Muscles Flashcards
Name the three muscle types.
-skeletal muscle
-cardiac muscle
-smooth muscle
What are the functions of skeletal muscle?
- Produce skeletal movement
- Maintain posture & body position
- Support soft tissues
- Guard body entrances & exits
- Maintain body temperature
- Store nutrients
What are the features of skeletal muscle?
- Myocytes are large, tubular cells
- Multinucleate
- Cells are banded or striated
- Sarcolemma
- Sarcoplasm
- Transverse or T tubules
→ Continuous with the sarcolemma
Extend deep into sarcoplasm
When action potential depolarises sarcolemma this impulse is transferred to rest of fibre via T tubules- allows the muscle cell to contract all at once
· Myocytes contain many chains of myofibril
· Shortening of myofibrils produces contractions
· Myofibrils are composed of myofilaments
· Thin filaments made of actin
· Thick filaments made of myosin
· Elastic filaments made of titin
· Myofibrils anchored to sarcolemma
· Sarcoplasmic reticulum- involved in the maintenance of calcium concentrations in a cell
What are the events at the neuromuscular junction?
· Neuron axon terminal meets muscle fibre at NMJ
· Action potential reaches presynaptic neuron terminal
· Voltage gated Ca channels open – influx of Ca ions
· Causes ACh-containing vesicles to bind to presynaptic membrane
· ACh released across synapse
· Binds to AChR on muscle fibre, ligand-gated Na ion channel
· Influx of Na ions produces action potential in sarcolemma
· AChE removes ACh
Influx of Ca ions into presynaptic cytosol
AChR is a ligand gated ion channel
ACh either diffuses away or is broken down by acetylcholinesterase
What is the process of a muscle contraction?
- Action potential travels down sarcolemma and T tubules
- Ca2+ ions released from SR - bind to thin filament causing a change in its conformation
- Thick filaments composed of ~300 myosin molecules
- ATP binds to myosin head and is hydrolysed to ADP & P (active process)
- Activated mysoin head forms cross bridge with actin
- ADP & P are released and myosin head pivotsand it moves
- Slides thin filament towards centre of sarcomere
- ATP binds to myosin which detaches from thin filament
-Sliding filament theory
How does the muscle fiber shorten?
Once ATP has been hydrolysed the myosin head is cocked or activated
Slides thin filament towards centre of sarcomere – sarcomere shortening
During this contraction the entire muscle shortens producing a pull or tension on the tendons at either end
What are isotonic contractions?
· Tension remains constant, muscle length changes
· Walking, running, lifting an light object
· Skeletal muscle obeys ‘all or none’ law
What are isometric contractions?
· Tension increases, muscle length remains constant
· Tension produced cannot exceed load
· Carrying a heavy object
How do muscles keep energy?
· ATP stores are not sufficient to maintain muscle contraction
· ATP is generated at the same rate it is used
· At rest, ATP transfers energy to creatine to produce phosphocreatine- stored in your muscle cells
· During contraction, myosin breaks down ATP to ADP & P, phosphocreatine is then used to ‘recharge’ the ADP concerting it to ATP
· Creatine kinase is the enzyme involved
· Muscle damage results in high blood CK levels
What is muscle fatigue?
· Depletion of metabolic reserves – energy provided through aerobic metabolism, fatigue doesn’t occur until stores of glycogen, lipids and aa are depleted. Affects endurance athletes after hours of exertion
· Anaerobic respiration – glycolysis produces pyruvate which is converted to lactic acid. Decrease in pH which inhibits muscle contraction
· Cumulative – effects are more pronounced as more fibres become affected
· Lactic acid build up this stops the sliding filament theory from taking place
What is myasthenia gravis?
· Autoimmune disease of the NMJ
· Antibodies damage AChR on skeletal muscle fibres
· ACh is released but there are insufficient receptors to bind to
· Action potential is not propagated down muscle fibre
· Muscle weakness/fatigue
· Treat with AChE inhibitor
→ Treat with AChE inhibitor – allow conc of ACh to build up in the synapse → Pyridostigmine used for treatment – is an AChE inhibitor
