April 7th 2014 - blogs.ubc.ca

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!