The Musculo-Skeletal System

Animal SkeletonsFunctions: •Support •Protection •Movement (T11.2.U1)

all movement results from: muscle working against a skeleton

3 Types of skeletons •hydrostatic (e.g. worm, cnidarians) •exoskeleton (e.g. arthropods) •endoskeleton

Thigh bone connected to the...• The mammalian skeleton is built from more than 200 bones

• Some fused together and others connected at joints by ligaments that allow freedom of movement

• are bones alive?• osteoblasts, osteocytes

Types of synovial joints in the appendicular skeletonT11.2.U2

Vertebrate Skeletal Muscle

Contraction

Muscles move the skeleton• What is always the action of muscle cells? • Skeletal muscles are attached to the skeleton in antagonistic

pairs (T11.2.U3) • Flexors vs Extensors • If one contracts, the other relaxes

Vertebrate Skeletal Muscle

Vertebrate Skeletal Muscle

• Muscle fibers contain many myofibrils (T11.2.U5) • each myofibril = many sarcomeres

• contractile subunit (T11.2.U6)

Vertebrate Skeletal Muscle

Sliding filament theory

• Mechanism of contraction? • thick and thin filaments... • slide past one another.

• Mechanism of sliding filaments? • Interaction between...

• actin and myosin: • The “head” of a myosin molecule binds to an actin

filament • Forming a cross-bridge and pulling the thin filament

toward the center of the sarcomere

T11.2.U7-9

Sliding filament theory

•ATP binds to myosin head •Head releases from actin site

Sliding filament theory

•Energized by phosphoryllation of ATP, •Myosin head now able to bind to actin site

Sliding filament theory

Sliding filament theory

•Binding to actin site releases ADP + Phosphate •Myosin head bends into low E config

Sliding filament theory

ActinTropomyosin Ca2+-binding sites

Troponin complex

(a) Myosin-binding sites blocked

• If ATP available, why doesn’t muscle just keep contracting? • Regulation

• Contraction stimulated by: • a motor neuron

• At rest, myosin-binding sites blocked by: • tropomyosin (regulatory protein)

The Role of Calcium and Regulatory Proteins

The Role of Calcium and Regulatory Proteins

Ca2+

Myosin-binding site

(b) Myosin-binding sites exposed

• What do calcium ions (Ca2+) do? • Bind to the troponin complex, • which uncovers myosin-binding sites

• Where does Ca2+ come from? How do Ca2+ ions get released? • Action potential (AP) in a motor neuron that synapses w/

the muscle fiber… • releases acetylcholine (n.t.)… • depolarizes the muscle and causing it to produce an AP • causes the sarcoplasmic reticulum (SR) to release Ca2+

• “SR?” • = modified ER

The Role of Calcium and Regulatory Proteins

Types of Muscle FibersFast TwitchSlow Twitch