Ch 12 Muscle Three types of muscle Skeletal

Ch. 12 Muscle • Three types of muscle • Skeletal muscle – Anatomy – Arrangement of muscle fibers – Filaments in muscle fibers – Proteins in muscle filaments – Other parts of muscle fibers

Three types of muscle • Skeletal muscle • Cardiac muscle • Smooth muscle

Muscles generate movement and force. Sometimes muscles generate force without movement. The muscle is a collection of muscle cells. Each muscle cell is called a muscle fiber. The muscle contains muscle fibers, blood vessels, nerve fibers, connective tissue. The muscle cell (fiber) is multinucleated.

Motor unit • A single motor neuron and all the muscle fibers that it innervates.

Muscle fiber anatomy: lots of myofibrils

Myofibrils are contractile elements. Thick filaments: myosin Thin filaments: actin

Sarcomere Thick filament (myosin) Thin filament (actin) M line

M line

(F-actin polymers)

Other components of the muscle fiber

Nebulin, not elastic, helps to align the actin filament Titin, a huge elastic molecule stabilizes the position of the contractile proteins And returns sarcomere to its resting length after stretching.

Skeletal muscle contraction • • Contraction, force and tension Sliding Filament Theory of Contraction cycle Regulation of the contraction cycle

Muscle contraction • Movement or resist a load (force) • Load is the weight or force that opposes the contraction of a muscle • Tension is the force created by a muscle • Need ATP to generate tension

Observations during muscle contraction Muscle shortens when it moves a load. (When muscle contracts, it does not always shorten)

Observations during muscle contraction: A band does not shorten during contraction.

Sliding filament theory of contraction: movement and force Resting length How about force without movement?

During contraction • • • Z discs move closer together Sarcomere shortens A band same length I band reduced H band reduced

What pushes the actin filaments into the myosin? • Cross-bridges link myosin to actin • Power stroke: myosin head binds to actin myosin head release actin. Repeated many times. • Myosin molecules are flexible • ATP causes movement of myosin molecules

Myosin • A motor protein • Converts chemical bond energy of ATP to mechanical energy of motion • Each myosin as ATPase • Energy from ATP hydrolysis is stored as potential energy in the myosin molecule, and is used to create the power stroke.

Why don’t actin and myosin continuously bind together? • ATP is usually available • Actin’s binding site for myosin is revealed only during cross-bridge (binding). • During relaxation, actin’s binding site for myosin is concealed