Muscular Physiology

Question 1

What is a motor unit?

Answer 1

• Made up of motor neuron and all the muscle fibers it controls
• Neuromuscular junctions: where the nerve lands on the muscle fibers
• Involves the somatic nerve neuron
• Originates from the CNS of the spinal cord

Question 2

What is a neuromuscular junctions?

Answer 2

• Where the nerve lands on the muscle fibers sarcolemma
• Myelinated axon of motor neuron -> has action potential (down a chemical gradient) -> travel towards axon terminal -> land on the sarcolemma
• All muscles must contact -> action
• In between sarcolemma and neuromuscular junction -> synaptic cleft
• Changes of neuromuscular junction/sarcolemma -> change in action potential

Question 3

What are the cell charges across the cell membrane?

Answer 3

• Plasma membrane: 2 phospholipid bilayer
• At resting membrane potential (no charge):
  o Extracellular: high concentration of Na+ ions
  o Intracellular: high concentration of K+ ions

Question 4

How does motor neuron stimulate a skeletal muscle fiber?

Answer 4

• Location: at neuromuscular junction, on the sarcolemma of muscle fiber
  1. Action potential arrives at the axon terminal of motor neuron -> brings Na+ in from the extracellular -> causing a positively charge axon terminal -> causes Ca2+ channels to open
  2. Voltage-gated Ca2+ channels open -> Ca2+ enters the axon terminal
  3. Ca2+ entry -> causes synaptic vesicles -> to release the acetylcholine (ACh) content by exocytosis
  4. ACh (neurotransmitter)  diffuses across the synaptic cleft due to high concentration of ACh in presynap than postsynap cleft -> ACh binds to receptors on the sarcolemma
  5. ACh binding opens postsynaptic membrane ion channels -> allow simultaneous passage of Na+ in & K+ out of muscle fiber
  6. Action potential in presynaptic cleft -> transferred to postsynaptic cleft -> allows movement of muscles
  7. ACh effect terminated by -> enzymatic breakdown in the synaptic cleft -> by acetylcholinesterase -> postsynaptic membrane ion channel closed

Question 5

What is the membrane potential and initiation of muscle contraction?

Answer 5

• Starts at axon terminal, action potential (electrical impulses at sarcolemma)
• Ca entry triggers ACh release and binds to receptor
• Open the voltage gated Na channels -> depolarization -> Action Potential
• K+ channels open
• Resting potential -> before another action potential

Question 6

What are the steps of depolarization?

Answer 6

1. Opening of the Na+ channels (enter from out into in)
   a. Potential becomes from negative to positive -> depolarization
   b. Reaches a peak of +30
2. K+ channels
   a. Moves out  repolarization
   b. Potential becomes from positive to negative
3. K+ channels closes at -90
   a. Hyperpotential
4. Resting potential
   a. Before another action potential

Question 7

What makes up a sarcomere?

Answer 7

• Titin holds the myosin filament in place

-	During contraction:
o	actin comes closer to the myosin
o	myosin remains statin
o	titin will contract  bringing both ends closer
o	Z discs come nearer to M line

• During relaxation:
  o Muscle recoil
  o Bring actin to original

Question 8

Describe the thick filament myosin

Answer 8

• each thick filament -> has many myosin molecules -> head protrude at opposite ends of the filament
• double stranded

Question 9

Describe the thin filament actin?

Answer 9

• Two strands of actin subunits twisted into helix
• Presence of 2 types of regulatory proteins: regulates the muscle contractions
  o Troponin
• Holds tropomyosin in place and stabilizes it

o Tropomyosin

Question 10

What happens during muscle contraction?

Answer 10

• Ca2+ release from sarcoplasm/T tubules -> troponin detaches -> tropomyosin forms a flat line -> active sites of actin exposed -> myosin head locked on actin site -> sliding theory -> muscle contraction

Question 11

What happens during the excitation contraction coupling process?

Answer 11

• Contraction of action potential along the sarcolemma -> leads to sliding of myofilaments

Question 12

What is the steps in the excitation contraction coupling process?

Answer 12

1. Action potential is propagated along the sarcolemma and down the T tubules
   a. T tubules has Ca2+ channels
   b. T tubules allow some Na+ to enter -> become + charged membrane -> Ca2+ release -> activate the propagation of action potential/muscle contraction
2. Ca2+ ions are released
   a. Presence of calcium ions in the cisternae of sarcoplasmic reticulum -> Ca2+ move out and filtered by T tubules
3. Tropomyosin blocking the active sites of actin
4. Ca2+ reacts with troponin -> pushes tropomyosin away off binding site -> active site of actin exposed -> myosin binds -> pull actin -> causes and overlap -> contraction

Question 13

What are the roles of Ca2+?

Answer 13

• Neuromuscular junction release acetylcholine (ACh)

- Reacts with troponin

Question 14

What is the recycling process of calcium to sarcoplasmic reticulum?

Answer 14

1. Cross bridge formation
   a. The influx of calcium triggering the exposure of myosin binding sites on actin

b. Actin binds to myosin

2. The power (working) stroke
   a. Power stroke of the myosin head -> causes the thin filament to slide

b. Binding of ATP to myosin head -> disconnects from actin
3. Cross bridge detachment

4. Cocking of myosin head
   a. Hydrolysis of ATP re-energizes and repositions the myosin head

b. Breaks down ATP to ADP and P
5. Cycle repeats and another calcium ion recycled

Question 15

What happens when the muscle fiber relaxes?

Answer 15

• Individual actin and myosin length will remain the same
• Shortening is due to the overlapping of actin and myosin
• No change in A band (due to overlapping)

Question 16

What happens when the muscle fiber contracts?

Answer 16

• Distance of Z disk decreases -> comes closer

- In present of ATP and Ca2+

Question 17

What are the different types of muscle contraction?

Answer 17

• Isotonic

- Isometric