The Musculoskeletal System
The Muscular System
Skeletal Muscle
• Voluntary movement (innervated by somatic system)
• Appears as striped or striated since the actin and myosin are arranged in repeating units
called sarcomeres.
• Multinucleated since individual muscle cells are fused into long rods
• Myoglobin: Oxygen carrier that uses iron in heme group to bind oxygen. Red colour
• Red Fibers or Slow-Twitch Fibers: high myoglobin and get energy aerobically
o Contain lots of mitochondria to carry out oxidative phosphorylation
• White Fibers or Fast-Twitch Fibers: less myoglobin, so there is a lighter colour.
• Both fibers mixed in a specific muscle, but if muscle is meant to contract slowly then it
contains more red fibers. Muscles that contract rapidly and fatigue quickly contain
mostly white fibers
Smooth Muscle
• Involuntary action and controlled by the autonomic nervous system.
• Found in respiratory tree, digestive tract, bladder, uterus, blood vessel walls, etc.
• Actin and Myosin fibers are not well organized so no striations are seen.
• Capable of more sustained contraction as compared to skeletal muscles.
• Tonus: is a constant state of low muscle contraction.
• Myogenic Activity: smooth muscle contracts without input from nervous system.
o Contract directly in response to stretch or stimuli.
Cardiac Muscle
• Both skeletal and smooth muscle characteristics. Has involuntary control, but are
striated
• Primarily have one nucleus
• Muscle cells are connected by intercalated discs that contain gap junctions. These are
connections between the cytoplasm of adjacent cells and allows for the flow of ions
directly between cells.
o Allow for more rapid and coordinated muscle cell depolarization and efficient
contraction
• Are able to define and maintain their own rhythm through myogenic activity.
o Sinoatrial Node is where depolarization starts, goes to Atrioventricular nodes.
Then spreads to bundle of His and then to the Purkinje Fibers.
o This happens automatically, but nervous and endocrine system can also alter
heart rate
• Vagus nerve provides parasympathetic innervation in order to slow down heart rate.
• NE from the sympathetic neuron or epinephrine from the adrenal medulla can bind to
the adrenergic receptors of the heart in order to increase its rate.
