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Nervous System: The Neuron and the Transmission of a Nerve Impulse. Why do animals need a nervous system?. What characteristics do animals need in a nervous system? fast accurate reset quickly. Remember… think about the bunny… . Poor bunny !. Nervous system cells. Neuron a nerve cell.

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Nervous System: The Neuron and the Transmission of a Nerve Impulse

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Nervous system the neuron and the transmission of a nerve impulse l.jpg

Nervous System: The Neuron and the Transmission of a Nerve Impulse


Why do animals need a nervous system l.jpg

Why do animals need a nervous system?

  • What characteristics do animals need in a nervous system?

    • fast

    • accurate

    • reset quickly

Remember…think aboutthe bunny…

Poor bunny!


Nervous system cells l.jpg

Nervous system cells

  • Neuron

    • a nerve cell

signal

direction

dendrites

  • Structure fits function

    • many entry points for signal

    • one path out

    • transmits signal

cellbody

axon

signal direction

synaptic terminal

myelin sheath

dendrite  cell body  axon

synapse


Fun facts about neurons l.jpg

Fun facts about neurons

  • Most specialized cell in animals

  • Longest cell

    • blue whale neuron

      • 10-30 meters

    • giraffe axon

      • 5 meters

    • human neuron

      • 1-2 meters

Nervous system allows for 1 millisecond response time


Transmission of a signal l.jpg

Transmission of a signal

  • Think dominoes!

    • start the signal

      • knock down line of dominoes by tipping 1st one

         this triggers the signal

    • propagate the signal

      • do dominoes THEMSELVES move down the line?

         no, just a wave through them!

    • re-set the system

      • before you can do it again, have to set up dominoes again

         reset the axon


Transmission of a nerve signal l.jpg

Transmission of a nerve signal

  • Neuron has similar system

    • protein channels are “set up”

    • Once the first one is opened, the rest openin succession

      • all or nothing response

    • a “wave” action travels along neuron

    • have to re-set channels (dominos) so neuron can react again


Cells surrounded by charged ions l.jpg

= protein

Na+

Na+

K+

Na+

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Na+

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Na+

Na+

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Cl-

Cl-

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aa-

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aa-

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Cl-

Cl-

Cells: surrounded by charged ions

  • Cells live in a sea of charged ions

    • anions (negative)

      • are more concentrated within the cell

        • Cl-,

        • charged amino acids (aa-), proteins, phosphate groups, etc.

    • cations (positive)

      • are more concentrated in the extracellular fluid(outside the cell)

        • Na+

channel leaks K+

K+

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K+


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Because there’smore negative charges on the inside of the cellandmore positive charges on the outside of the cell…

  • cells have voltage!

  • Opposite charges on opposite sides of cell membrane

    • membrane is polarized

      • negative inside; positive outside

      • IF the charges could, they would “attempt to REACH EQUILIBRIUM

      • Thus, a charge gradient exists (also called a voltage gradient)

        • Remember concentration gradients?? It’s the same thing, only with electrical charge

      • This gradient is like stored / potential energy (like a battery)


Measuring cell voltage l.jpg

Measuring cell voltage

Voltage is measured as the charge difference between the outside and the inside of the cell.

In the case of an unfired neuron, this charge difference is -70 millivolts

unstimulated neuron = resting potential of -70mV


How does a nerve impulse travel l.jpg

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How does a nerve impulse travel?

  • 1st, it has to get started:

    • Stimulus: nerve is stimulated

      • This stimulus must be significant enough to reach threshold potential

        • This means that If the stimulus is strong enough, it will cause a Na+ channel in the cell membrane of the neuron to open

        • if it’s not strong enough, nothing will happen

      • Na+ ions will diffuse into the cell through the open channel at that location

    • Thus, at that point on the neuron, the electrical charges are reversed!

      • More positive inside; more negative outside

      • cell is now depolarized at that location

      • The “first domino” has gone down!

The 1stdomino goesdown!


How does a nerve impulse travel11 l.jpg

Gate

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channel open

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wave 

How does a nerve impulse travel?

  • Wave: nerve impulse travels down neuron

    • The initialchange in chargeopens the next Na+ gate down the line

      • “voltage-gated” channels –

        • channels that are triggered

          to open by a nearby change in voltage

        • this means that the voltage change

          caused when the first Na+ channel opened is

          the very thing that causes the next Na+

          channel to open, and so on….

    • As these Na+ ions continue to diffuse into cell

      a “wave” of depolartization moves down neuron = action potential

The restof thedominoes fall!


Impulse must go in the right direction toward the end of the axon l.jpg

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wave direction 

Impulse MUST go in the right direction – toward the END of the AXON

  • Re-set of the “dominos”: a 2nd wave travels down neuron

    • This time, K+ channels open

      • These K+ channels open right behind

        the Na+ channels that opened

    • K+ (POSTIVE!) ions diffuse OUT of cell

    • RE-SETS the voltage gradient so it’s like it was when it started

      • Positive to the outside

      • negative to the inside

Why is this resetting of the

dominos (voltage gradient)

important?

This Re-setting of the voltage

gradient right behind the

impulse keeps the impulse

From going backwards

Setdominoesback upquickly!


How does a nerve impulse travel13 l.jpg

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wave 

How does a nerve impulse travel?

  • Combined waves travel down neuron

    • wave of opening ion channels moves down neuron

    • signal moves in one direction     

      • ALWAYS from DENDRITEStoEND OF AXON

      • flow of K+ out of cell stops activation of Na+ channels in the wrong direction

Readyfornext time!


How does a nerve impulse travel14 l.jpg

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wave 

How does a nerve impulse travel?

  • Action potential propagates

    • wave = nerve impulse, oraction potential

    • Goes from brainto finger tips in milliseconds!

Action Potential

In theblink ofan eye!


Voltage gated channels l.jpg

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Voltage-gated channels

  • Ion channels open & close in response to changes in charge across membrane

    • Na+ channels open quickly in response to depolarization & close slowly

    • K+ channels open slowly in response to depolarization & close slowly

Structure& function!


But houston we have a problem l.jpg

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wave 

But Houston, We have a Problem…

  • Even though we’ve reset the dominos (- inside, + outside)…

  • The Na+ and K+ are now on the WRONG SIDES of the membrane from where they started!

    • Na+ needs to move back out

    • K+ needs to move back in

    • To do so, both must move against their concentration gradients

      • We need a pump!!

A lot ofwork todo here!


How does the nerve re set itself l.jpg

How does the nerve re-set itself?

  • Sodium-Potassium pump

    • active transport protein in membrane

      • requires ATP

    • 3 Na+ pumped out

    • 2 K+ pumped in

    • re-sets proper Na and K

      concentrations across membrane

ATP

That’s a lot of ATP !

Feed me somesugar quick!


Neuron is ready to fire again l.jpg

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Neuron is ready to fire again

resting potential


Action potential graph l.jpg

Action potential graph

  • Resting potential

  • Stimulus reaches threshold potential

  • DepolarizationNa+ channels open; K+ channels closed

  • Na+ channels close; K+ channels open

  • Repolarizationreset charge gradient

  • UndershootK+ channels close slowly

40 mV

4

30 mV

20 mV

Depolarization

Na+ flows in

Repolarization

K+flows out

10 mV

0 mV

–10 mV

3

5

Membrane potential

–20 mV

–30 mV

–40 mV

Hyperpolarization

(undershoot)

Threshold

–50 mV

–60 mV

2

–70 mV

1

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Resting

Resting potential

–80 mV


Myelin sheath l.jpg

Myelin sheath

  • Axon coated with Schwann cells

    • insulates axon

    • speeds signal

      • signal hops from node to node

      • saltatory conduction

        • Move by hopping over large chunks of neuron rather than having to touch each bit of it…

    • Impulse travels at

      • 150 m/sec rather than 5 m/sec(330 mph vs. 11 mph)

signal

direction

Wow!

myelinsheath


Slide21 l.jpg

action potential

saltatory

conduction

Na+

myelin

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axon

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Na+

  • Multiple Sclerosis

    • immune system (T cells) attack myelin sheath

    • loss of signal


Synapse l.jpg

What happens at the end of the axon?

Synapse

Impulse has to jump the synapse!

  • space between neurons

  • Impulse has to jump quickly from one cell to next

How does the wavejump the gap?


Chemical synapse events l.jpg

  • action potential reaches the end of the axon and depolarizes membrane there.

  • Depolarization causes Ca++ channels at end of axon to OPEN

  • Calcium ions stimulate vesicles containing neurotransmitters to fuse with the membrane at the end of the axon.

  • Vesicles then release neurotransmitter into synapse

  • Neurotransmitters  DIFFUSE across synapse

  • neurotransmitter binds with protein receptor on cell on OTHER SIDE of synapse

    • Causes ion-gated channels on other side open

  • neurotransmitter in the receptor is then degraded or recycled

Chemical Synapse Events:

axon terminal

action potential

synaptic vesicles

synapse

Ca++

neurotransmitteracetylcholine (ACh)

receptor protein

muscle cell (fiber)

We switched…

from an electrical signal

to a chemical signal


Nerve impulse travels to next cell l.jpg

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binding site

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Nerve impulse travels to next cell

K+

  • Next cell MAY be:

    • Post-synaptic neuron or effector/muscle cell (cell that actually DOES something besides transmitting an impulse)

  • Neurotransmitter is received by and opens ion-gated channels

    • Na+ diffuses into cell

    • K+ diffuses out of cell

      • switch back to voltage-gated channel

Here wego again!


Some well known neurotransmitters l.jpg

Some well known Neurotransmitters:

  • Acetylcholine

    • transmit signal to skeletal muscle

  • Epinephrine (adrenaline) & norepinephrine

    • fight-or-flight response

  • Dopamine

    • widespread in brain

    • affects sleep, mood, attention & learning

    • lack of dopamine in brain associated with Parkinson’s disease

    • excessive dopamine linked to schizophrenia

  • Serotonin

    • widespread in brain

    • affects sleep, mood, attention & learning

Different neurotransmitters are involved in different responses


Neurotransmitters l.jpg

Neurotransmitters

  • Weak point of nervous system

    • any substance that affects neurotransmitters or mimics them affects nerve function

      • gases: nitrous oxide, carbon monoxide

      • mood altering drugs:

        • stimulants

          • amphetamines, caffeine, nicotine

        • depressants

          • barbiturates

      • hallucinogenic drugs: LSD

      • SSRIs: Prozac, Zoloft, Paxil

      • poisons


Acetylcholinesterase l.jpg

Acetylcholinesterase

  • Enzyme which breaks downacetylcholine neurotransmitter

    • acetylcholinesterase inhibitors = neurotoxins

      • snake venom, sarin, insecticides

neurotoxin in green

active site in red

snake toxin blockingacetylcholinesterase active site

acetylcholinesterase


Questions to ponder l.jpg

Questions to ponder…

  • Why are axons so long?

  • Why have synapses at all?

  • How do “mind altering drugs” work?

    • caffeine, alcohol, nicotine, marijuana…

  • Do plants have a nervous system?

    • Do they need one?


Slide29 l.jpg

Ponder this…Any Questions??


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