1. Transmission of Nerve Impulses
UNIT B
Chapter 12: Nervous System
Section 12.2
Transmission of Nerve Impulses
Nerve impulse: how nervous system conveys information
The nervous system uses the nerve impulse to convey information.
nerve impulse: a type of impulse used by the nervous system to convey information
Oscillascope: one electrode inside, one outside axon
On a voltmeter, voltage is displayed as a trace (pattern) over time
the two points are in the inside (axoplasm) and the outside of the axons
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2. Resting and Action Potential
Resting potential: More NA+ outside , more K+ inside axon.
oscilloscope records resting potential of -65mV. Inside to be neg compared to outside due to ions.
Action Potential: Na+ gates open, Na+ moves in causing depolarization, repolarization occurs when K+ gates open and K+ moves out.

3. Voltage: electrical potential difference between two points (mV)
UNIT B
Chapter 12: Nervous System
Section 12.2
Voltage: electrical potential difference between two points (mV)
Resting Membrane Potential: is the potential difference across the membrane in a resting neuron
-70mV
The polarity of the resting axonal membrane is due to a difference in ion distribution on each side.
Greater [ Na+] outside
Greater [K+] inside
Why?
resting potential: the membrane potential of an inactive neuron
In an axon that is not conducting an impulse, the voltmeter records a potential difference across an axon membrane equal to -70mV.
This reading, known as the resting potential, shows that the inside of the axon is negative compared to the outside (there is polarity across the axonal membrane)
The resting potential is the potential difference across the membrane in a resting neuron
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4. Why? UNIT B Chapter 12: Nervous System Section 12.2
The concentration of Na+ is greater outside the axon than inside
The concentration of K+ is greater inside the axon than outside
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5. Q: Why is there a Potential Difference Across a Membrane?
UNIT B
Chapter 12: Nervous System
Section 12.2
Q: Why is there a Potential Difference Across a Membrane?
A: Sodium-potassium pumps
membrane is permeable to Na+ and K+
but more permeable to K+,
therefore there are
always more positive
ions outside
membrane than inside
sodium-potassium pump: a carrier protein that moves sodium ions to the outside of a cell and potassium ions to the inside; especially in nerve and muscle cells.
This unequal distribution is maintained by carrier proteins called sodium-potassium pumps, which actively transport Na+ out of the axon and K+ into the axon
The pumps are always working because the membrane is permeable to Na+ and K+
The membrane is more permeable to K+, therefore there are always more positive ions outside the membrane than inside
Negatively charged organic ions on the inside of the axon also contribute to the polarity across a resting axonal membrane
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6. Action Potential: Rapid change in polarity across axonal membrane.
UNIT B
Chapter 12: Nervous System
Section 12.2
Action Potential:
Rapid change in polarity across axonal membrane.
Requires two gated channel proteins:
Sodium channel lets Na+ into the axon
Potassium channel lets K+ out
action potential: electrochemical changes that take place across the axon membrane; the nerve impulse
Rapid change in polarity across an axonal membrane as the nerve impulse occurs.
Is an all-or-none phenomenon: if a stimulus causes the membrane to depolarize to a certain level (threshold), an action potential occurs
The strength of an action potential does not change, but an intense stimulus can cause an axon to fire (start an action potential) more often
Requires two gated channel proteins in the membrane:
One channel protein allows Na+ to pass into the axon
One channel protein allows K+ to pass out of the axon
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7. All-or-none phenomenon:
A certain level of stimulus
(threshold) must be reached
before the voltage gated
channels will open.
Strength of an action potential does not change
But frequency can: an intense stimulus can cause an axon to fire more often (causing more action potentials)
Figure Action Potential. c. The changes in the transmembrane potential of the axon are a result of sodium ions flowing into the axon and potassium ions flowing out. An action potential lasts only a few seconds

8. Action Potential: Sequence of Events
UNIT B
Chapter 12: Nervous System
Section 12.2
Action Potential: Sequence of Events
1. Sodium Gates Open (Depolarization)
Na+ flows down its concentration gradient into the axon
 membrane potential changes from -70 mV to +35 mV
Called depolarization because the charge inside the axon changes from negative to positive
When an action potential begins, sodium channel gates open, and Na+ flows down its concentration gradient into the axon
As Na+ moves inside the axon, the membrane potential changes from -70 mV to +35 mV
This is called depolarization because the charge inside the axon changes from negative to positive
Figure Action Potential. a. The action potential begins as the sodium gates (purple) open and Na+ ions move into the axon through facilitated diffusion. This is depolarization as the membrane potential jumps from −70 to +35 millivolts.
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9. Action Potential: Sequence of Events
UNIT B
Chapter 12: Nervous System
Section 12.2
Action Potential: Sequence of Events
2. Potassium Gates Open (Repolarization)
K+ flows down its concentration gradient
 the action potential becomes more negative again (repolarization)
During this time, it briefly becomes slightly more negative that its original resting potential (hyperpolarization)
Figure Action Potential. b. The repolarization of a neuron occurs as the potassium gates (orange) open and K+ ions move out of the axon through facilitated diffusion.
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10. What stops the signal from going both ways?
UNIT B
Chapter 12: Nervous System
Section 12.2
Action potentials in non-myelinated axons:
Travels down axon one small section at a time, depolarizing and repolarizing.
What stops the signal from going both ways?
As action potential travels down an axon, each successive portion depolarizes and then repolarizes. Each preceding portion cause an action potential in the next section.
refractory period: in non-myelinated axons, a period in which sodium gates are unable to open following an action potential
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11. Q: What stops the signal from going both ways?
A: Refractory period:
Period of time when sodium gates are unable to open
Prevents action potential from moving backward; it always moves down an axon
Limiting factor for next impulse is the speed with which sodium ion can be pumped back outside the neuron membrane.
Refractory period:
Occurs in section of axon after action potential has moved on
sodium gates are unable to open
Prevents action potential from moving backward; it always moves down an axon
When the refractory period is over, the sodium-potassium pump has restored the ion distribution by pumping Na+ out of the axon and K+ into the axon

12. Action potentials in myelinated axons
UNIT B
Chapter 12: Nervous System
Section 12.2
Action potentials in myelinated axons
The gated ion channels that produce an action potential are concentrated at the nodes of Ranvier
Ion exchange only occurs at these nodes, therefore the action potential travels faster than in non-myelinated axons
The action potential appears to “jump” from node to node (saltatory conduction)
Latin saltare, to hop or leap
saltatory conduction: occurs in myelinated axons when the action potential “jump
Reaction TimeMany factors have been shown to affect reaction times, including age, gender, physical fitness, fatigue, distraction, alcohol, personality type, and whether the stimulus is auditory or visual.
As sodium rushes into the node it creates an electrical force which pushes on the ions already inside the axon. This rapid conduction of electrical signal reaches the next node and creates another action potential, thus refreshing the signal. In this manner, saltatory conduction allows electrical nerve signals to be propagated long distances at high rates without any degradation of the signal. Although the action potential appears to jump along the axon, this phenomenon is actually just the rapid, almost instantaneous, conduction of the signal inside the myelinated portion of the axon.
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13. Myelin Sheath Increases speed nerve impulse,
helps in reduces energy expenditure over the axon membrane as a whole, (less Na+ and K+ needed to be pumped to bring the concentrations back to the resting state after each action potential)

14. Crash Course: Action Potential

15. An action potential cannot cross a synapse, so how does the action potential get to the next neuron?
Every axon branches into endings that have a small swelling called an axon terminal
Each terminal lies close to the dendrite or cell body of another neuron or a muscle cell
This region of close proximity is called a synapse or chemical synapse
Membrane of the first neuron: presynaptic membrane
Membrane of the second neuron: postsynaptic membrane
Two neurons at a synapse do not physically touch each other; they are separated by a tiny gap called the synaptic cleft

16. Every axon branches into endings that have a small swelling called an axon terminal
Each terminal lies close to the dendrite or cell body of another neuron or a muscle cell
This region of close proximity is called a synapse or chemical synapse
Membrane of the first neuron: presynaptic membrane
Membrane of the second neuron: postsynaptic membrane
Two neurons at a synapse do not physically touch each other; they are separated by a tiny gap called the synaptic cleft

17. Transmission Across a Synapse
UNIT B
Chapter 12: Nervous System
Section 12.2
Transmission Across a Synapse
axon terminal – small swelling at end of axon
synapse - region of close proximity (chemical or electrical)
presynaptic membrane- membrane of the first neuron
postsynaptic membrane - membrane of the second neuron:
synaptic cleft – gap between
neurotransmitters - chemicals stored in the synaptic vesicles in axon terminals
synapse: the region of close proximity between an axon terminal and the dendrite or cell body of another neuron or muscle cell
synaptic cleft: a tiny gap separating two neurons at a synapse
Every axon branches into endings that have a small swelling called an axon terminal
Each terminal lies close to the dendrite or cell body of another neuron or a muscle cell
This region of close proximity is called a synapse or chemical synapse
Membrane of the first neuron: presynaptic membrane
Membrane of the second neuron: postsynaptic membrane
Two neurons at a synapse do not physically touch each other; they are separated by a tiny gap called the synaptic cleft
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18. When an action potential arrives at an axon terminal:
UNIT B
Chapter 12: Nervous System
Section 12.2
When an action potential arrives at an axon terminal:
Gated channels for Ca2+ open, Ca2+ flows in,
Ca2+ binds to contractile proteins, which contract and pull the synaptic vesicles to the presynaptic
membrane synaptic vesicles
merge with the presynaptic
membrane  exocytosis NT released
Neurotransmitter molecules released into synaptic cleft  diffuse across the synapse to the postsynaptic membrane, where they bind to specific receptors
Many NT can bind to more than one type of receptor  inhibitory or excitatory
Types of Receptors: Ionotropic (ligand gated) Na or Ca allowed in – excitatory and are brief and local.
Metabotropic (not ion channels ) instead activate second messengers. Slower but larger effect, exponential spread.
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19. Excitatory signals: cause a depolarizing effect
UNIT B
Chapter 12: Nervous System
Section 12.2
Excitatory signals: cause a depolarizing effect
Inhibitory signals: cause a hyperpolarizing effect
Synaptic integration is the summing up of the excitatory and inhibitory signals in a postsynaptic neuron. If the combined signals cause membrane potential to rise above threshold, an action potential occurs
integration: the summing up of excitatory and inhibitory signals by a neuron
Figure 12.6 Synaptic integration.
a. Inhibitory signals and excitatory signals are summed up in the dendrites and cell body of the. postsynaptic neuron. Only if the combined signals cause the membrane potential to rise above threshold does an action potential occur. b. In this example, threshold was not reached
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20. Q: What happens to neurotransmitter after it has done its job?

21. Broken down by enzymes in the postsynaptic membrane
UNIT B
Chapter 12: Nervous System
Section 12.2
Neurotransmitters
To prevent continuous stimulation, neurotransmitter needs to be removed:
Broken down by enzymes in the postsynaptic membrane
Reabsorbed (reuptake) by presynaptic membrane
Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft.
This prevents continuous stimulation (or inhibition) of postsynaptic membranes
In some synapses, the postsynaptic membrane contains enzymes that break down the neurotransmitter
In other synapses, the presynaptic membrane reabsorbs the neurotransmitter for repackaging in synaptic vesicles
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22. Neurotransmitters
Amino acids: GABA, glutamate, aspartate, D-serine,, glycine. Monoamines: dopamine, norepinephrine, epinephrine , histamine, serotonin, Peptides: Opioids, endorphins Gasotransmitters: nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S) Other: Acetylcholine (used by motor neurons for muscle contraction)
glutamate, aspartate, D-serine, γ-aminobutyric acid (GABA), glycine.
Gasotransmitters:
nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S)
Monoamines:
dopamine (DA), norepinephrine (noradrenaline; NE, NA), epinephrine (adrenaline), histamine, serotonin (SER,)
Peptides;
Opioids, endorphins

23. Agonist
chemical that binds to and activates the receptor to produce a biological response.
causes an action.
Antagonist blocks action
Eg. Botox prevents the release of the chemical acetylcholine

24. Drugs effect on Neurotransmitters:
UNIT B
Chapter 12: Nervous System
Section 12.2
Drugs effect on Neurotransmitters:
Enhance release
Block release
Mimic the NT
Block the receptor
Interfere with the removal
Example:
Sarin gas is a chemical weapon that inhibits acetylcholinesterase (AChE), an enzyme that is responsible for the breakdown of acetylcholine (ACh)
prolonged convulsive spasms
Many drugs that affect the nervous system act by interfering or enhancing the action of neurotransmitters.
Serotonin is a inhibitory neurotransmitter (and mood stabilizer). SSRI’s eases depression by increasing levels of serotonin in the brain. SSRIs block the reabsorption (reuptake) of serotonin in the brain, making more serotonin available.
Botox
Cumarin
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25. Opioids
Block pain receptors. Morphine, heroin  analgesia Inhibit pain signal at multiple steps in the pathway (brain, spinal cord and periphery) . In the brain opioids have mood altering effects, cause sedation, and can even decrease emotional response to pain. ENDOPHINS (ENDO genous moRPHIN e)
Pain - nosireceptors
glutamate and substance P
Opioids receptors in both pre and post synaptic membrane.
Pre: decreases amount of Ca released  less exc NT released.
Post: decrease response to any NT coming in. less pain.
Why would we even have opiod receptors? We have a built it (endogenous) analgesic system that modulates pain signals, like endorphins (ENDOgenous morphine). Exogenous opiods are stronger.
Mu, kappa, delta receptors. Opioid agonsit

26. Pain

27. Addiction
SciShow

28. Golden poison frog (Phyllobates terribilis)
Harbours enough poison to kill 10 grown men.
called neurotoxins, affect the nervous system; others, ... and symptoms include muscle paralysis and a low heart rate (bradycardia). ... venomous, such as puffer fish, blue-ringed octopuses and poison dart frogs.
batrachotoxin

29. Sydney Funnel Web Spider
atracotoxin does the opposite. It hyper-stimulates the nervous system to the point of overload. As the toxin works its way through a victim’s body, it elevates blood pressure, eventually causing the millions of air sacs in the lungs to burst (pulmonary edema), a condition which causes you to effectively drown on dry land. 


the nervous system to the point of overload. As the toxin works its way through a victim’s body, it elevates blood pressure, eventually causing the millions of air sacs in the lungs to burst (pulmonary edema), a condition which causes you to effectively drown on dry land. Harmless to most mammals except primates.

30. Brazilian Wandering Spider
PhTx3 targets the calcium channel in neural synapses with the effect of causing involuntary muscle control, especially around the respiratory system, leading to asphyxiation by preventing your diaphragm from contracting. Basically, you suffocate to death

31. Muscle Paralysis Found in the Cone Snail
toxin works by exciting acetylcholine receptors which eventually causes a heart attack

32. Puffer Fish
Tetrodotoxin 100 times more deadly than potassium cyanide, is found in blue ringed octopus, a few newt species, and an entire family of sea snails. actually produced by bacteria which has developed a symbiotic relationship with all of these different marine animals.

33. Check Your Progress UNIT B
Chapter 12: Nervous System
Section 12.2
Check Your Progress
Describe the activity of the sodium-potassium pump present in neurons.
Explain how the changes in Na+ and K+ ion concentrations that occur during an action potential are associated with depolarization and repolarization.
Define refractory period, saltatory conduction, and synaptic integration.
ANSWERS
1. The sodium-potassium pump in neurons are always transporting Na+ ions to the outside and K+ ions to the inside of the cell. This is active transport and requires an expenditure of ATP energy.
2. During an action potential, the sodium voltage-gated channel proteins open and Na+ ions enter the cell, causing a depolarization. The potential difference across the membrane rises from −70 mV to +35 mV. Then the sodium gates close, the potassium voltage-gated channel proteins open, and K+ ions enter the cell. This causes a repolarization and, briefly, a slight hyperpolarization.
3. refractory period—in nonmyelinated axons, a period in which sodium gates are unable to open following an action potential; saltatory conduction—a type of conduction in mylenated axons when the action potention “jumps” from node to node”; synaptic integration—the summation of all incoming excitatory and inhibitory messages at the cell body of a neuron.
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