ARRANGEMENT
1. What is the difference between the origin and the insertion of a skeletal muscle?
2. What is the difference between a flexor and an extensor skeletal muscle?
3. What is the difference between an adductor and an abductor skeletal muscle?
The function of muscles is to contract or shorten.
In doing this, they move a body part or body contents, or they provide resistance to some movement. A primary consideration in determining what muscles accomplish is their cell arrangement. Accordingly, the muscle cells might be arranged in sheets, sheets rolled into tubes, bundles, rings (sphincters), or cones, or they might remain as discrete cells or clusters for more precise or less forceful action. The emptying of visceral structures (e.g., urinary bladder, stomach, heart) or the conveyance of intestinal contents or organ secretions, as provided by smooth and cardiac muscle, is accomplished because of their intimate association with the affected part. Apart from the skeletal muscle sphincters, the effects of skeletal muscle may be noted at a point some distance from their location. This means that their contraction must be transmitted somehow to the affected part. For this to happen, one end of the muscle must be relatively fixed or anchored and the other end must be attached directly or by a tendon to the movable part. Accordingly, the anatomic description of a skeletal muscle sometimes refers to its origin and insertion, the origin being the least movable end and the insertion the most movable end. Contraction of skeletal muscle brings the origin and insertion closer together and, when attachments involve two bones, one or both of the bones will move.Skeletal muscles are often described according to the type of movement performed. They are flexors if they are located on the side of the limb toward which the joint bends when decreasing the joint angle. They are extensors if they are located on the side of the limb toward which the joint bends when increasing the joint angle. Adductors are muscles that pull a limb toward the median plane and abductors pull a limb away from the median plane. Sphincters are arranged circularly to constrict body openings. Muscles are strategically located to best serve the structure they affect. Lack of adduction occasionally occurs in the hindlimbs of cows after parturition or calving. The adductor muscles are supplied by the obturator nerves (one to each leg), each of which passes through an opening (obturator foramen) in the birth canal (see Chapter 7). Its injury during the calving process can be followed by the inability to adduct one or both of the hind legs (obturator paralysis).
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