1. The Nervous System
AP Biology
Unit 6

2. Branches of the Nervous System
There are 2 main branches of the nervous system
Central Nervous System
Brain
Spinal Cord
Peripheral Nervous System
All nerves leading to rest of body

3. Anatomy of a Neuron
Dendrites = where a signal is received by the neuron
Cell body = contains the organelles, nucleus of the cell
Axon = signal travels down this to get to the other end of the neuron

4. Anatomy of a Neuron Myelin = surrounds the axon to speed up the signal
Synaptic Terminal = end of the neuron
Synapse = gap/space between neurons

5. Question… What is the general pathway of a signal through a neuron?
Dendrites  cell body  Axon  Synaptic Terminals (then into the synapse to get to the next neuron or other cell)

6. Sending Signals
The “signal” sent through a neuron is an electrical signal
Based on the movement of ions into and out of the cell
Causes changes in the + and – charges inside the cell

7. A Neuron at Rest
A neuron at rest (unstimulated) has a difference in charge (voltage) across the plasma membrane = -70 mV = resting potential
This means that it is more negative inside than outside
The resting potential is caused by the distribution of ions on either side of the membrane

8. A Neuron at Rest
Resting potential (-70 mV) is maintained by the Sodium-Potassium Pump
Pumps Na+ out of cell
Pumps K+ into the cell
Active transport

9. Ion Concentrations at Rest
Inside neuron
Outside neuron
Na+
Lower
Higher
Cl- K+
At rest, there are also open K+ channels in the membrane  Allows some K+ to escape
Leaves negatively charged molecules behind (Cl- ions, etc.)  more negative on the inside than on the outside.

10. Sending a Signal: Action Potential
Na+ channels are embedded in the membrane of the neuron
Usually, these Na+ channels are closed, but can be triggered to open when the correct stimulus is received
Voltage gated channels = open in response to a particular change in voltage (charge)
Chemical gated channels = open in response to a chemical binding to them

11. Action Potential
STEP 1: To start an action potential, some kind of stimulus (light, pressure, chemical, etc.) causes Na+ channels in the dendrite to open.
This causes Na+ to flood into the neuron from outside  DEPOLARIZATION

12. Questions… Why does Na+ diffuse in from the outside?
Higher concentration on the outside
When depolarization occurs, how is the charge inside the neuron affected?
Becomes more positively charged inside
What would happen if Cl- channels are also opened?
Cl- would also flow in– makes the inside more negative (cancels out the charge from Na+ coming in) -- HYPERPOLARIZED

13. Action Potential
STEP 2: The change in voltage triggers the next Na+ channel (voltage gated channel) to open.
STEP 3: As Na+ diffuses down the neuron, it continues to trigger voltage gated Na+ channels to open.
This is what sends a signal down the neuron towards the axon terminal.

14. Action Potential
STEP 4: Na+ voltage gated channels only open temporarily. After a short period of time, they close and an inactivation gate opens to prevent them from opening again for a little while  REFRACTORY PERIOD

15. Action Potential
STEP 5: The neuron is “reset” (REPOLARIZED) by the opening of voltage gated K+ channels.
K+ flows out of the neuron, making the inside more negative again.
Why does K+ flow out?
Higher K+ concentrations inside neuron
The Na+/K+ pump also helps reestablish resting potential.

16. Saltatory Conduction
Depolarization & Repolarization happens over and over down the axon, so the nerve impulse travels.
Myelin sheaths insulate the axon, keeping ions from flowing out except at Nodes of Ranvier.

17. Saltatory Conduction
Wider axons yield faster conduction because there is less resistance.
Action potentials jump from one Node of Ranvier (space between myelin sheaths) to the next, speeding up the signal.

18. Communication between Neurons
When the signal reaches the axon terminal, it triggers voltage gated Ca2+ channels to open.
This causes vesicles that contain neurotransmitter molecules to fuse with the plasma membrane and expel the neurotransmitters into the synaptic cleft (space between neurons)

19. Communication between Neurons
The neurotransmitters will diffuse across the cleft and bind to receptors on the next neuron (postsynaptic neuron).
This triggers a Na+ chemical gated channel to open on the postsynaptic neuron, triggering an action potential in that neuron.

20. Communication between Neurons
After the signal has been sent, neurotransmitters are eliminated from the synaptic cleft by
Diffusion = diffuse away
Reuptake
Enzyme degradation

21. Communication between Neurons
Reuptake
Neurotransmitters are actively transported back into the presynaptic neuron  repackaged into vesicles to be released again
Recycling neurotransmitters 
Enzyme degradation
Enzymes in the synaptic cleft break down the neurotransmitter
Some medications work by inhibiting reuptake– ex. Prozac

22. Question…
Why is it important that our bodies / medications control nerve communication?
So that signals are only sent to neurons when needed.

23. Control of Communication
How can nerve communication be varied?
Change conduction of impulse
Change synaptic cleft size
Change volume of neurotransmitters or vesicles
Change number of ligand-gated ion channels on post synaptic neuron
Add a chemical that binds to ligand-gated ion channels to block them or always keep them open
Add a chemical that binds to neurotransmitters, so they cannot bind to the ligand-gated ion channels