Muscle

Introduction to Muscles Microanatomy

Muscles and Forces • Muscle cells convert chemical energy into force • Three major types – Skeletal – attached to bony skeleton – Smooth – surrounds hollow organs and tubes, stomach, intestinal tract, blood vessels – Cardiac muscle – pumping muscle of the heart

• Muscle such as biceps made up of muscle cells, bound together by connective tissue and attached to bone at either end by tendon

Three layers of connective tissue •

– epimysium – around entire muscle – perimysium- divides muscle into clumps of muscle fibers – endomysium- surrounds each muscle fiber

Skeletal Muscle Structure • Muscle cells are gigantic cells, 10 to 100 µm in diameter, up to 20 cm long • Adult muscle cell formed by fusion of undifferentiated mononucleate cells (myoblasts) to form a multinucleate cell. • Damaged muscle cells generally do not regenerate; some replacement by undifferentiated satellite cells associated with endomysium

Skeletal Muscle Structure • Muscle cells are activated by motor neurons. One motor neuron (in spinal cord) will innervate (connect to) many muscle fibers. • The collection of muscle fibers innervated by one motor neuron is called a motor unit – it is the contractile unit of a muscle.Typically there are many motor units in a single muscle.

Muscle Ultrastructure • In the light microscope, a series of light and dark bands can be observed in histological slides and in polarized light in living muscle – striations, hence striated muscle. Within a muscle cell there are many cylindrical - elements (1-2 µm in diameter) called myofibrils, that have striations. • Each striation is called a sarcomere

Muscle Ultrastructure • Each sarcomere is composed of two major proteins • Myosin, a long shaft with two cross bridges (‘lollipops’) at one end. Myosin molecules bundled together so that the cross bridges stick out at regular intervals around the bundle’s circumference (15 nm diameter). Individual myosin molecules connected together at the center of the sarcomere by a specialized protein at the M line; one bundle faces to the left, the other to the right.

Muscle Ultrastructure • The bundle of myosin molecules is called a ‘thick filament’. An array of thick filaments makes up the region of the sarcomere called the A band. • Actin, a major cytoskeletal protein, is composed of globular subunits that hook together like beads on a necklace to form a long filament. Two actin filaments wrap around each other to form a ‘thin filament’ (5 nm). • On the surface of the thin filament a long protein, tropomyosin, wraps around it, attached at regular intervals by the protein troponin.

Muscle Ultrastructure • The thin filaments are bound to a protein lattice called the Z disk at either end of the sarcomere; the thin filament region is called the I band. The thick filaments are also attached to the Z disk proteins by a protein called titin. • Thick and thin filaments are interdigitated with six thin filaments around a thick filament.