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Action Potentials
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- 1What is an action potential?A brief, pulse-like electrical signal in a small portion of cell membrane
- 2How does it travel?By propagation from one portion of membrane to the next
- 3What happens in an action potential?The membrane potential reverses in polarity: the inside of the axon becomes briefly more positive than the outside
- 4How is an action potential triggered?when a local electrical signal exceeds a threshold
- 5Size of spike?Generally, each spike in a cell always has the same size ('all-or-nothing events')
Larger stimulus = earlier spike - 6What are the stages in a spike?Resting Potential
Threshold
Rising phase
Falling phase
Recovery - 7What happens in the threshold phase?-A few Na+ channels are opened permitting Na+ to enter the neuron
-This increases # of positive ions in cell--> membrane potential depolarises (less negative)
-If depolarisation reaches threshold - more Na+ channels open - 8What happens in the Rising phase?-As Na+ ions rush into the cell, voltage across the membrane reaches the most positive value
-At this peak, many Na+ channels start to close + voltage gated K+ channels open (distinct from those responsible for resting potential) - 9What happens in the falling phase?-Positive charges leave the cell
-Membrane repolarises BEYOND RESTING POTENTIAL
-This undershoot occurs because more K+ channels are open at this point than during resting potential -->More K+ leave the cell
-These extra channels then close and resting potential is maintaines - 10Dendrites and spikes?Dendrites collect signals from other neurons
They do not make spikes, but make smaller electrical signals
Signals from different dendrites collect together at the AXON HILLOCK
These signals may sum together to exceed the spike threshold and trigger a spike - 11What is the function of spikes?-Amplify a small electrical signal
-Carry electrical excitation along an axon
-Neurons are poor CONDUCTORS of ELECTRICAL SIGNALS, spikes boost the signal ensuring it can travel along long axons - 12What does the speed of a spike depend on?-Axon width
-Temperature
-Myelin sheath
(all in mammals) - 13Axon width?Wide axons = larger electrical currents = faster
- 14Temperature?Warmer animals = faster spike
- 15Myelin sheath?Not in invertebrates
-Insulates axon meaning less electrical energy lost
-Spikes travel node to node (saltatory conduction)
-Multiple sclerosis = myelin sheath interrupted so spikes not as quick - 16Summary of spike mechanism?1. If excited, voltage gated Na+ channels open
2. Na+ enters by diffusion
3. Makes inside of axon less negative
4. This opens more voltage gated Na+ channels
Regenerative process (back to 2)
5. Na+ channels close and deactivate
6. Voltage gated K+ channels open --> K+ exits cell by diffusion - 17Voltage gated Na+ channels vs Voltage gated K+ channels?Voltage gated Na+ channels open faster
- 18Why is inactivation of Voltage gated Na+ channels important?Keeps spikes DISCRETE - a period where another spike cannot be triggered: the REFRACTORY PERIOD
- 19Why do Na+ ions move into the axon when the Na+ channels start to opens?Net direction of diffusion driven by ELECTROCHEMICAL GRADIENT for Na+
- 20Why is the peak of the spike at a membrane voltage of +50 mV?+50 mV is the EQUILIBRIUM POTENTIAL for Na+
The concentration and electrical gradients balance each other - 21How do we know the mechanism for a spike?-JZ Young studied giant axons in squid - told us a lot about how a spike is made
-Hodgekin and Huxley found:
That the polarity reverses during a spike
The concentration of ions in and out of axon - 22What theory did Hodgekin and Huxley come up with?Came up with a theory that:
-there are ion channels across membrane that are voltage gated
-that there is a sequence of events
-They used a mathematical model to show it would work - later it was proved
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- 2nd Year (Animal Physiology) Biology Fichas sobre Action Potentials, creado por Phoebe Stockton el 19/08/2016.
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- Phoebe Stockton
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