Muscle Function

1. Muscle Function • Muscle plays four important roles in the body: • Produces movement • Maintains posture • Stabilizes joints • Generates heat

2. Producing Movement • Movements of the human body are a result of muscle contraction • Mobility of the body reflects the activity of skeletal muscles • Skeletal muscle is responsible for all locomotion • Allows for response to stimuli and emotional expression

3. Maintaining Posture/Stabilizing Joints • Skeletal muscle has to function continuously in or to maintain posture. • Similar to involuntary action • Stabilizing joints • As skeletal muscle contract and pull on bones to cause movement, they also stabilizes joints • Tendons are important in stabilizing joints with poor articulating surfaces (shoulder)

4. Generating Heat • A by-product of muscle activity • About 75% of energy during muscle contraction escapes as heat • Skeletal muscle is most responsible for generating heat (40% body mass)

5. Skeletal Muscle Activity • Irritability • the ability to receive and respond to a stimulus • Contractility • The ability to shorten (forcibly) when an adequate stimulus is received • Skeletal muscle cells must be stimulated by nerve impulses in order to contract. • A single motor neuron (nerve cell) may stimulate anywhere from a few to hundreds of cells

6. One neuron and all the skeletal muscle cells it stimulates are a motor unit • Neuron (nerve fiber/axon) reaches the muscle and branches into axonal terminals that form junctions with the sarcolemma of different cells (neuromuscular joints). • The nerve endings and cells never touch, the gap between the two are called a synaptic cleft and is filled with fluid (interstitial)

7. Neurotransmitter • A chemical that is released when a nerve impulse reaches the axonal terminals • Acetylcholine – neurotransmitter that stimulates skeletal muscle cells ACh • ACh diffuses across the synaptic cleft and attaches to receptors (membrane proteins) and on part of the sarcolemma • If enough ACh is released the sarcolemma becomes temporarily permeable to sodium ions that rush into the muscle cell. • This causes an excess of positive ions inside the cell and generates an electrical current (action potential) • The action potential travels over the sarcolemma to either end of the cell and results is muscle contraction

8. Oxygen Debt • Not being able to take in enough oxygen to keep the muscles supplied when they are working strenuously • In this case, blood supply aids in reducing fatigue • A lack of oxygen causes lactic acid begins to accumulate and ATP levels decrease • This combination results in less effective contractions and finally stop all together • True muscle fatigue is rare • Oxygen debt always occurs to some extent during vigorous muscle activity

9. Muscle Fatigue and Oxygen Debt • Muscle Fatigue • Occurs when a muscle is unable to contract, even when it’s being stimulated • Without rest, active/working muscles begins to tire and contracts more weakly until it finally fails to contract • Results from oxygen debt that occurs during prolonged muscle activity

10. Types of Muscle Contractions • Isotonic – same tone/tension • Most familiar • Myofilaments successful in sliding movements, muscle shortens and movements occurs • Examples: bending of elbow, knee; rotating ankle, wrist, smiling • Isometric – same measurement/length • Contractions in which the muscles do not shorten • Myofilaments can’t slide and move due to immoveable object, therefore the tension builds in the muscle