1. Chapter 49 –Sensory and Motor Mechanisms

2. 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

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

4. 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 for forearm rotation at the elbow and head movement from side to side

5. 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

6. 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

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

8. 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)

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

10. 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
Ca2+ then binds to troponin complex triggering a muscle contraction

11. Review of muscle contraction
ACh released, triggers action potential in muscle fiber
Propagated along membrane and down T tubules
Triggers Ca2+ release from SR
Ca2+ bind to troponin, change shape, exposing myosin-binding sites
Myosin cross-bridges attach to actin and detach pulling actin towards center of sarcomere
Ca2+ is removed by active transport into SR
Tropomyosin blocks myosin binding sites, muscle fiber relaxes

12. Muscle Contraction Summary
Action potentials and muscle contraction animation
Part 2