Chapter 49 Sensory and Motor Mechanisms Animal Skeletons

• Chapter 49 –Sensory and Motor Mechanisms

Animal Skeletons • Function in support, protection, and movement • Hydrostatic – fluid held under pressure in a closed body compartment (cnidarians, flatworms, nematodes, and annelids) • Exoskeleton – hard encasement deposited on the surface of an animal (molluscs = mantle, calcium carbonate; arthropods = cuticle, chitin) • Endoskeleton – hard supportive elements embedded within the soft tissue

Human Skeleton • 2 main parts: – Axial – skull and vertebral column – Appendicular – limbs, pelvic and pectoral girdles

Human Joints • Ball and socket – where the humerus contacts the shoulder girdle and the femur contacts the pelvic girdle, enable arms and legs to rotate and legs in several planes • Hinge joints – between the humerus and head of the ulna, restrict movement in a single plane • Pivot joints – allows forearm rotation at the elbow and head movement from side to side

Vertebrate Skeletal Muscle • Attached to bone, responsible for movement • Bundles of long fibers running parallel to the length of the muscle • Each fiber is a single cell with multiple nuclei • Myofibrils – smaller fibers that make up a muscle fiber • Myofilaments – make up myofibrils, two types – Thin filaments – actin – Thick filaments - myosin

Vertebrate Skeletal Muscle (cont. ) • Striated – because of regular arrangement of filaments causing light and dark bands • Sarcomere – contractile unit of a muscle • Z line – sarcomere border • M line – center or middle of the sarcomere • I band – only actin, a region of no overlap • A band – actin and myosin protein fibers overlap • H zone – only myosin, center of sarcomere, no overlap

Sliding-Filament Model • Sarcomere length reduced • Myosin = purple, actin = orange • (a) relaxed muscle fiber – the I band H zone are relatively wide • (b) contracting muscle fiber – actin and myosin slide past each other, reducing I band H zone width • (c) fully contracted muscle fiber – H zone and I band are eliminated

Actin/Myosin Interaction • Myosin head is bound to ATP and is in low energy conformation • Myosin head hydrolyzes ATP to ADP and is in high energy conformation • Myosin head binds to actin forming a cross-bridge • Myosin returns to low energy conformation by releasing ADP, slides actin • Binding of new ATP releases myosin from actin and begins a new cycle • Extra energy for contractions stored as creatine phosphate (transfers P to ADP) and glycogen (broken down to make ATP)

Muscle Contraction Regulation • Relaxation: tropomyosin blocks myosin binding sites on actin • Contraction: calcium binds to troponin complex; tropomyosin changes shape, exposing myosin binding sites

Muscle Contraction Regulation (cont. ) • T (transverse) tubules – travel channels in plasma membrane for action potential • Sarcoplasmic reticulum – a specialized endoplasmic reticulum, stores calcium • Stimulated by action potential in a motor neuron, opens calcium channels allowing calcium to enter the cytosol • Ca 2+ then binds to troponin complex triggering a muscle contraction

Review of muscle contraction • ACh released, triggers action potential in muscle fiber • Propagated along membrane and down T tubules • Triggers Ca 2+ release from SR • Ca 2+ bind to troponin, change shape, exposing myosin-binding sites • Myosin cross-bridges attach to actin and detach pulling actin towards center of sarcomere • Ca 2+ is removed by active transport into SR • Tropomyosin blocks myosin binding sites, muscle fiber relaxes

Muscle Contraction Summary • Action potentials and muscle contraction animation • Part 2