Microscopic Structure Skeletal Muscle Cells: muscle fibers (what are bone cells called?) Highly organized and grouped together Each fiber contains protein filaments which are threadlike structures which are threadlike structures Two types of proteins: a) actin (thin myofilaments) a) actin (thin myofilaments) b) myosin (thick myofilaments) b) myosin (thick myofilaments)

Microscopic (cont) Sarcomere (basic functional unit) (basic building block) (basic building block) (? What is the basic unit called in the skeletal system) (? What is the basic unit called in the skeletal system) Sarcomere is the contractile unit of skeletal muscles Actin and myosin overlap each other Sarcomeres are distinct from each other at Z lines Refer to the diagram on page 136 and draw a replica in your notes (use diagram A)

Motor Unit Combination of specialized nerve cell known as a motor neuron and a muscle fiber Neuromuscular junction; space between muscle and motor neuron Chemicals known as neurotransmitters assist the impulse to transfer to the muscle

Muscle Fiber Contraction (Sliding Filament Model) Nervous system stimulates a muscle fiber Calcium is released into the fiber’s cytoplasm ATP must also be supplied Actin and myosin myofilaments form bridges that act as levers to pull the filaments past each other This causes the fiber to shorten (muscle contraction) (muscle contraction) Refer to page 136 and draw a diagram of a relaxed and maximally contracted sarcomere

Questions Critical Thinking Page 158 #16 Page 158 #16 Chapter Test #5,6,7 and 95

Muscle Stimulus All or None Rule A muscle will contract only when an applied stimulus reaches a certain level known as the threshold. A threshold stimulus is the minimal level of stimulation required to cause a muscle fiber to contract.

Muscle Stimulation (cont) All or None REsponse Once a muscle fiber is stimulated to the threshold level, it will completely contract This is called the All or None Response

Muscle Stimulation (cont) Graded Response Not all fibers within a muscle have the same threshold level Some fibers may require more or less of a stimulus in order for the fiber tho contract This allows for a “graded response” by the entire muscle This way we are able to control such things as heavy vs.light lifting or walking vs. running

Fatigue If muscle cells are repeatedly stimulated; without rest…the strentgth of the contraction will decrease If continued; the muscle may lose its ability to contract

Oxygen Debt Paying back decreased energy reserves through labored breathing, after the activity has ceased Homeostasis is maintained Gets rid of lactic acid that built up from exercising with less oxygen Lactic acid can cause muscle soreness

Skeletal muscle Contractions Flexion/extensionAbduction/adductionRotation Supination /pronation Dorsiflexion/plantar flexion Show your partner each of these movements Show your partner each of these movements