Nervous Tissue

1. Nervous Tissue Martini Chapter 12 Bio 103 Part 2

2. How do neurons communicate? • Electrically: Action Potentials • All or Nothing • ALWAYS EXCITATORY!

3. How do neurons communicate? • Electrically: Action Potentials • All or Nothing • ALWAYS EXCITATORY! • Chemically: Neurotransmitters • various types • can stimulate or depress electrical activity • can have long impact on post synaptic cellular function

4. Electricity Terminology Voltage potential energy generated by separated charge Current the flow of electrical charge from one point to another Resistance hindrance to the current insulators = high resistance conductors = low resistance

5. Electricity Terminology Voltage potential energy generated by separated charge Current the flow of electrical charge from one point to another Resistance hindrance to the current insulators = high resistance conductors = low resistance • Ohm’s Law • Current (I) = voltage (V) / resistance (R) • OR • V = I x R

6. The Transmembrane Potential All cells have a difference in charge across their membranes resulting in potential energy. measured in voltage mVolts

7. An Analogy Membrane Potential is like a damned up lake, except, instead of water trying to get through, its ions.

8. The Transmembrane Potential All cells have a difference in charge across their membranes resulting in potential energy. GENERALLY: extracellular fluid is high in Na+ high in Cl-

9. The Transmembrane Potential All cells have a difference in charge across their membranes resulting in potential energy. GENERALLY: extracellular fluid is high in Na+ high in Cl- intracellular fluid is high in K+ high in proteins (A-)

10. The Transmembrane Potential All cells have a difference in charge across their membranes resulting in potential energy. THUS, the intracellular environment is relatively more negative neurons usually -70 mV at rest

11. The Resting Membrane Potential The voltage across the membrane when the cell is at rest. RMP for neurons usually around -70 mV

12. How is the Transmembrane Potential Created and Maintained? • If the cell membrane were freely permeable, diffusion would eventually distribute the ions and proteins evenly across the membrane.

13. How is the Transmembrane Potential Maintained? • If the cell membrane were freely permeable, diffusion would eventually distribute the ions and proteins evenly across the membrane. • BUT: • Ions must pass through ion channels or be transported by an active (ATP requiring) mechanism • the large, mostly negative proteins inside the cell cannot cross the selectively permeable membrane

14. How is the Transmembrane Potential Created? • Passive forces create voltage across the membrane, which the cell uses as potential energy: • Chemical Gradient • concentrations of a molecule differ across the membrane • molecules diffuse to the areas of lower concentration • Electrical Gradient • charge differs across the membrane • opposites (+/-) attract, molecules diffuse towards opposite charge • Electrochemical Gradient • the sum effect of electrical and chemical forces

15. How is the Transmembrane Potential Created? • The electrochemical gradients of Na+ and K+ are the primary factor determining the transmembrane potential in neurons

16. How is the Transmembrane Potential Created? • The electrochemical gradients of Na+ and K+ are the primary factor determining the transmembrane potential in neurons • Na+ and K+ must cross the cell membrane through channels, or via active transport

17. Ion Channels • leaky (passive) ion channels • ions can always pass through these protein channels • both Na+ and K+ have leak channels in neurons, but K+ has significantly more

18. Ion Channels • gated ion channels • these channels open and let ions pass only under specific circumstances

19. Ion Channels • chemical-gated channels • regulated by chemical signals that bind to the channel

20. Ion Channels • voltage-gated channels • regulated by changes in voltage across the membrane

21. Ion Channels • mechanical-gated channels • regulated by changes in pressure

22. Ion Channels and Chemical Gradients Na+ Na+ Na+ Na+ K+ Which direction will Na+ or K+ move through leak or gated channel? Na+ K+ K+ NEURON K+ K+ K+ Na+ K+ K+ K+ Na+ Na+ Na+ Na+ Na+

23. The membrane is more permeable to K+ relative to Na+ Na+ Na+ Na+ Na+ K+ Na+ K+ K+ NEURON K+ K+ K+ Na+ K+ K+ K+ Na+ Na+ Na+ Na+ Na+

24. The membrane is more permeable to K+ relative to Na+ Na+ Na+ Na+ Na+ K+ Potassium leaves faster than sodium enters, and the cell become more negative. Na+ K+ K+ NEURON K+ K+ K+ Na+ K+ K+ K+ Na+ Na+ Na+ Na+ Na+

25. The membrane is more permeable to K+ relative to Na+ Na+ Na+ Na+ Na+ K+ Potassium leaves faster than sodium enters, and the cell become more negative. This is one factor that contributes to the transmembrane potential. Na+ K+ K+ NEURON K+ K+ K+ Na+ K+ K+ K+ Na+ Na+ Na+ Na+ Na+

26. Electrochemical Gradient of K+ • chemically K+ wants OUTA LOT • electrically K+ want INA Little • together, the net effect is to move out of the cell

27. Electrochemical Gradient of Na+ • chemically Na+ wants IN A LOT • electrically Na+ want INA Little • together, the net effect is to move into the cell

28. Equilibrium Potential • The transmembrane potential at which there is no net movement for a specific ion • the voltage at which the gradient for an ion is eliminated potassium = -90 mV close to resting membrane potential sodium = +66 mV far from resting membrane potential

29. Maintaining the Resting Membrane Potential AT REST: • cell is highly permeable to K+ • large positive charge leaves the cell • cell has little permeability for Na+ • only slight positive charge enters cell • HOWEVER, • eventually enough Na+ will leak across to eliminate the resting membrane potential (-70mV)

30. Maintaining the Resting Membrane Potential • In order to maintain the electrochemical gradients of Na+ and K+, they must be actively transported across the membrane

31. The Sodium-Potassium Exchange Pump

32. The Sodium-Potassium Exchange Pump • The exchange pump maintains the electrochemical gradients for sodium and potassium, thus maintaining the resting membrane potential.

33. Overview of Resting Potential

34. Overview of Resting Potential • membrane is highly permeable to K+, so the RPM is close to K+ equilibrium potential

35. Overview of Resting Potential • membrane is not very permeable to NA+, so RMP is not close to NA+’s equilibrium potential

36. Overview of Resting Potential • the sodium-potassium exchange pump maintains the RMP • 3 NA+ out • 2 K+ in

37. Overview of Resting Potential • at rest, the passive and active transport mechanisms are in balance and the RMP is stable • neuron usually -70 mV

38. When a neuron is excited • It’s membrane potential changes • 3 states of membrane potential • resting potential • at rest (-70 mV) • graded potential • some excitation (-69 to -61 mV) • action potential • excitation above threshold (-60 to -55 mV)

39. TERMINOLOGY • EXCITATION • when potential is made more positive (from -70mV to a more positive #) it is called depolarization • when resting potential (-70mV) is restored after depolarization it is called repolarization • INHIBITION • if potential is made more negative (from -70mV to a more negative #) it is called hyperpolarization.

40. How Do Changes in Membrane Potential Occur? • Gated Ion Channels open and close

41. Graded Potentials • excitation or depolarization of the transmembrane potential that doesn’t spread far from the site of stimulation • The stimulation was not strong enough to cause an action potential

42. An example of Graded Potential • The neuron begins at rest

43. An example of Graded Potential • a chemical (e.g., Acetylcholine), binds to its receptor on a chemically-gated Na+ channel

44. An example of Graded Potential • Na+ rushes into the opened channels, causing a local current, depolarizing portions of the membrane

45. 4 Characteristics of the Graded Potential • membrane potential is most impacted at the site of stimulation • change in charge only spreads locally (local current) • change in voltage can be: • depolarizing e.g., if Na+ channels open • hyperpolarizing e.g., if K+ channels open

46. Graded Potentials:actual terminology • Excitatory Postsynaptic Potential (EPSP) • Inhibitory Postsynaptic Potential (IPSP) At any point in time a neuron may be receiving numerous EPSP’s and IPSP’s It is the summation of these individual inputs that determines if a neuron will send a message in the form of an action potential

47. Action Potential • propagated excitation of the transmembrane potential • chain reaction of depolarizing events • neurons receives enough stimulation (graded potentials) to cross a threshold of voltage • If the threshold is exceeded voltage-gated Na+ channels open • Na+ rushes into the cell setting of the chain reaction that is an action potential

48. 30 0 Membrane Potential (mV) threshold -55 -70 1 2 3 4 5 6 7 8 Time (ms) Action Potential is ALL-OR-NONE • An action potential only happens if enough excitatory stimulation occurs to bring the transmembrane potential above a threshold • Threshold typically -60 to -55 mV • Threshold is the voltage that opens the voltage-gated Na+ channels

49. 30 0 Membrane Potential (mV) threshold -55 -70 1 2 3 4 5 6 7 8 Time (ms) Action Potential is ALL-OR-NONE • Once the threshold voltage is exceeded an action potential will take place • Action potentials have only 1 level of strength • The amount that the threshold is exceeded will not affect the strength or speed of an action potential action potential

50. 30 0 Membrane Potential (mV) threshold -55 -70 1 2 3 4 5 6 7 8 Time (ms) Generation of an Action Potential • stimulation ABOVE threshold • increased Na+ permeability causes depolarization • decreased Na+ AND increased K+ permeability cause repolarization • prolonged increase in K+ permeability causes undershoot (hyperpolarization) • return to normal membrane permeability and RMP