april 7th 2014 - blogs.ubc.ca
TRANSCRIPT
Date
April 7th 2014
Neurons - Excitable Cells
✤ Excitable cells = electrically active cells
✤ There is polarity, or a difference in electrical charge between the inside and outside of the neuron => called membrane potential
✤ Inside the cell: more negatively charged
Axomembrane Ion Channels & Pumps
✤ Ions involved - Sodium (outside of neuron) & Potassium (inside of neuron)
✤ Voltage-gated channels (do not use ATP) are sensitive to voltage changes and facilitate transportation of Na+ and K+ across the membrane
✤ Sodium-potassium pumps (use ATP) actively pump Na+ & K+ across the membrane to maintain the membrane potential (negative interior of the cell)
Axomembrane Ion Channels
Axomembrane Ion Pumps
Maintenance of Membrane Potential
✤ https://www.youtube.com/watch?v=NCE8baQaiK8
Nerve Impulse Transmission
✤ As electrical impulses travel through neurons, there is a series of membrane potential shifts
✤ These shifts are due to the movement of Na+ & K+ ions (charged molecules) across the membrane
Nerve Impulse Transmission: Action Potential
✤ If a stimulus (e.g. pressure, sound, etc.) is strong enough, a nerve impulse is initiated => action potential
✤ Action potential: when the membrane potential rapidly rises and falls - this acts as the signal/message to be repeated & sent along the neuron's axon
Action Potential Stages
✤ 1. Resting State
✤ 2. Threshold
✤ 3. Depolarization
✤ 4. Repolarization
✤ 5. Recovery (refractory) Period
✤ 6. Back to Resting State
1. Resting Stage
✤ The sodium & potassium voltage-gated channels in the membrane are closed
✤ Hence, membrane is not permeable to these ions - cannot move in or out of the neuron
✤ Inside the cell: more negative
✤ Resting membrane potential: -70mV
2. Threshold
✤ A stimulus triggers the opening of some sodium voltage-gated channels and Na+ flow into the neuron
✤ Inside the cell: becomes less negative
✤ If the membrane potential reaches -55mV => threshold has been reached and an action potential is initiated
3. Depolarization
✤ Once a stimulus surpasses the threshold, an action potential is triggered
✤ The membrane becomes permeable to sodium => all sodium voltage-gated channels open and Na+ rush into the neuron
✤ Inside the cell: becomes more positive (+30mV at peak)
4. Repolarization
✤ When depolarization peaks at +30mV, potassium voltage-gated channels open and K+ rush out of the neuron
✤ Sodium gates begin closing
✤ Inside the cell: becomes negative again => restoring the resting potential at -70mV (but ions are in reverse positions)
5. Recovery Period
✤ At -70mV, all sodium & potassium voltage-gated channels close
✤ Inside the cell: becomes more negative than the resting membrane potential
✤ To restore Na+ and K+ ions to their original conditions, sodium-potassium pumps (using ATP) actively pump Na+ & K+ back across the membrane until the resting state is re-established
6. Return to Resting State
✤ Re-establishment of resting state allows the conduction of another impulse
Action Potential Propagation
✤ https://www.youtube.com/watch?v=Sa1wM750Rvs
Action Potential Propagation
All or None Response
✤ If the threshold (-55mV) has been reached, an action potential will be generated
✤ Each action potential is equal to all other action potentials (stronger stimulus ≠ bigger impulse)
✤ Stronger stimuli produce a greater # of impulses i.e. more neurons involved or one neuron conducting a series of impulses
Myelination & Action Potential Propagation
Myelination & Action Potential Propagation
✤ https://www.youtube.com/watch?v=DJe3_3XsBOg
Reminder
✤ PNS & Neuron Quiz on Wednesday
✤ Make flashcards to help you study!