nerve impulse -
action potential
Across the membrane of an stimulated axon, there is a rapid change in the electrical potential difference
Membrane potential - Difference of charges across the
plasma membrane
Resting potential
- Resting cells are (-) inside and (+) outside
- Larger amounts of Na+ outside the axon than K+ inside the axon
- Un-stimulated axon / nerve fibre
- Polarized membrane
Impulse due to unequal concentration of positive ions across nerve membrane.
Negative ions inside axon cannot move across membrane.
- Sodium and Potassium ions diffuse across the membrane through the Sodium/Potassium Pump.
- Sodium and Potassium channels are voltage-gated.
The phases of an action potential:
- Resting Potential
- Depolarization
- Rising phase of action potential
- Falling phase of action potential
- Hyperpolarization
- Resting Potential
1. Resting State
- positive outside axon - negative inside axon
- Sodium channel closed
- Potassium channel closed
Threshold level - voltage required to initiate Action Potential
ALL-OR-NONE principle - neurons fire all maximally or none at all
ALL-OR-NONE principle - neurons fire all maximally or none at all
- Increasing the intensity of the stimuli above the critical threshold value will not produce an increased response
- The intensity of the nerve impulse and speed of transmission remain the same.
- Sodium channels open
- Positive inside axon - Negative outside axon
- Potassium channels begin to open at 0mV (slowly)
3. Repolarization
- Sodium channels inactive
- Potassium channels open
- Positive outside axon - Negative inside axon
4. Hyperpolarization
- Sodium channels close
- Potassium channels close SLOWLY
- Positive outside - Negative inside (Resting Potential)
saltatory conduction
Unmyelinated Axon
Impulses carried continuously along the axon
Myelinated Axon
Impulses carried from node to node
Advantages
Impulses carried continuously along the axon
Myelinated Axon
Impulses carried from node to node
Advantages
- Saves on ATP/energy (Sodium / Potassium Pump)
- Increases the speed of conduction
Now that we have an understanding of the nerve impulse, let's take a closer look at how the action potential travels from one axon to another axon through the synapse.
Resources:
DiGiuseppe, Maurice et al. (2003). Nelson Biology 12. Nelson Canada ELHI (1 Edition). Toronto, Ontario: Thomson Canada Limited
DiGiuseppe, Maurice et al. (2003). Nelson Biology 12. Nelson Canada ELHI (1 Edition). Toronto, Ontario: Thomson Canada Limited