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Nervous System

AP Biology

AP Biology

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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!

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Nervous system cells

dendrites

cell body

axon

synaptic terminal

  • Neuron
    • a nerve cell
  • Structure fits function
    • many entry points for signal
    • one path out
    • transmits signal

signal direction

signal

direction

dendrite → cell body → axon

synapse

myelin sheath

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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

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Transmission of a signal

  • Think dominoes!
    • start the signal
      • knock down line of dominoes by tipping 1st one

→ trigger the signal

    • propagate the signal
      • do dominoes 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

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Transmission of a nerve signal

  • Neuron has similar system
    • protein channels are set up
    • once first one is opened, the rest open in succession
      • all or nothing response
    • a “wave” action travels along neuron
    • have to re-set channels so neuron can react again

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Cells: surrounded by charged ions

  • Cells live in a sea of charged ions
    • anions (negative)
      • more concentrated within the cell
      • Cl-, charged amino acids (aa-)
    • cations (positive)
      • more concentrated in the extracellular fluid
      • Na+

Na+

Na+

Na+

Na+

Na+

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Na+

Na+

K+

Na+

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Cl-

K+

Cl-

Cl-

Cl-

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aa-

K+

Cl-

Cl-

aa-

aa-

aa-

aa-

aa-

K+

K+

channel �leaks K+

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–

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Cells have voltage!

  • Opposite charges on opposite sides of cell membrane
    • membrane is polarized
      • negative inside; positive outside
      • charge gradient
      • stored energy (like a battery)

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AP Biology

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Measuring cell voltage

unstimulated neuron = resting potential of -70mV

AP Biology

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How does a nerve impulse travel?

  • Stimulus: nerve is stimulated
    • reaches threshold potential
      • open Na+ channels in cell membrane
      • Na+ ions diffuse into cell
    • charges reverse at that point on neuron
      • positive inside; negative outside
      • cell becomes depolarized

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Na+

The 1st�domino �goes�down!

AP Biology

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How does a nerve impulse travel?

  • Wave: nerve impulse travels down neuron
    • change in charge opens �next Na+ gates down the line
      • “voltage-gated” channels
    • Na+ ions continue to diffuse into cell
    • “wave” moves down neuron = action potential

Gate

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wave →

The rest�of the�dominoes �fall!

AP Biology

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How does a nerve impulse travel?

  • Re-set: 2nd wave travels down neuron
    • K+ channels open
      • K+ channels open up more slowly than Na+ channels
    • K+ ions diffuse out of cell
    • charges reverse back at that point
      • negative inside; positive outside

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Na+

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wave →

Set�dominoes�back up�quickly!

AP Biology

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How does a nerve impulse travel?

  • Combined waves travel down neuron
    • wave of opening ion channels moves down neuron
    • signal moves in one direction → → → → →
      • flow of K+ out of cell stops activation of Na+ channels in wrong direction

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Na+

wave →

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Ready�for�next time!

AP Biology

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How does a nerve impulse travel?

  • Action potential propagates
    • wave = nerve impulse, or action potential
    • brain → finger tips in milliseconds!

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In the�blink of�an eye!

AP Biology

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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

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Structure�& function!

AP Biology

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How does the nerve re-set itself?

  • After firing a neuron has to re-set itself
    • Na+ needs to move back out
    • K+ needs to move back in
    • both are moving against concentration gradients
      • need a pump!!

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wave →

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A lot of�work to�do here!

AP Biology

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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 charge�across �membrane

ATP

That’s a lot �of ATP !

Feed me some�sugar quick!

AP Biology

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Neuron is ready to fire again

Na+

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aa-

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resting potential

AP Biology

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Action potential graph

  • Resting potential
  • Stimulus reaches threshold potential
  • Depolarization �Na+ channels open; �K+ channels closed
  • Na+ channels close; �K+ channels open
  • Repolarization�reset charge gradient
  • Undershoot�K+ channels close slowly

–70 mV

–60 mV

–80 mV

–50 mV

–40 mV

–30 mV

–20 mV

–10 mV

0 mV

10 mV

Depolarization

Na+ flows in

20 mV

30 mV

40 mV

Repolarization

K+ flows out

Threshold

Hyperpolarization

(undershoot)

Resting potential

Resting

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Membrane potential

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Myelin sheath

signal

direction

  • Axon coated with Schwann cells
    • insulates axon
    • speeds signal
      • signal hops from node to node
      • saltatory conduction
    • 150 m/sec vs. 5 m/sec�(330 mph vs. 11 mph)

myelin sheath

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myelin

axon

Na+

Na+

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action potential

saltatory

conduction

Multiple Sclerosis

    • immune system (T cells) attack myelin sheath
    • loss of signal

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Synapse

Impulse has to jump the synapse!

    • junction between neurons
    • has to jump quickly from one cell to next

What happens at the end of the axon?

How does � the wave�jump the gap?

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Chemical synapse

axon terminal

synaptic vesicles

muscle cell (fiber)

neurotransmitter�acetylcholine (ACh)

receptor protein

Ca++

synapse

action potential

  • Events at synapse
    • action potential depolarizes membrane
    • opens Ca++ channels
    • neurotransmitter vesicles fuse with membrane
    • release neurotransmitter to synapse → diffusion
    • neurotransmitter binds with protein receptor
      • ion-gated channels open
    • neurotransmitter degraded or reabsorbed

We switched…

from an electrical signal

to a chemical signal

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Nerve impulse in next neuron

  • Post-synaptic neuron
    • triggers nerve impulse in next nerve cell
      • chemical signal opens ion-gated channels
      • Na+ diffuses into cell
      • K+ diffuses out of cell
        • switch back to �voltage-gated channel

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Na+

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ion channel

binding site

ACh

Here we�go again!

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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

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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
          • quaaludes, barbiturates
      • hallucinogenic drugs: LSD, peyote
      • SSRIs: Prozac, Zoloft, Paxil
      • poisons

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Acetylcholinesterase

  • Enzyme which breaks down�acetylcholine neurotransmitter
    • acetylcholinesterase inhibitors = neurotoxins
      • snake venom, sarin, insecticides

snake toxin blocking�acetylcholinesterase active site

acetylcholinesterase

active site �in red

neurotoxin �in green

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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?

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Ponder this…�Any Questions??

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