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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


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

  • 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

  • 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

  • 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

  • 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


= protein

Na+

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Cl-

Cl-

Cl-

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aa-

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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

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


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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!


Gate

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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!


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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!


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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!


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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!


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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!


Na+

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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?

  • 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!


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Neuron is ready to fire again

resting potential


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

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5

Membrane potential

–20 mV

–30 mV

–40 mV

Hyperpolarization

(undershoot)

Threshold

–50 mV

–60 mV

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–70 mV

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Resting

Resting potential

–80 mV


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


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


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?


  • 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


Na+

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ACh

binding site

ion channel

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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:

  • 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

  • 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

  • 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…

  • 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?


Ponder this…Any Questions??


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