Neuromuscular structure and function

Question 1

Why do neuronal cells have a neg resting membrane potential?

Answer 1

• Potassium has the greatest intracellular potential(K+)
• Na+ greater extracellular potential (Na+)
• 70 mv
• Active transport and diffusion processes control the resting membrane potential
• AT has the biggest influence
  3 Na+ in, 2 K+ out changes membrane potential

Question 2

Explain the basis of resting membrane potential

Answer 2

• In all cells, Na+ and K+ are constantly pumped across the cell membrane by Na+, K+, - ATPase
• • Maintains:
• High Na+ conc in the ECF and low Na+ conc in the ICF
• High K+ conc in thr ICF and low K+ conc in the ECF
• Constant diffusion of Na+ into the cell by:
• open K+ channels that are always open (leaky)
• When a cell is at rest, the pumping of the Na+, K+, -ATPase = the diffusion of Na+ and K+

Question 3

Describe what happens during an Action Potential

Answer 3

1.) Na+/K+ ion pump active maintaining resting potential (-70mV)
2.Stimulus causes some Na+ channels to open allowing some Na+ ions into axon
3.Membrane potential is less neg as Na+ ion diffuse into axon (-55mV) : threshold is reasched
4.All Na+ ion channels open = rapid influx of Na+ ions
5. All Na+ channels close at +40mV
6. All K+ channels open
7. Rapid efflux of K+ ions - electrical charge inside axon falls as K+ ions move out so axon membrane starts to be repolarised back to original -70mV
8. K+ ions channel remain open = hyper-polarisation
9. K+ ions channels close at -80/90mV
10. Na+/ K+ ion pump allows membrane potential to go from hyper-polarised back to resting potential of -70mV

Question 4

How are motorneruons activated?

Answer 4

• Iontropic mechanism
• Neuromodulation

Question 5

What is the iontropic mechanism?

Answer 5

• Generate synaptic currents
• Initiate action potential

Question 6

What is neuromodulation?

Answer 6

• Modification of the voltage/ ligand-gated channels
• can result in a lowering of activation threshold

Question 7

What are important neuromodulators in the motor system?

Answer 7

• Serotonin (5-HT)
• Norepinephrine (NE) are important neuromodulators in the motor system

Question 8

What happens when an action potential arrives at an axon terminal?

Answer 8

• Voltage gated Ca2+ channels open
• Ca2+ enters the presynaptic neuron
• Ca2+ signals to neurotransmitter vesicles
• Vescivles move to the membrane and dock
• Neurotransmitters released via exocytosis
• Neurotransmitters bind to receptors
• Signal initiated in postsynaptic cell

Question 9

What ACH release at the NMJ dependent on?

Answer 9

• depolarisation of the presynaptic membrane of the motor neurone
• Depolarisation results in Ca2+ ion influx via voltage-dependent Ca2+ channels
• Ca2+ ions promote fusiion of the ACh vesicles with the presynaptic membrane
• Activation of nAChRs by ACh (2 molecules per receptor) leads to depolarisation of the muscle fibre membrane- via net influx of Na+ ions
• Release of a single vesicle of ACh results in a miniature end-plate potential whereas release of several causes an end plate potential or EPP

Question 10

What is the relationship between T tubules and the SR?

Answer 10

• T tubules are deep inviginations of the muscle cell membrane and are placed at the junction of the A bands and the I bands
• They provide a mechanism for changes in membrane potential to be communicated right to the inners of the muscle fibre
• In striated muscle, each T tubule comes into close apposition with SR at several different levels
• When the membrane on the T tubule is depolarised, this triggers release of calcium from the SR
• T tubules and SR allow calcium conc in the sarcoplasm to rise in the area where they are needed