BIOL 2457 1st Edition Lecture 23Outline of Last Lecture Muscle tissues Outline of Current LectureMuscular system Muscle Attachment Sites: Origin & Insertion Skeletal muscles cause movements by exerting force on tendons, which pulls on bones or other structures. Articulating bones usually do not move equally in response to contraction. The attachment of a tendon to the stationary bone is called the origin. The attachment of the muscle’s other tendon to the movable bone is called the insertion. The action(s) of a muscle are the main movements that occur during contraction (e.g., flexion or extension). Lever Systems A lever is a rigid structure that can move around a fixed point called a fulcrum. A lever is acted on at two different points by two different forces: The effort, which causes movement The load or resistance, which opposes movement. The effort is the force due to muscular contraction; the load is the weight that is moved or some resistance to an object to being moved (e.g., weight of a book to be overcome before you can pick it up). Motion occurs when the effort applied to the bone at the insertion exceeds the load. Types of levers There are 3 types of levers that differ on the positions of the fulcrum, effort, and load. First-class levers are not common: the fulcrum is between the effort and the load. Second-class levers are uncommon: the load is between the fulcrum and the effort. Third-class levers are common: the effort is between the fulcrum and the load. Effects of Muscle Fascicle Arrangement All muscle fibers are parallel to one another within a single fascicle. Fascicles, however, form patterns with respect to the tendons. Parallel Fusiform Circular Triangular Pennate Effects of Muscle Fascicle Arrangement Muscle fascicles have a compromise that they must make. They must compromise between power and range of motion. The longer the fibers in a muscle, the greater the range of motion it can produce. The power of a muscle depends not on length but on its total cross-sectional area. Coordination Among Muscles Muscles do not work in isolation; movements are often the result of several muscles acting as a group Most muscles are arranged in opposing (antagonistic) pairs at joints Prime mover (agonist) – contracts to cause an action Antagonist – opposes the action of the prime mover Synergists – prevent unwanted movement or otherwise aid the movement Fixators – stabilize the origin of the prime mover Depending upon the movement required, many muscles may act as prime movers, antagonists, synergists, or fixators Muscles of Facial Expression Muscles of facial expression Lie within the subcutaneous layer Usually originate in the fascia or skull bones & insert into the skin. Because of their insertions, the muscles of facial expression move the skin rather than a joint when they contract. Extrinsic Eye Muscles Six extrinsic eye muscles control movements of each eyeball. They are calledextrinsic because they originate on the outside of the eyeballs in the bony orbit and insert on the outer surface of the sclera. Those muscles with the word “rectus” in their name have obvious actions (the inferior rectus muscle moves the eye inferiorly so that you would be looking downward). The actions of the two oblique muscles cannot be deduced from their names. To understand how they move the eye, you must know the origin, insertion, and the unusual ‘path’ that each follows (see
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