WEEK4 - Muscular system (2) learning contents

Topic 2.5: Physiology of muscle contraction

 

- The neuromuscular junction

The neuromuscular junction is a connection between an axon terminal of a somatic motor neuron and a muscle fiber that is the route of electrical stimulation of the muscle cell.

 

1.   The axon terminal and muscle fiber are separated by a narrow gap, the synaptic cleft.

 

2.   Within the axon terminal are synaptic vesicles containing the neurotransmitter acetylcholine, or ACh; while junctional folds of the muscle cell contain millions of ACh receptors.

 

3.   A motor neuron stimulates a skeletal muscle fiber when a nerve impulse causes the release of ACh to the synaptic cleft, which diffuses across the cleft and binds to ACh receptors on the sarcolemma, creating electrical events that lead to the generation of an action potential.

 

4.   After acetylcholine binds to ACh receptors, an enzyme in the synaptic cleft, acetylcholinesterase, breaks down acetylcholine to acetic acid and choline, to prevent continued contraction in the absence of stimulation.

 

- Generation of an Action Potential across the Sarcolemma

Generation of an Action Potential across the Sarcolemma

1.   Generation of an end plate potential occurs when ACh binds to ACh receptors at the neuromuscular junction, causing chemically gated ion channels to open: more Na+ diffuses in than K+ diffuses out, and the membrane depolarizes.

 

2.   The end plate potential triggers an action potential, which propagates along the sarcolemma by causing the opening of voltage gated Na+ channels.

 

3.   Repolarization occurs when voltage gated Na+ channels close, and voltage gated K+ channels open, restoring the resting polarity to the sarcolemma.

 

a.   During repolarization, the muscle cell is in a refractory period and may not be depolarized until repolarization is complete.

 

- Excitation-contraction coupling

Excitation-contraction coupling is the sequence of events by which an action potential on the sarcolemma results in the sliding of the myofilaments.

1.   A nerve impulse reaches the axon terminal, causing release of ACh to the synaptic cleft.

2.   ACh binds to ACh receptors in the sarcolemma, and a net influx of sodium ions causes the generation of an end plate potential.

3.   Voltage-gated sodium channels open, allowing the generation and propagation of an action potential on the sarcolemma.

4.   Transmission of the action potential along the T tubules, stimulating the release of calcium ions from the sarcoplasmic reticulum to the cytosol.

 

-Cross Bridge Cycling

Muscle Fiber Contraction: Cross Bridge Cycling 

1. As calcium levels in the cytosol increase, calcium binds to troponin, which causes tropomyosin to slide away from the binding sites for myosin on the actin filaments.

2. Energized myosin heads bind to actin and perform a power stroke, causing actin to slide over myosin.

 

 

-Isotonic and Isometric Contractions

Isotonic contractions produce uniform tension in a muscle, once a load has been overcome, and result in movement occurring at the joint and a change of length of muscles.

a.   Concentric isotonic contractions result when a muscle generates force when it shortens, while in eccentric isotonic contractions, the muscle generates force as it lengthens.

 

2.  Isometric contractions result in increases in muscle tension, but no lengthening or shortening of the muscle occurs, and often are used to maintain posture or joint stability while movement occurs at other joints.

 

 

-Force of Muscle Contraction

Force of Muscle Contraction

1.   As the number of muscle fibers stimulated increases, force of contraction increases.

2.   Large muscle fibers generate more force than smaller muscle fibers.

3.   As the rate of stimulation increases, contractions sum up, ultimately producing tetanus, allowing the external tension generated by the connective tissue elements to approach internal tension generated by the muscle fibers, increasing contractile force.

4.   The length–tension relationship optimizes the overlap between the thick and thin filaments that produces optimal contraction.

 

Types of skeletal muscle fibers

Velocity and Duration of Contraction

There are three muscle fiber types: slow oxidative fibers, fast glycolytic fibers, and fast oxidative fibers.

a.   Slow oxidative fibers contract slowly, rely mostly on aerobic respiration, and are highly fatigue resistant.

b.   Fast glycolytic fibers contract rapidly, use anaerobic respiration, depend heavily on glycogen, but fatigue quickly.

c.   Fast oxidative fibers are a less common, intermediate type of fiber that provide rapid contraction, but have excellent capillary penetration for oxygen and nutrient delivery, and rely on aerobic respiration.

All muscles have varying amounts of all fiber types and, while the proportion of each type is a genetically influenced trait, that proportion can be modified by specific types of exercise.

As the load on a muscle increases, velocity and duration of contraction decreases.

Recruitment of additional motor units increases velocity and duration of contraction.

 

- Providing Energy for Contraction / Muscle ATP metabolism

Providing Energy for Contraction

Muscles contain very little stored ATP, and consumed ATP is replenished rapidly through phosphorylation by creatine phosphate, anaerobic glycolysis, and aerobic respiration.

 

1.   As muscle metabolism transitions to meet higher demand
during vigorous exercise, consumed ATP is regenerated by transferring a phosphate to consumed ATP from creatine phosphate (direct phosphorylation), a molecule unique to muscle tissue.

2.   As stored ATP and creatine phosphate are consumed, ATP is produced by breaking down blood glucose or stored glycogen in glycolysis, an anaerobic pathway that precedes both aerobic and anaerobic respiration. If adequate oxygen is not available to support aerobic respiration, anaerobic glycolysis converts the pyruvate formed from glycolysis into lactic acid.

a.   This pathway produces only about 5% the ATP from each glucose compared to the aerobic pathway, but ATP production occurs 2½ times faster.

b.   Most of the lactic acid produced is released to the bloodstream and taken to the liver, heart, or kidneys for use, but the lactic acid that remains in the muscle contributes to muscle soreness following exercise.

3.   Aerobic respiration provides most of the ATP during light to moderate activity, includes glycolysis, along with reactions that occur within the mitochondria, and produces 32 ATP per glucose, as well as water, and CO2, which will be lost from the body in the lungs.

How does it work in practice:
Muscles function aerobically as long as there is adequate oxygen and nutrient delivery to support it, but when exercise demands for ATP exceed the production ability of aerobic reactions, the cell will switch to anaerobic pathways

 

 

- Endurance vs Resistance exercise

A.   Aerobic exercise promotes an increase in capillary penetration, the number of mitochondria, and synthesis of myoglobin, leading to higher efficiency and endurance, while possibly converting fast glycolytic fibers to fast oxidative fibers.

B.   Resistance exercise, such as weight lifting or isometric exercise, promotes an increase in the number of mitochondria, myofilaments and myofibrils, and glycogen storage, producing hypertrophied cells that may change from fast oxidative to fast glycolytic fibers.

 

 

 

 

 

 

 

https://youtu.be/NfEJUPnqxk0

https://www.youtube.com/watch?v=qccUQGZbyi8&list=PLTCXr1VpsHmt-ZFEPUxQWxEBUzFTThyK9&index=7 

https://www.youtube.com/watch?v=PTjzD3pqM3o 

https://www.youtube.com/watch?v=0qFy-vl11CM 

https://aclandanatomy.com/index.aspx

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