Download 2 Skeletal muscle contractions - delano

Chapter 6—Skeletal Muscle
Anatomy and Contractions
What does the microscopic anatomy of a skeletal muscle look like?
What happens during a contraction of a skeletal muscle?
What is an action potential?
What is a motor unit?

Microscopic Anatomy
 Sarcolema—plasma membrane of a muscle cell
 Myofibril—long ribbon-like organelles of muscle cells
 Light Bands (I) and Dark Bands (A)—give the muscle it’s striped
appearance
 Sarcomere—Tiny, contractile bands within the Myofibril
 Sarcoplasmic Reticulum—specialized smooth ER in muscle cells
*Stores and released Calcium when a fiber is stimulated to
contract
Myosin and Actin
Thick Filament—Myosin filament
Composed of the protein Myosin and ATPase enzymes
Thin Filament—Actin filament
Composed of Actin and other proteins
Cross bridges—link the thick and thin filaments during contraction
Skeletal Muscle Special Functions
Irritability—ability to receive & respond to stimuli
Contractility—ability to shorten when a stimulus is received
Nerve Stimulus—Skeletal muscles need to be stimulated by nerve
impulses
Where a muscle meets a nerve….
Motor Unit—one neuron (nerve cell) and all the skeletal muscle
cells it stimulates
**could innervate a few cells, or hundreds
Axon—Long fiber-like, thread-like extension of the neuron
Axonal Terminal—forms junction with sarcolema of different
muscles
Neuromuscular Junction—nerve / muscle junction, but the muscle
does not touch the nerve
Synaptic Cleft—space at Neuromuscular junction filled with
interstitial fluid
What is an ACTION POTENTIAL??
Impulse triggers a release of neurotransmitter chemical: Acetylcholine (Ach)
ACh diffuses across the synaptic cleft (space)
ACh attaches to receptors
If enough ACh is released, Na+ ions rush into the cell
Excess positive ions changes the conditions of the sarcolema
Action Potential is triggered (electrical current that travels down the sarcolema)
Action Potential is unstoppable, so it affects the whole sarcolema (end to end)
A muscle contraction results
 An Action Potential is comparative example to:
Lighting a fuse on a stick of dynamite
 What two things slow a contraction to a stop?
1.) Diffusion of Potassium ions (K+) out of the cell
2.) Activation of the Na / K active transport pump
that moves ions back to normal positions
What causes muscle proteins to slide?
1.) Cross bridges attach to myosin binding sites on the thin filaments
2.) Calcium Ions are released into the Sarcoplasm (from the SR)
3.) Sliding begins
4.) Cross bridges act like oars to attach and detach several times during the
contraction
5.) The thin filaments are pulled toward the center of the sarcomere
6.) Simultaneously, this occurs in all the sarcomeres of the cell
7.) The muscle shortens (Contraction)
8.) Calcium ions are reabsorbed into the storage areas inside the cell
9.) Muscle cell relaxes
Actin, Myosin and Cross Bridges
ATP is needed!!!
ATP is needed for energy by the cell when contracting
*There are 3 pathways for muscles to regenerate ATP
1.) Creatine Phosphate and ADP
2.) Anaerobic Glycolysis
3.) Aerobic Respiration
Creatine Phospate +ADP
Phosphorylation of ADP by Creatine Phosphate:
CP + ADP  Creatine + ATP
CP is found in muscle fibers
CP supply is exhausted in about 20 seconds
Anaerobic Glycolysis (No Oxygen!)
Lactic Acid Formation (O2 is depleted & Musc. activity is intense)
1.) Glycolysis  Pyruvic Acid + ATP
2.) Pyruvic Acid is then converted to Lactic Acid
3.) Lactic Acid builds up and causes fatigue and soreness
4.) 2 ATP molecules are produced per each glucose (only 5%
efficiency compared to Aer. Resp)
(lasts for about 30 seconds of activity)
Aerobic Respiration
**use Oxygen to regenerate ATP
Glucose is broken down into Water, Carbon Dioxide and ATP
36 ATP molecules are produced per each glucose
*This is a slow process depending on Oxygen and Nutrients
present*
What is fatigue?
Muscle Fatigue and Oxygen Debt
Muscle Fatigue inability to contract, even with stimulation
Oxygen Debt O2 is not taken in fast enough to keep up with demands
Anaerobic Glycolysis & Lactic Acid formation is initiated
Muscles contract weaker and weaker, and their contractions
become less and less effective
**Oxygen Debt must be paid back—Cause of rapid, deep breathes
Deep Breathes Continue until Oxygen rids the muscles of Lactic Acid
and ATP/CP reserves are replenished
http://www.youtube.com/watch?v=SCasruJT-DU
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