receptors leads to depolarization of the sarcolemma. The depolarization is led into the interior of the muscle fiber by T tubules. The sarcoplasmic reticulum liberates Ca2+ ions that initiate contraction of the sarcomeres.


Acetylcholinesterase enzyme is concentrated in the basement membrane, and about 30% of released ACh is hydrolyzed without reaching the postsynaptic membrane. Following hydrolysis, the choline moiety is returned to the axoplasm.


Also in terminal boutons are some dense-cored vesicles containing one or more peptides (Figure 12.2C). Best known is calcitonin gene-related peptide, a potent vasodilator.


Details of the muscle fiber contraction process are in Box 10.2.



Jun 10, 2016 | Posted by in NEUROLOGY | Comments Off on Innervation of muscles and joints

Full access? Get Clinical Tree

Get Clinical Tree app for offline access