50 Locomotion 2009

50 Locomotion 2009 - Biol 61 Muscle Contraction (Chapter...

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Biol 61 – Muscle Contraction (Chapter 50) 5/4/09 For this chapter we are going to focus on the vertebrate endoskeleton and how muscles contract, pages 1105-1110, starting off with material found on pages 1112 and 1114. Types of Skeletal Systems 1. hydrostatic skeleton – this is essentially a water-filled cavity that provides structure to an organism and also acts as a non-compressible region for muscles to push against. 2. exoskeleton – this is an outside or external skeleton. It provides more protection than a hydrostatic skeleton and, in addition, this strong external covering is used as a site for muscle attachment, allowing the organism to be more mobile (see Fig. 50.32). One of the drawbacks of an exoskeleton is that they are very heavy, and this can limit the size of the organism that has one (except in water, where the water can help support the weight of the exoskeleton) and they can’t grow along with the organism (exoskeletons are secreted by the skin cells beneath and then harden up, so if an organism wants to grow it has to molt). 3. endoskeleton – this is an internal skeleton, found in vertebrates, for example. In vertebrates it acts as a site for muscle attachment, used for support of the body and as a protective structure (though it may not protect as much area as an exoskeleton). It is much lighter than an exoskeleton, so organisms with an endoskeleton can grow to much larger sizes. In addition, the vertebrate endoskeleton is made of living tissue and can grow along with the organism. The Vertebrate Skeleton The skull and the backbone, including the ribs, make up the axial skeleton . This skeleton provides an important protective function (think of your skull and what it protects!). The rest of your body contains an appendicular skeleton supporting the appendages (arms and legs, hips {pelvic girdle} and shoulders {pectoral girdle} See Fig. 50.34). The primary role of the appendicular skeleton is locomotion. Joints The site where bone movement occurs is at the joints , where one bone meets another. As we have discussed, muscles can only contract and relax (they cannot “push” to extend). Since muscles are attached to bones that lie across joints, this contraction results in the movement of the bones. The arrangement of the bones at joints restrict the types of movements that the skeleton can make. Some joints, like those at the shoulder and hip, have a ball-and- socket arrangement that allows them to have a greater degree of free rotation than hinge joints or pivot joints (you can spin your arm in a circle around your shoulder, for example). Muscles are attached to bone by tendons . Muscles can only contract . When a muscle contracts, one end (called the origin) will be stationary, while the other end (the insertion) is at the bone that moves upon contraction of that muscle. Muscle attachments are across joints – the origin and insertion of a muscle are not found on the same bone! In general, your muscles will be found in pairs: one for
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50 Locomotion 2009 - Biol 61 Muscle Contraction (Chapter...

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