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

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

2. 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!

3. 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

4. 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

5. 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

6. 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

7. = protein Na+ Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ K+ Cl- Cl- Cl- aa- aa- K+ aa- Cl- aa- aa- aa- K+ 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+ + – K+

8. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – 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)

9. 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

10. + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ 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!

11. Gate + + – + channel closed channel open + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ 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!

12. K+ + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ 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!

13. K+ + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ 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!

14. K+ + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ 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!

15. K+ + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ wave  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!

16. Na+ Na+ Na+ Na+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ Na+ Na+ Na+ + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – K+ Na+ Na+ K+ Na+ K+ Na+ K+ Na+ K+ K+ Na+ K+ K+ K+ Na+ 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!

17. 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!

18. Na+ Na+ Na+ Na+ Na+ Na+ + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ Na+ Na+ Na+ Na+ Na+ Na+ K+ K+ K+ K+ aa- aa- K+ aa- K+ aa- aa- aa- K+ K+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ Neuron is ready to fire again resting potential

19. 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 6 Resting Resting potential –80 mV

20. 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

21. action potential saltatory conduction Na+ myelin + – axon + + + – + Na+ • Multiple Sclerosis • immune system (T cells) attack myelin sheath • loss of signal

22. 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?

23. 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

24. Na+ Na+ ACh binding site ion channel + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – K+ Na+ K+ Na+ 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!

25. 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

26. 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

27. 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

28. 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?

29. Ponder this…Any Questions??