1. Electrochemical Impulse
SBI 4U
Electrochemical Impulse

2. Some Interesting Facts about the Neuron
Longevity – can live and function for a lifetime
Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception
High metabolic rate – require abundant oxygen and glucose

3. How do nerve cells pass along a message?
Nerve impulses remain as strong at the end of a nerve as they were at the beginning
Use cellular energy to generate current (which is the message)
Deals with a change in electrical potential energy across a membrane

4. Action vs Resting Potential
Action potential  voltage difference across a membrane when a nerve is excited
Resting potential  voltage difference during resting stage

5. How do nerve cell membranes become charged?
Molecular Level of the Nerve Cell
Neurons have a large amount of both + and – ions both inside and outside cell (unlike most cells)
Negative ions don’t contribute much charge (too big)
Key Focus:
Electrochemical message is caused by an unequal concentration of + ions across the nerve cell membranes

6. How do nerve cell membranes become charged?
Note
Sodium likes to diffuse into cell
Postassium likes to diffuse out of cell
Both diffuse at the same time
However, diffusion is unequal; cell more permeable to potassium, so more potassium diffuse out
Since more K+ out, exterior of membrane more positive than interior
Conversely the excess negative ions accumulate along the inside of the membrane
Creates a polarized membrane

8. How a charge is generated
Ion gates allow ions into and out of cell
Steps
Nerve excited
Na+ gates open allowing more sodium ions inside cell, while K+ gates close
Rapid inflow of + ions causes charge reversal (a.k.a. Depolarization)
Inside of cell now positive so Na+ gates close
Sodium-potassium pump restores resting membrane by transporting 3 Na+ out for every 2 K+ in (a.k.a. Repolarization)

9. Process of Depolarization and Repolarization
Note how the action potential is moving away from the site of origin

10. Refractory Period
Before a nerve can produce another action potential, it must repolarize
This recovery time is called the refractory period
Often lasts from 1 to 10 ms

11. Movement of Action Potential
Once a nerve is stimulated, the message needs to be carried along the length of the axon
Therefore, depolarization has to move from the zone it initiated in to adjacent regions
How does this happen?

12. Movement of Action Potential
Once an action depolarization has occurred, there are now more positive ions on the inside of the membrane
These positive ions are attracted to the negative ions in the adjoining regions that have not been stimulated
As these positive ions move toward the negative ions (the resting membrane), the nerve impulse is carried with them
This resting membrane then undergoes depolarization

13. Movement of Action Potential

14. Movement of Action Potential
Once depolarization happens, it stimulates the sodium channels to open which results in the movement in the action potential
This wave of depolarization therefore moves along the length of the nerve membrane
Depolarization of the membrane causes the sodium channels to close, the potassium ions to reopen
Note that every wave of depolarization is followed by a repolarization and a refractory period

15. Movement of Action Potential

16. Nodes of Ranvier How does one action potential move down the axon?
Nodes of Ranvier are located between myelinated sections of the axon
Figure 8.15, p.358
Nodes contain many Na+ channels
Nodes are the specific site of triggering an action potential
Remember: sodium ions ENTER via the channels
This DEPOLARIZES the membrane
THRESHOLD occurs
Prior membrane can not be stimulated. Why not?
Next membrane in front can be. Once Threshold occurs, action potential is triggered. Signal continues down axon.
This process can also be referred to as : Saltatory Conductions
No jumping in non-myelinated axons. So which is faster?

17. Movement of an Action Potential, cont’d

18. Threshold Levels Threshold Level
A minimum level of stimulus is required to produce a response
Varies depending on the neuron

19. Higher threshold of B, so two neurons are excited
All-or-None Response
Once you reach the threshold level, adding more stimulus will not elicit a greater response
A nerve muscle fibre responds completely or not at all to a stimulus
Note: the intensity of the response will change depeding on:
If the brain recognizes a change in the frequency of responses
if neurons that have a higher threshold are excited
Higher threshold of B, so two neurons are excited

20. Synaptic Transmission
Messages need to be transmitted between neurons
Occurs via synapses
Small spaces between neurons, or between neurons and effectors (e.g. Muscles)
As impulse moves along axon it reaches the endplate and releaes vesicles that contain neurotransmitters (via exocytosis)
Neutrotransmitters are released from the presynaptic neuron, travel across the synaptic cleft, and create a depolarization of the post synaptic neuron

21. Synaptic Transmission

22. Example of Neurotransmitter
Acetylcholine
Acts as excitatory neurotransmitter on many post synaptic neurons
Opens sodium channels, helping with depolarization
Potential problem  keeps sodium channels open, so keeps cell in constant state of depolarization; not able to respond to next impulse since no refractory period
Solution: membrane enzyme cholinesterase destroys acetylcholine
Real world application: insecticides blocks cholinesterase so insect heart remains contracted