Movement
How does an organism support itself, protect itself, and move?
Three kinds of skeletons
- Hydrostatic skeleton
- exoskeleton
- endoskeleton
o all serve one or more of these functions
§ support
§ protection
§ locomotion
Hydrostatic skeleton
- fluid under pressure in a closed compartment
- works well in aquatic environment or lying on ground, but cannot support animal above ground
- fluid doesn’t compress very much, so squeezing one side makes another side change shape
o worms of various kinds
o bands of muscles squeeze and change shape of animal
Exoskeleton
- hard skeleton on outside of body
o Mollusks
§ Chitin strengthened with minerals like calcium salts
o Insects
§ Chitin
- Big problem with exoskeletons is that animal needs to shed them to grow in size, during which time it is vulnerable.
Endoskeleton
- Vertebrates, sponges, echinoderms (starfish and others)
- internal skeleton
o sponges and echinoderms have protein/mineral structures
§ plates or other shapes (spicules)
o cartilage
§ cells in gelatinous semisolid matrix
o bone
§ cells in a solid matrix mainly calcium phosphate and protein collagen
Movement
Is produced by muscles attaching to skeleton
How does an organism move?
- must attach muscle to two things, so can pull
- attached to bone or exoskeleton
o tendons attach muscles to bones
- at a joint, two bones come together.
Muscle types
in all muscle cells, two proteins, actin and myosin, are organized into filaments
o they slide past each other, and this is the basis of muscle movement
- skeletal muscle
o voluntary movements under voluntary controls
o cells fuse together to form a fiber
o a muscle is a bunch of muscle fibers arranged in bundles wrapped by connective tissue (Fig. 46.3a)
o striated – because can see lines in fiber
- cardiac muscle
o in heart
o cardiac muscle cells branch – this is unique to these cells
o connections between cells are intercalated discs
§ tight junctions
o connected with gap junctions
o some are specialized to be pacemaker cells – initiate own depolarizations and cause heartbeat (in SA node)
- smooth muscle
o contractile force for most internal organs – digestive, emptying of bladder, lining of arteries
o not under voluntary control
o arranged in sheets with gap junctions between
Contraction in skeletal muscle
- muscle fibers are arranged into sarcomeres – these are the product of an arrangement of actin and myosin (Fig. 46.3b, 46.5)
- myosin in big bunch in middle
- actin filaments surround – 6 around each myosin bunch
- actin filaments anchored to other proteins which make up the end of the sarcomere
- one sarcomere is between two of these (two Z lines in Fig46.3b)
- Huxley proposed sliding filament model for muscle contraction
o actin and myosin slide past each other as the muscle contracts
o causes each sarcomere to shorten (Fig. 46.4)
o therefore, whole muscle shortens
How do actin and myosin slide? (Fig. 46.7)
- actin and myosin interact
How is this initiated? (Fig. 46.10, 46.9)
- a single neuron can make neuromuscular junctions
- when neurotransmitter released, opens Na+ channels and causes depolarization
- spreads along fiber membrane and into the fiber by T tubules (a specialized invagination of plasma membrane)
- this depolarization causes sarcoplasmic reticulum to release Ca2+
- so Ca2+ concentration in sarcoplasm increases
- this binds to the protein troponin (Fig.46.8)
- troponin and tropomyosin are proteins which bind to actin, and in absence of Ca2+, thy cover binding sites of myosin
- troponin changes shape, leading associated tropomyosin to change position
- this uncovers binding sites for myosin on actin, and away we go with contraction