Understanding the Nervous System Functions and Neurons

1. Nervous system

2. Nervous system • 2 main parts • 1) Central Nervous System (CNS) • Brain: central processing center • Spinal cord: extends down back below brain

3. Nervous system • 2 main parts • 2) Peripheral Nervous System (PNS) • Connects CNS to muscles, sensory cells, etc. • 12 pairs of cranial nerves (attached to brain) • _________ pairs of spinal nerves (attached to spinal cord)

4. Nervous system • Functions: • 1) Monitors changes in external and internal environment • Receptors: gather information

5. Nervous system • Functions: • 1) Monitors changes in external and internal environment • Receptors: gather information • 2) Sends messages in response to information gathered • Effectors: cells that respond to message (glands, muscles)

6. Nervous system • 3 types of nerve cells (neurons) • Sensory neurons (act as receptors) • Found in skin, organs. Cell body of neuron outside of CNS. Long dendrites, ____________ axons

7. Nervous system • 3 types of nerve cells (neurons) • Motor neurons (carry message to effectors) • Cell bodies contained within CNS. Short dendrites, long axons

8. Nervous system • 3 types of nerve cells (neurons) • Association neurons (often link sensory and motor neurons) • Cell bodies in CNS. Connect motor and sensory neurons. Connect sensory neurons to ____________

9. The neuron • Functional unit of nervous system • Neuron: dendrite (incoming messages), cell body, axon (outgoing message)

10. Neuroglia • Neuroglia (supporting cells) • Schwann cells (PNS) and oligodendrocytes (CNS) • Form myelin sheath, Nodes of Ranvier

11. Neuroglia • Myelinated axons are whitish: called white matter in nerves/spinal cord/brain • Unmyelinated axons/dendrites and cell bodies are grayish: called gray matter

12. Neuroglia • Secrete myelin (80% ___________) • Cells wrap axon in a whitish “jelly roll”: myelin sheath

13. Neuroglia • Functions: • 1) support and protect nerve cell from injury • 2) nourish nerve cell (lies between blood vessels and neurons) • 3) electrical insulator: increases rate of impulse, insulates against firing of other neurons • 4) aids in regeneration of sensory nerves after ______________

14. Neuroglia • Example of importance: Multiple Sclerosis (MS) • 2 million people worldwide • Body’s immune system attacks myelin sheaths • Result: poor nerve impulse conduction. Lose ability to _____________ muscles • Cause? Uncertain: genetics, viral infection

15. How a nerve works • Nerve impulse: • electrochemical signal • great speed: up to _________ miles per hour • resting nerve has charge difference across membrane: membrane “polarized” • impulse: wave of depolarization caused by sudden influx of Na+ into axon • followed by repolarization: return to original polarized state

16. How a nerve works • Resting neuron • Lots Na+ outside membrane and K+ on inside • Charge is + outside and - inside due to Cl- and proteins on inside of membrane • Maintained by Na-K pumps

17. How a nerve works • Resting neuron • Neuron said to be polarized • Charge difference across membrane called _________ potential. About -70 millivolts (mV)

18. How a nerve works • Resting neuron movie

19. How a nerve works • Firing neuron • stimulus arrives • if stimulus is enough, causes neuron to fire • firing is production of an action potential across membrane: change in _____________ distribution • goes from -70 mV (resting potential) to +30 mV (at peak of action potential)

20. How a nerve works • Firing neuron • 1) membrane channels open to allow Na+ to rush in • 2) extra K+ channels open and lots of K+ flows out • This repolarizes membrane • 3) Refractory period: time during which original state is regenerated by Na-K pumps. During this time, neuron __________ fire again.

21. How a nerve works • The firing neuron movie

22. How a nerve works • Nerve firing: • All or none response. A nerve either fires or it doesn’t • Very brief. Lasts about ________ millisecond • Transmitted along sections of axon like “the wave” in a stadium

23. How a nerve works • Myelin sheaths speed impulse movement • allow action potential to jump between nodes • action potential only produced at nodes, not in between

24. How a nerve works • Myelin sheaths speed impulse movement • like stadium with sections of empty seats. “Wave” jumps to next filled section • called saltatory conduction

25. How a nerve works • Note speed of myelinated axons • Note also that axon diameter influences speed of impulse. Larger axon, ___________ speed.

26. Neuron to neuron transmission • 2 neurons usually don’t touch • Share synapse: tiny intercellular space (synaptic cleft)

27. Neuron to neuron transmission • Chemical transmission of impulse: • 1)Action potential in firing axon stimulates release of synaptic vesicles into synapse

28. Neuron to neuron transmission • Chemical transmission of impulse: • 2) Vesicles contain neurotransmitters • 3) Neurotransmitters diffuse across cleft to _________ proteins in postsynaptic membrane (dendrite of other neuron or muscle cell as shown here)

29. Neuron to neuron transmission • Chemical transmission of impulse: • 4) Receptor proteins cause start of action potential in postsynaptic membrane • 5) Enzymes ______________ neurotransmitters when transmission is completed. Prepares synapse for the next impulse.

30. Neurotransmitters • Vital substances in transfer of messages between neurons

31. Neurotransmitters • 2 main types • 1) excitatory transmitters: cause depolarization of postsynaptic membrane • 2) inhibitory transmitters: stabilize membrane against depolarization

32. Neurotransmitters • Acetylcholine: Neurotransmitter between motor nerve axon and skeletal muscle cell • Example of excitatory neurotransmitter • Causes depolarization of muscle cell membrane and stimulation of ________________

33. Neurotransmitters • How stop stimulus? Acetylcholinesterase: enzyme breaks down acetylcholine • What happens if acetylcholinesterase inhibited? Muscle contraction uncontrolled. • This is _________...... • Some chemicals designed to kill by inhibiting acetylcholinesterase

34. Neurotransmitters • Nerve gas: works by inhibiting acetylcholinesterase • Example, VX gas featured in “The Rock” VX gas spheres in “The Rock”

35. Neurotransmitters • Nerve gas: works by inhibiting acetylcholinesterase • Example, sarin gas • Used in Tokyo subway attack by religious cult in 1995 (12 dead)

36. Neurotransmitters • Nerve gas: works by inhibiting acetylcholinesterase • Example, sarin gas • Used by Saddam Hussein and his pal “Chemical Ali” in 1988 against Kurds at Halabjah (5,000 killed)

37. Neurotransmitters • Nerve gas: works by inhibiting acetylcholinesterase • Example, sarin gas • US Army used incinerator at Anniston AL to destroy some sarin gas (Sept. 2003)

38. Neurotransmitters • GABA: an inhibitory neurotransmitter • Allows ________ into neuron membrane, making it even more negative inside. • Thus harder to develop action potential

39. Neurotransmitters • Some sedatives take advantage of GABA mechanism • Ex, Valium: enhances binding of GABA to its receptors. Harder for neurons to fire.

40. Neurotransmitters • Serotonin: Involved in sleep regulation and emotional states • Insufficient serotonin production involved in depression, Parkinson’s disease

41. Neurotransmitters • Prozac: blocks reabsorption of serotonin to make up for _________ of production

42. Neurotransmitters • Serotonin • LSD: blocks serotonin receptors in portion of brain to produce hallucinations or “trip”

43. Neurotransmitters • Cocaine: Affects neurons in brain’s “pleasure pathways” (limbic system)

44. Neurotransmitters • Slows reabsorption of transmitters, so pleasure messages intensified

45. Neurotransmitters • Addiction: body adjusts to drug. Example, cocaine • Decrease number of neurotransmitter receptors • When drug removed, synapse _________ sensitive

46. Neurotransmitters • Addiction example, nicotine

47. Neurotransmitters • Nicotine binds to brain receptors that normally bind acetylcholine • Influences a number of other neurotransmitters and their receptors • Result is stimulating

48. Neurotransmitters • Problem: brain cells adjust numbers and sensitivities of many brain receptors • If nicotine removed (stop smoking), system unbalanced. Effects unpleasant

49. Neurotransmitters • Example, nicotine • Takes time to recover

50. Synaptic Integration • Recall that there are two types of neurotransmitters: excitatory and inhibitory • So, synapses can be either excitatory or inhibitory, depending on the neurotransmitter produced • CNS neurons often receive input from many other neurons