Muscle Physiology Chapter 1

Objectives Describe the micro and macrostructure of muscle Describe the sliding filament theory Differentiate types of muscle actions Discuss factors that affect force production

Huxley?

KEY POINT The muscular system is the organ system responsible for movement. Muscles develop tension, causing bones to act as levers and rotate around an axis.

Neuromuscular Components Neuromuscular junction- the point where a motor neuron joins muscle fibers, the motor end plate Motor unit- a motor neuron and all the muscle fibers it innervates All or none law- all the muscle fibers in a motor unit contract, or none contract

Structure of a Muscle Cell Sarcoplasm- the cytoplasm of a muscle fiber, contains mitochondria, sarcoplasmic reticulum Sarcomere- smallest contractile unit of muscle tissue Sarcoplasmic Reticulum – contains calcium.

Structure of a Muscle Cell Myofibrils- long fibers in the sarcoplasm of muscle cells Actin (thin) Myosin (thick) Crossbridges- globular heads on the myosin molecules

Structure of Skeletal Muscle Contractile proteins- actin and myosin Myofibril- muscle fiber, one muscle cell Regulatory proteins- troponin and tropomyosin Sarcomere- Z-line to Z-line Sarcoplasmic reticulum/T-tubules- “communications” system in the muscle cell

Sliding Filament Theory of Muscle Contraction 1. Resting phase- calcium is segregated in the sarcoplasmic reticulum (SR), few crossbridges are attached, the muscle cell is “at rest”

Sliding Filament Theory of Muscle Contraction 2. Excitation phase- contraction coupling phase- calcium ions are released from the SR, troponin and tropomyosin undergo a conformational change, allowing actin to bind to myosin, causing flexion of the myosin head

Sliding Filament Theory of Muscle Contraction 3. Contraction phase- ATP causes myosin heads to detach from actin, ATP is hydrolized which “cocks” the myosin head, the myosin head attaches to the next active site

Sliding Filament Theory of Muscle Contraction 4. Recharge phase- as long as ATP is present to break the attraction of actin to myosin, and calcium is present, the cycle continues causing tension to develop in the muscle

Sliding Filament Theory of Muscle Contraction 5. Relaxation phase- stimulation from the motor nerve stops, calcium is re-segregated into the sarcoplasmic reticulum, troponin and tropomyosin return to their original shape and block the attraction of actin and myosin, the muscle returns to the resting unbound state

Cycle Time

Sliding Filament Animation http://www.sci.sdsu.edu/movies/actin_myosin_gif.html

Summary of Muscle Contraction Acetylcholine (ACH) released from terminal portion of motor neuron ACH crosses the gap at the neuromuscular junction Muscle fiber membrane depolarizes through transverse tubules to the sarcoplasmic reticulum

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