1. Martini Chapter 12 Bio 103 Part 2
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 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+
NEURON K+ K+ K+
Na+ K+ K+ K+
Na+
Na+
Na+
Na+
Na+

24. The membrane is more permeable to K+ relative to Na+
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+
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 OUT A LOT
electrically K+ want IN A 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 IN A 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 INHIBITION
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. Action Potential is ALL-OR-NONE
An action potential only happens if enough excitatory stimulation occurs to bring the transmembrane potential above a threshold
Membrane Potential (mV)
Time (ms)
-70
-55 30
threshold
Threshold typically -60 to -55 mV
Threshold is the voltage that opens the voltage-gated Na+ channels

49. 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
Membrane Potential (mV)
Time (ms)
-70
-55 30
threshold
action potential

50. 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
Membrane Potential (mV)
Time (ms)
-70
-55 30
threshold

51. Step 1 Stimulation of a Resting Neuron
via excitatory chemicals binding to receptors on the post-synaptic neuron
post-synaptic
pre-synaptic

52. Step 2: Voltage-Gated Na+ Channels Open
At rest, the voltage-gated Na+ channels are closed.

53. Step 2: Voltage-Gated Na+ Channels Open
DEPOLARIZTION PHASE:
When a neuron is stimulated above threshold (-60 to -55 mV) voltage-gated Na+ channels open and Na+ rushes INTO the cell making it even more positive.

54. Step 3: Na+ channels are inactivated and K+ channels open
REPOLARIZATION PHASE
At the peak of the action potential curve (~30 mV), the inactivation gate (ball) on the Na+ channel snaps shut, stopping the rush of Na+ into the cell.
As long as this gate is shut, the neuron cannot fire another action potential. It is in a REFRACTORY PERIOD

55. Step 3: Na+ channels are inactivated and K+ channels open
REPOLARIZATION PHASE
At the same time, voltage-gated K+ channels are opening, allowing K+ to rush out of the cell.
at 30 mV, both electrical and chemical gradients favor K+ movement out of the cell.

56. Step 4: Return to Normal Permeability
HYPERPOLARIZATION PHASE
a.k.a. undershoot
K+ channels remain open beyond the point of reaching -70 mV causing an undershoot of the RMP to about -90 mV (equilibrium of K+).
When the membrane potential reaches -70mV, the inactivation gate on the Na+ channel (ball) snaps back open

57. Step 4: Return to Normal Permeability
Eventually the voltage-gated K+ channels close and the membrane is again at the RMP of -70 mV.

58. The Refractory Period
There is a period after an action potential when a neuron cannot (1), or is unlikely (2) to produce another action potential.

59. The Refractory Period
Absolute Refractory Period: As long as the Na+ channel is deactivated, the neuron cannot fire another action potential.

60. The Refractory Period
Relative Refractory Period: At this point the deactivation gate on the sodium channels has reopened, and the neuron can technically produce another action potential.
HOWEVER:
it has to overcome the membrane hyperpolarization!
Threshold = -60 mV
RMP = -70 mV
Hyperpolarized membrane = -90 mV

61. Na+/K+ exchange pump and Action Potentials
1 action potential does not change ionic concentrations enough to require the pump to reset the RMP
However, a neuron can fire as many as 1000/second
Thus, the exchange pump is necessary to maintain RMP

62. Action Potential Overview

63. Comparing Graded Potentials and Action Potentials

64. Action Potential Propagation
Unlike graded potentials, action potential spread across the entire neuron from soma to synapse.
continuous propagation (axon is unmyelinated)
saltatory propagation (axon has myelin – see picture below)

65. Continuous Propagation (unmyelinated axon)
action potential in initial segment
local current depolarizes adjacent membrane

66. Continuous Propagation (unmyelinated axon)
another action potential fires in this region, while the initial segment enters a refractory period

67. Continuous Propagation (unmyelinated axon)
a local current depolarizes the adjacent portion of the membrane…and so on…..

68. Saltatory Propagation (myelinated axon)
An action potential fires in the initial segment of the neuron.

69. Saltatory (leaping) Propagation (myelinated axon)
The adjacent node is depolarized, skipping the myelinated segment.

70. Saltatory (leaping) Propagation (myelinated axon)
an action potential fires in node 1
the process, with action potentials being fired at each node
Because saltatory propagation leaps across internodes that may be 1-2 mm apart, propagation is much faster and consumes less energy (i.e., fewer Na ions have to be pumped back out).

71. Animation of Propagation

72. Axon Diameter and Propagation Speed
Larger diameter = faster propagation
less membrane/surface area to impede flow of current
Squids have GIANT axons (100 X’s diameter of humans), to help them propel away from predators.

73. Axons are classified by diameter, myelin, and speed
Type A fibers
diameter of 4-20 mm
myelinated
300 mph propagation speed
Type B fibers
diameter of 2-4 mm
40 mph propagation speed
Type C fibers
diameter of 2 mm or less
unmyelinated
2 mph propagation speed

74. Type A Fibers What do they do? Carry sensory information to CNS
position, balance, delicate touch and pressure
Carry motor commands to the skeletal muscles

75. Type B and C Fibers What do they do?
Carry sensory information to the CNS:
temperature, pain, general touch and pressure
Carry motor output information to smooth and cardiac muscles:

76. Why aren’t all fibers Type A?
Compromise between speed and size
only 1/3 of fibers are myelinated
Critical information is carried over Type A