Skeletal Muscle - Contraction and Training Flashcards
What stimulates contraction in skeletal muscle?
Somatic motor neuron.
What is a motor unit?
One nerve fibre and the muscle fibres it innervates.
What is the connection between the muscle and the nerve called?
A neuro-muscular junction (NMJ); the gap in-between is the synaptic cleft.
What is contained within the axon terminal of the motor neuron and why?
There are many vesicles of acetyl choline (ACh) neurotransmitters to allow the action potential to pass on to the sarcolemma.
*Electrical impulses cannot cross the synaptic cleft hence the need for chemical signals in the form of neurotransmitters.
Where are T-tubules found and what surrounding structures are there? What is it’s function?
T-tubules run around myofibrils and are themselves surrounded by sarcoplasmic reticulum (SR). It connects with the outer membrane and contains calcium ion channels (gated).
Where are Ca ions stored before contraction?
In the terminal cisternae which are part of the sarcoplasmic reticulum.
What is required by thin filaments to initiate contraction?
Calcium ions.
Describe contraction (general)?
The movement of thick and thin filaments of the sarcomere relative to each other, to cause active shortening of a muscle fibre.
Describe the sequence of events at the pre-synaptic neuro-muscular junction.
- Action potential is propagated along the somatic motor neuron to the presynaptic axon terminal.
- This causes the opening of voltage-gated calcium channels - causing in influx of Ca from the extracellular fluid.
- Increased concentration of Ca causes vesicles containing ACh to fuse with the presynaptic membrane, releasing ACh into the synaptic cleft (exocytosis).
Describe the sequence of events at the post-synaptic neuro-muscular junction.
- ACh binds to receptors on the post-synaptic membrane (sarcolemma), opening ligand-gated channels on the membrane surface.
- Na ions (abundant in extracellular fluid) then flow through these channels into the sarcolemma (where there are high concentrations of K ions), causing the membrane to become more positive.
Describe the sequence of events after the neuro-muscular junction.
- Sodium ions flow through the ligand-gated channels on the sarcolemma, causing the membrane to become more positive; eliciting a muscular action potential.
- The action potential travels along the sarcolemma and propagates down the T-tubules causing calcium channels to open. *Mediated by ryanodine receptors on SR surface.
- These channels open due to a change in voltage.
- Calcium then moves from the terminal cisternae into the T-tubules and to the thin filaments.
What is the thick filament of the sarcomere?
Myosin (which has two heads);
- One to bind to ATP
- One to bind to actin sub-units on the thin filament.
What is the thin filament of the sarcomere?
Actin with two regulatory proteins;
Troponin - binds to Calcium
Tropomyosin - blocks myosin binding sites on actin.
Describe the Sliding Filament Theory.
- Calcium binds to Ca binding sites on the TnC sun-unit on troponin.
- This causes a conformational change in shape of troponin and as it is attached to tropomyosin, pulls the tropomyosin with it; uncovering the myosin binding sites.
- ATP simultaneously binds to tropomyosin causing hydrolysis of ATP and thus a conformational change in shape of myosin, moving it closer to the actin sub-unit. ADP remains attached.
- Myosin now binds to actin forming cross bridges enabling it to pull the thin filaments closer to the midline (centre of the sarcomere). This causes the release of ADP.
- This move towards the midline allows actin to generate tension and contraction in muscle.
- Actin towards M-line and myosin towards Z-line.
What happens when no more stimuli is received by muscle fibres?
- Calcium release channels close
- Intracellular Ca levels return to normal
- Contraction stops
Calcium restored to reservoirs in the SR by active transport pumps moving Calcium back into the terminal cisternae for storage. - New ATP binds to myosin when ADP is released, causing myosin to detach to the actin and is ready for ATP hydrolysis again.
How much ATP is stored in muscles before contraction?
Not much - only enough to sustain a few twitches.
How is contractile activity sustained?
Metabolism must produce ATP molecules as quickly as they are broken down during contractile activity.
What are the ways to regenerate ATP for contraction?
There are 3 ways:
- Phosphorylation of ATP by creatine phosphate
- Phosphorylation of ATP by oxidative phosphorylation in mitochondria
- Phosphorylation of ATP by glycolytic pathways in the cytosol
Describe Creatine Phosphate.
- High energy molecule in high concentrations in sarcoplasm.
- Used when muscle demand for energy > available ATO supply
- Creatine phosphate and ADP couple up in the presence of creatine kinase producing ATP (reversible reaction)
ADP + CT > C + ATP - The energy released when the chemical bond is broken between creatine and phosphate is similar to that of ATP breakdown.
Yield - 1 ATP per CT molecule - Used for short bursts of vigorous exercise (15 seconds of muscle contraction)
What are the benefits and limitations to ATP regeneration by creatine phosphate?
- Rapid formation however limited to the amount of creatine phosphate available
- Use of CP at the start of contraction provides time for the slower processes to increase their rates of ATP formation to match the rate ATP breakdown.
Describe Oxidative Phosphorylation.
- Moderate levels of activity will mostly have ATP regenerated using this method.
- First 5-10mins; breakdown of muscle glycogen into glucose to provide ATP
- Next 30mins; blood-bone glycogen and fatty acids breakdown is more dominant
- After this; mostly fatty acid breakdown.
- Conversion of glucose into ATP is slower but more complete.
- Aerobic
Yield - 36 ATP / glucose
Describe glycolysis.
- Produces small ATP quantities but produces them rapidly when enough enzymes and substrates are available.
- Anaerobic
Glucose obtained from;
1. Blood
2. Stores of glycogen with contracting muscle fibres - Side production of pyruvate acid which converts into lactic acid.
What is a major determinant for the fibre Type composition of an individual?
Genetics.
This will determine was type of training/activity is best for you.
What Type of fibres are associated with a high force output?
Type II.