Chapter 44 Neurons and Nervous Systems

1. Chapter 44Neurons and Nervous Systems

2. Nervous System Cells • Neuron • a nerve cell dendrites signal direction • Structure fits function • many entry points for signal • one path out • transmits signal cellbody axon signal direction synapse dendrite  cell body axon  terminal branches 

3. Types of Neurons cell body sensory neuron “afferent” cell body axon interneuron “associative” dendrites dendrites cell body motor neuron “efferent”

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 ~ millisecond response times

5. Transmission of a Nerve 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

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

7. 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 • more concentrated within the cell • Cl-, charged amino acids (aa-) • cations • more concentrated in the extracellular fluid • K+, Na+ channel leaks K+ K+ + – K+

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

9. Measuring Cell Voltage unstimulated neuron = resting potential of ~60 mV

10. The 1stdomino goesdown! + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ 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

11. Gate + + – + channel closed channel open The restof thedominoes fall! + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ wave  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

12. Setdominoesback upquickly! K+ + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ wave  How does a nerve impulse travel? • Re-set: 2nd wave travels down neuron • K+ channels open • K+ channels up more slowly than Na+ channels • K+ ions diffuse out of cell • charges reverse back at that point • negative inside; positive outside

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

14. In theblink ofan eye! K+ + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – Na+ wave  How does a nerve impulse travel? • Action potential propagates • wave = nerve impulse, oraction potential • brain  finger tips in milliseconds!

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

16. Na+ Na+ Na+ Na+ Na+ A lot ofwork todo here! 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  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!!

17. That’s a lot of ATP! Feed me some sugar quick! Wait a second… I’m nothing more than crystallized high fructose syrup! uh-oh… How does the nerve re-set itself? • Na+ / K+ pump • active transport protein in membrane • requires ATP • 3 Na+ pumped out • 2 K+ pumped in • re-sets chargeacross membrane ATP

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 • K+ channels open; Na+ channels close; • Repolarizationreset charge gradient • Undershoot: K+ 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 by Schwann cells • insulate axon • speeds signal • signal hops from node to node • saltatory conduction • 150 m/sec vs. 5 m/sec(330 mph vs. 11 mph) signal direction myelinsheath

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

22. How does the wavejump the gap? What happens at the end of the axon? synapse • Impulse has to jump the synapse! • junction between neurons • has to jump quickly from one cell to next

23. We switched… from an electrical signal to a chemical signal • Events at synapse • action potential depolarizes membrane • opens Ca++ channels • neurotransmitter vesicles fuse with membrane • release neurotransmitter to synaptic cleft • neurotransmitter binds with protein receptor • ion-gated channels open • neurotransmitter degraded or reabsorbed The Synapse axon terminal action potential synaptic vesicles synapse Ca++ neurotransmitteracetylcholine (ACh) receptor protein muscle cell (fiber)

24. Acetylcholinesterase • Enzyme which breaks down acetylcholine neurotransmitter • neurotoxins = inhibitors • snake venom, sarin, insecticides neurotoxin in green active site in red snake toxin blockingacetylcholinesterase active site acetylcholinesterase

25. Na+ Na+ ACh binding site Here wego again! ion channel + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – + + – – K+ Na+ K+ Na+ 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

26. Neurotransmitters • Acetylcholine • transmit signal to skeletal muscle • Dopamine • widespread in brain • lack of dopamine in brain associated with Parkinson’s disease • excessive dopamine linked to schizophrenia • pleasure & reward pathways • Serotonin • widespread in brain • affects sleep, mood, attention & learning

27. 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 • hallucinogenic drugs • Prozac • poisons

29. 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?

30. Muscle Contraction • Nerve signal stimulates muscle cell’s sarcoplasmic reticulum (SR) to release stored Ca+2

31. Ca+2 Triggers Muscle Action • At rest, tropomyosin blocks myosin-binding sites on actin • Ca+2 binds to troponin complex • shape changecauses movement of tropomyosin-troponin complex • exposes myosin-binding sites on actin

32. How Ca+2Controls Muscle • Sliding filament model • exposed actin binds to myosin • fibers slide past each other • ratchet system • shorten muscle cell • muscle contraction • muscle doesn’t relax until Ca+2 is pumped back into SR • requires ATP ATP ATP

33. How it all works… • Action potential causes Ca+2 release from SR • Ca+2 binds to troponin • Troponin moves tropomyosin uncovering myosin binding site on actin • Myosin binds actin • uses ATP to "ratchet" each time • releases, "unratchets" & binds to next actin • Myosin pulls actin chain along • Sarcomereshortens • Z lines move closer together • Whole fiber shortens  contraction! • Ca+2 pumps restore Ca+2 to SR relaxation! • pumps use ATP ATP ATP

34. Put it all together… 1 2 3 ATP 7 4 6 ATP 5

35. associative neurons nerve cords radial nerve nerveribs nerve net FlatwormPlatyhelminthes Cnidarian Echinoderm Cephalization = Brain Evolution Cephalization= clustering of neurons in “brain” at front (anterior) end of bilaterally symmetrical animals  where sense organs are More organization but still based on nerve nets; supports more complex movement Simplest, defined central nervous system more complex muscle control Simplest nervous system no control of complex actions

36. giant axon central nervous system brain brain ventral nerve cords peripheral nerves Mollusk Earthworm Arthropod Cephalization = Brain Evolution • increase in interneurons in brain region More complex brains connected to all other parts of body by peripheral nerves More complex brains in predators most sophisticated invertebrate nervous system Further brain development ganglia = neuron clusters along CNS

37. Shark Frog Crocodile Cat Human Spinal cord Hind: Medulla oblongata Hind: Cerebellum Optic tectum Bird Midbrain Fore: Cerebrum Olfactory tract Evolution of Vertebrate Brain forebraindominant cerebrum forebrain hindbrain forebrain

38. Any Questions??