48. Question - Diagram a skeletal muscle myofiber as seen with the light microscope - not electron microscope. Diagram the organization of myofibers into skeletal muscles.
Skeletal muscle cells (myofibers) are long multinucleate "cylinders" that are bound together to form fascicles primarily by the endomysium which contains the nutrient capillaries. The perimysium that surrounds the individual fascicles is loose and allows the fascicles to move relative to each other and contains distributing blood vessels and nerve fascicles. Epimysium is of variable thickness and surrounds the entire muscle.
49. Question - Diagram a skeletal muscle myofiber as seen with the electron microscope.
Myofibrils are bundles of thick (myosin) and thin (actin) myofilaments. Each myofibril is surrounded by sarcoplasmic reticulum and, at the A-I junction, T-tubules, which are continuous with the sarcolemma. Be able to identify on your drawing triad, sarcomere, I-band, A-band, H-band, Z-line, M-line, actin filaments and myosin filaments.
50. Question - Diagram a cardiac myocyte. How is its fine structure different from that of skeletal muscle?
Compared with skeletal muscle cardiac muscle T-tubules are larger and are located at the Z line instead of the A-I junction. Myofibrils are larger and more irregular in cross section. Mitochondria are more numerous and have more cristae. It is said to be mononucleate although as many as half may be binucleate. Gap junctions connect cardiac and smooth muscle cells electrically.
51. Question - Diagram a smooth muscle fiber including its innervation and control.
Nerve fibers of the autonomic nervous system have bulbous expansions and endings near smooth muscle cells but specialized junctions are not seen. Synaptic vesicles are found at these sites and they release neurotransmitter in the region to influence smooth muscle activity. Contraction impulses sweep over the smooth muscle because the cells communicate with each other by way of nexus (gap) junctions.
52. Question - Diagram a skeletal muscle motor unit and a neuromuscular junction (motor end plate or myoneural junction). What is the relationship between a motor unit and a muscle fascicle?
A motor unit consists of a nerve cell (alpha motor neuron) plus all the muscle fibers (extrafusal fibers) it innervates (10 to >100 fibers). There are two units represented in this diagram. The muscle fibers of a motor unit may be found in several different fascicles and the fibers in a fascicle may belong to several different motor units. Muscle fibers belonging to the same unit are of the same type - red, white or intermediate. The motor end plate drawing is below the motor units.