PBL Topic 3 Case 6 Flashcards
What is a diffusion potential?
- The potential difference between the inside and outside of the membrane
What is the Nernst potential?
- The diffusion level that exactly opposes the net diffusion of a particular ion through a memrbane
What is the role of the sodium potassium pump?
- Moves 3Na+ to the outside of the cell
- For every 2K+ to the inside of the cell
- In order to restore the resting membrane potential
What are leak channels and how do they differ in permeability to different ions?
- Channels through which ions leak
- Greater leakage of potassium because it is more permeable
What is the value of the resting potential in large nerve fibres?
- -90 millivolts
- Meaning the potential inside is 90 millivolts more negative than the potential in the extracellular fluid
Explain why the Nernst potential for sodium and potassium differs
- Potassium Nernst potential is -94 millivolts because there is a high ratio of potassium ions inside to outside
- Sodium Nernst potential is +61 because there is a lower ratio of sodium ions inside to outside
What is the resting stage of an action potential?
- Membrane is polarised due to -90 millivolts generated by sodium potassium pump
What is the depolarisation stage of an action potential?
- Membrane becomes permeable to sodium
- Large number of sodium ions diffuse into axon
- Overshoot caused by excess of sodium entering
What is the repolarisation stage of an action potential?
- Sodium channels close
- Potassium channels open
- Rapid diffusion of potassium out
Identify the two types of gating
- Voltage gating involving opening of a protein channel as a result of changing potential
- Chemical ligand gating involving opening of a protein channel due to binding of a ligand e.g. ACh
Describe the gates of a voltage gated sodium channels
- Activation M gate opens when membrane potential becomes less negative than the resting potential
- Inactivation H gate closes as a result of same increase in voltage but it closes slower
What is meant by threshold for stimulation and what is its value?
- Membrane potential becomes less negative than resting potential
- To around -65 millivolts
- Which causes explosive development of an action potential
Explain the process behind the propagation of an action potential
- Excited portion of nerve fibre causes local circuit of current flow to adjacent resting membrane areas
- Positive electrical charges are carried by inward diffusion sodium ions
- Increasing threshold in adjacent areas above threshold for stimulation
What is meant by the all or nothing principle?
- Action potential only occurs if sufficient voltage to stimulate the next area of the membrane builds up
- Above the threshold
- Or it does not travel at all
What is the importance of ATP in nerve impulse?
- Sodium potassium pump re-establishes sodium and potassium concentration differences
- Which requires ATP
What is the ratio of myelinated to unmyelinated nerve fibres?
- 1:2
What is the velocity of conduction in a small unmyelinated nerve fibre compared to a large myelinated fibre?
- Small unmyelinated: 0.25m/sec
- Large myelinated: 1m/sec
Describe the structure of a myelinated nerve fibre
- Central core is the axon filled with axoplasm
- Surrounded by a myelin sheath
Outline the process of myelination
- Membrane of Schwann cell envelops the axon
- Schwann cell rotates around axon many times
- Depositing many layers of sphingomyelin
- Which is an insulator that decreases ion flow through the membrane
What is saltatory conduction?
- Junction between Schwann cells is known as node of Ranvier
- Nerve impulse jumps between nodes of Ranvier
- Increasing the velocity of transmission
What is primary demyelination?
- Myelin sheath is destroyed
- Axons remains intact
What is secondary demyelination?
- Damage to axon
- Resulting in breakdown of of myelin
What happens to the debris of myelin breakdown?
- Phagocytosed by macrophages
- Transformed into droplets of neutral lipids (cholesterol esters)
What is an acute sub-threshold potential?
- Weak negative stimulus incapable of exciting a nerve fibre