E. Relaxation

E. Relaxation: When neural stimulation ends, all the membranes repolarize, allowing the sar coplasmic reticulum to sequester Ca from the sarcoplasm by active transport. This removes Ca" from the TnC and returns the TnI to a position in which it inhibits binding of the myosin head to the actin filament. Muscle phsiology

F. Energy Production: Muscles use glucose (from stored glycogen and from the blood) and fatty acids (from the blood) to form the ATP and phosphocreatine that provide chemical energy for contraction. When ATP is not available, actin-myosin binding become stabilized, accounting for rigor mortis, the muscular rigidity that occurs shortly after death.

G. Organization of the Skeletal Muscles: Named muscles leg, biceps brachii) are bundles of muscle fascicles surrounded by a sheath of dense connective tissue termed the epimysium. Each fascicle is a bundle of muscle fibers surrounded by a dense connective tissue sheath called the perimysium, which consists of septumlike inward extensions of epimysium. Each muscle fiber is a bundle of myofibrils surrounded by a delicate connective tissue sheath termed the endo mysium, which consists of a basal lamina and a loose mesh of reticular fibers. Each myofibril is a bundle of myofilaments surrounded by an investment of sarcoplasmic reticulum, with a triad at both A-I junctions of each sarcomere. The connective tissue investments are continuous with one another.

H. Muscle-Tendon Junctions: The attachment of muscle to tendon must be secure to prevent the muscle from tearing away during contraction. The tendon's collagen fibers blend with the epi mysium and penetrate the muscle along with the perimysium. Near the junction with the tendon, the ends of the muscle cells taper and exhibit many infoldings of their sarcolemmas. Collagen and reticular fibers enter the infoldings, penetrate the basal lamina, and attach directly to the outer surface of the sarcolemma. The attachment of actin filaments to the inner surface of the sar colemma helps stabilize the association between the collagen fibers and the muscle cell.

I. Pattern of Innervation: Each motor neuron has a single axon that may terminate on a single muscle fiber or undergo terminal branching (arborization) and terminate on multiple muscle fibers. A motor neuron and all the muscle fibers it innervates (one to > 100) is termed a motor unit. Muscles responsible for delicate movements leg, extraocular muscles) are composed of many small motor units; those responsible for coarser movements leg, gluteus maximus) are composed of a few large motor units.