General Information on Muscles
Muscles are often classified through their microscopic morphology. They are divided into skeletal, cardiac, and smooth muscle. Skeletal muscle is heavily striated and multi-nucleated. These muscles are generally associated with bones and cartilages. The skeletal muscles are controlled, voluntary muscles. Cardiac muscles are also striated, but are not multi-nucleated. These muscles can be identified by the branching on the cells and the intercalating disks that connect the cells. Cardiac muscle is found in the heart only and is an involuntary muscle. The cardiac muscle undergoes waves of contractions that create an electric impulse across the intercalated disks. The smooth muscle has no striation and is mainly associated with viscera. The smooth muscle is mono-nucleated, short and fusi-form in shape.
During embryonic development, muscles develop form three sources, the mesenchyme, the paired hypomere and the paraxial mesoderm. The mesenchyme cells give rise to the smooth muscles that make up the walls of blood vessels and some organs. Cells of the hypomere form the smooth muscle of the alimentary tract, as well as, the cardiac muscle of the heart. Paraxial mesoderm gives rise to the skeletal muscle.
In mammals, the muscle groups that make up the musculature are divided into groups based on their origin. These groups include: axial, dorsal, ventral, branchiomeric, and hypobranchial. The axial muscles are greatly reduced in mammals because they are no longer used for locomotion as in fishes. These muscles do make up the muscles of the back, which control the flexion of the vertebral column and the movement of the rib cage. The axial muscles also contribute to the shoulder and forelimb muscles by forming a sling that supports the body between the shoulder blades with the levator scapulae, rhomboideus complex and serratus muscles. The dorsal muscles contribute to the forelimb, hindlimb and abdominal muscles, as well as, the pectoral and pelvic girdles. Branchiomeric muscles, which arise from cranial paraxial mesoderm, give aid to the pectoral girdle through the trapezius. In mammals, the branchiomeric muscles make up many muscles of the face and throat. These muscles are innervated by cranial nerves. The hypobranchial muscles arise from the trunk paraxial mesoderm and are innervated by vertebral nerves. These muscles give rise to cervical muscles and some muscles of the throat.
Kardong, Kenneth. Vertebrates: Comparative Anatomy, Function, Evolution. NY: McGraw-Hill, 2002.
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