1. The Neuron & Action Potential
The basic building block of our nervous system and how it sends messages.

2. Cell Body & Nucleus Key words: Neuron; structures of neurons
Graphic From Image and lecture gallery

3. The Cell Body Contains the cell’s nucleus
round, centrally located structure
contains DNA
controls protein manufacturing
directs metabolism
no role in neural signaling
Graphic from Hockenbury slides
Key words: Cell body; soma; cell nucleus
Interesting facts:
The DNA in the nucleus of the cell has lost its ability to divide. therefore, when a neuron dies,for the most part, the adult brain cannot simply grow new neurons. (Note there are a few exceptions to this rule.)
The relative inability to grow new neurons leads to two interesting questions:
Q1: How do brain tumors (cancer) occur?
A: Unlike neurons, glial cells can divide and grow new cells throughout one's lifetime. Most brain tumors are limited to glial cells, not neurons.
Q2: If a person cannot grow new neurons, how does the brain change in order to accommodate new learning?
A: One mechanism by which the brain adapts to help you learn new information involves the structure on the next slide: the dendrites.

4. Dendrites Key words: Neuron; structures of neurons
Graphic From Image and lecture gallery

5. Dendrites Information collectors or receivers
Receive inputs or signals from neighboring neurons
Inputs may number in thousands
If enough inputs the cell’s AXON may generate an electrical output

6. Dendritic Growth Mature neurons generally can’t divide
But new dendrites can grow
Provides room for more connections to other neurons
New connections are basis for learning
Studies show people with higher education have more dendritic connections than someone that is a high school dropout.

7. Neural Networks

8. Axon Axon Key words: Neuron; structures of neurons
Graphic From Image and lecture gallery

9. Axon Where all the action is
Action Potential takes place – electrical charge is sent down the axon.
One axon per cell, 2 distinct parts
tube-like structure
branches at end (axon terminals) that branch out to dendrites of other cells
Hockenbury slides (Schulman)
Key words: axon; action potentials
Interesting facts:
- The diameter of an axon may vary from approximately 1mm-20mm.
- An axon may travel long distances to reach it's destination (longest axon is approximately 3 feet in humans and 10 feet in giraffes).

10. Myelin Sheath & Nodes of Ranvier
Key words: Neuron; structures of neurons
Graphic From Image and lecture gallery

11. Myelin Sheath White fatty casing on axon
Acts as an electrical insulator
Not present on all cells
When present, increases the speed of neural signals down the axon allowing the action potential to “jump” to each Node of Ranvier - like a paved highway (see video below to compare mylenated axons vs. non-mylenated axons
If this degenerates (dirt road), you have multiple sclerosis and can’t control your muscles.
Graphic from Hockenbury slides
Key words: myelin sheath; action potentials; axon
Interesting facts:
- The myelin sheath is NOT a part of the axon. The myelin sheath is actually formed of glial cells (oligodendricytes and Schwann cells) that wrap around the axon.
- You may have often heard the brain referred to as either white matter or gray matter. The myelin sheath appears white in nature. Hence, the term white matter refers to areas of the brain that are myelinated. Gray matter refers to areas of the brain that are unmyelinated.
- When you accidentally cut yourself, you often visually notice that you've cut yourself before you actually feel any pain from the cut. The reason for this is that visual information uses myelinated axons; whereas, pain information uses unmyelinated axons.
- The loss of myelin is a significant factor in the disease multiple sclerosis (MS). When myelin is lost, the high-speed transmission of information is slowed down or blocked completely, which could lead the person with the inability to walk, write or speak.
If time view this in a video click on the web address below (it will use QuickTime):
Mylenated Axon

12. Axon Terminal or Buttons
Axon Terminals
Key words: Neuron; structures of neurons
Graphic From Image and lecture gallery

13. Axon Terminal or Buttons
This is where the electrical impulse triggers synaptic transmission to the dendrites of a receiving neuron.
Let’s Review with this Quick Video.

14. Glial Cells They are the janitors of the neuron.
Support cells that provide neurons with structural support and nutrition.
They also remove cell wastes and enhance the speed of the neuron

15. How neurons send an electrical message
Action Potential
How neurons send an electrical message

16. How Neurons Communicate
Neurons communicate by means of an electrical signal called the Action Potential
Action Potentials are based on movements of ions between the outside and inside of the axon
When an Action Potential occurs, a molecular message is sent to neighboring neurons
Action Potential is an All or Nothing Process (like a gun firing)

17. Threshold: Triggering Action Potential
When a neuron is resting there is a balance of excitatory and inhibitory signals.
When one of these exceeds the other stimulus threshold is reached triggering the neuron to transmit an electrical impulse down its axon (action potential)
How do you feel something that is intense?
More neurons fire, the intensity of their electric impulse always stays the same.
Lou Gehrig’s Disease - too many inhibitory stimuli cause the muscles to freeze up.
Parkinson’s Disease - too many excitatory stimuli cause the muscles to move without control.

18. Steps to Action Potential

19. Step 1: Threshold is Reached
Axon at Resting Potential - fluid inside the axon is mostly negatively charged with positive on the outside (polarized)
An impulse is triggered in the neuron’s dendrite when stimulated by pressure, heat, light or a chemical messenger from another neuron (stimulus threshold).
This minimal level of stimulation that causes the axon to fire is called Stimulus Threshold

20. Resting Potential At rest, the inside of the cell is at -70 microvolts
Graphic, Hockenbury slides
At rest, the inside of the cell is at -70 microvolts
With inputs to dendrites inside becomes more positive
If resting potential rises above threshold, an action potential starts to travel from cell body down the axon
Figure shows resting axon being approached by an AP

21. Step 2: Action Potential Begins
When neuron fires, its axon membrane is selectively permeable.
Gates in the axon called ion channels open allowing positive sodium ions to enter the axon while potassium ions leave giving it a brief positive electrical charge (depolarized).
The brief positive charge is action potential.

22. Depolarization Ahead of AP
Graphic from Hockenbury slides
AP opens cell membrane to allow sodium (Na+) in
Inside of cell rapidly becomes more positive than outside
This depolarization travels down the axon as leading edge of the AP

23. Step 3: Refractory Period
As the next gates open allowing positive sodium ions in, the previous gates close and begin to pump the positively charged sodium ions out of the axon and potassium ions back inside. (repolarized).
This step is called the refractory period and the axon cannot fire again until it returns to resting potential (negative polarized state).
The entire process is like falling dominoes all the way down the axon except these dominoes can set themselves back up as soon as they fall over.
Why do you think the axon has to set itself back to a resting state so quickly (3 milliseconds)?
So the neuron can fire again and send another message immediately after the last one.

24. Repolarization follows
Graphic from Hockenbury slides
After depolarization potassium (K+) moves out restoring the inside to a negative voltage
This is called repolarization
The rapid depolarization and repolarization produce a pattern called a spike discharge

25. Finally, Hyperpolarization
Graphic from Hockenbury slides
Repolarization leads to a voltage below the resting potential, called hyperpolarization
Now neuron cannot produce a new action potential
This is the refractory period

26. Action Potential Within a Neuron

30. Animated Action Potential
To review this entire process click on the link below for a short video that helps explain this complex process:

31. DAILY
DOUBLE

32. How can a toilet represent Action Potential?
Full Toilet – Resting Potential
Push Flush Lever – Threshold Stimulus triggering Action Potential.
Toilet Refilling/Can’t Flush – Repolarization/Refractory Period
Sewer Pipes – One-way communication like action potential only goes from dendrite end to axon terminal end.