Advanced Physiology Exam 1 Topic 2

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Topic 2
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College: Fourth year
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physiology
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1

Membrane Voltage (Vm) is equal to

the differnce between the ICF and the ECF: V(ICF) - V(ECF)

2

g

refers to electrical conductance (another way to express permeability

3

chord equation (goldman-hodgkin-katz)

Vm= g(ion1)/g(tot)*E(ion1) + g(ion2)/g(tot)*E(ion2) etc.

In words, Vm is determined by the relative conductances of a cell to different ions

4

Intuition from the chord equation

a cell's Vm is always nearest to the equilibrium potential for the ion to which that cell is most permeable
(i.e. if cell is most permeable to K than Vm will be closest to Ek)

5

Ohm's law

V=IR; V=I/g; I=gV (recall that g=1/R)

ion version: i(ion)=X*g(ion)*(Eion-Vm)
where i(ion) is inward flux of the ion
where X is the charge of the ion

6

depolarization

an increase in Vm

7

hyperpolarization

a decrease in Vm below rest

8

repolarization

returning a cell from a depolarized Vm toward rest

9

Changes in Vm result from...

changes in ion conductances (permeability)

10

graded changes (graded signals)

signals that can vary in size and direction

11

excitability

the use of graded signals to trigger a huge, very rapid Vm change (known as action potential)

12

action potential (A.P)

1. a huge, rapid Vm change
2. is the key electrical signaling event in the body
3. action potentials process within 1 to 2 ms (not AP seen in cardiac muscle lasts much longer 500ms)
4. IS AN ALL OR NOTHING EVENT

13

axon hillock

the trigger zone of an action potential (at the base of the axon where it attaches to the cell body

14

threshold

the point above the trigger zone is depolarized

15

trigger phase of Na

Na voltage gates open; probability exceeds at about -55 to -50mV

16

positive feedback

increasing gNa further leads to more depolarization which increases gNa further, which leads to more depolarization etc...until it peaks near +60mV

17

triggering the positive feedback phase critical to generating an AP

positive feedback pahse will only be initiated if depolarization triggers a critical number of Na

18

when there are more Na channels present, its easier to reach critical number to trigger an AP

the voltage threshold for an AP will be lower than if there are fewer Na channels present
axon hillock has a very high density of Na and therefor has the lowest threshold for AP generation

19

inactivation gate

a second gate on a Na channel that is a ball and chain typ structure. The ball has many positively charged amino acids that are attracted to the normally negative Vm and thus repells the ball so that it gets lodged in the pore of the channel, blocking passage of Na (at rest hangs down)

20

gK is slower to start than gNa

1. the channels are simply slower
2. the cahnnels require more depolarization to open

21

gK lasts longer than gNa

because K channels dont have an inactivation gate, but as gk become greater than gNa the cell becomes hyperpolarized and the voltage gates of K close (AP is over)

22

refractory period

after AP, a brief period of time when its difficult or impossible to elicit another AP from a cell
absolute refractory period is where it is impossible to generate another AP
relative refractory period...possible to generate another AP, but at a higher threshold

23

absolute refractory period

where it is impossible to generate an AP because Na are mostly inactivated (inactivation means no AP)

24

relative refractory period

inactivation is dropping back down so AP is possible, but more difficult b/c gK is much higher than normal

25

excitotoxicity

cellsdying b/c they are excessively activated

26

rate-coding

the code used to represent information in the nervous system to regulate AP

27

dendrites

input...the site where synatpic inputs arrive and eventually generate graded signals int he neuron

28

cell body & axon hillock

integration...the site where graded potentials come together and initiate an AP

29

axon & synaptic teminals

the AP is porpagated down the acon from the cell body...reaches the synaptic terminal...generates synaptic inputs onto another cell

30

challenges to propagation (size)

size...decline in the size of voltage due to diffusion process and more importantly, due to Na leaks in the membrane

31

length constant

the distance required for a signal to drop to (1/e) = 37% of its original size

32

length constant is determined by

the ratio of resistance to Na flow:
a. higher membrane resistance...larger length constant...Na flows farther through cell
b. higher intracellular resitance...smaller length constant...Na flows less far through cell

33

Factors that determine length constant in an axon

a. axon diameter...larger diameter of an axon...lower intracellular resistance...larger length constant
b. membrane resistance of the axon membrane...larger membrane resistance...larger length constant

34

myelin sheath

makes membrane resistance larger, prevents Na from leaking

35

challenges to propagation (velocity of conduction)

cells would like signals to travel as quickly as possible from sit of synaptic input to the synaptic terminals

36

conduction velocity

speed of conduction

37

factors that determine volcity in an axon

a. lower intracellular resistance...faster conduction velocity; larger diameter axons...faster conduction
b. higher membrane resistance...faster conduction

38

active propagation

special mechanisms to maximize the length adn velocity of proagation

39

two mechanisms to accomplish active propagation

1. signal amplificatio by Na...mechanism is see in the axons of all neurons
2. Myeline sheath...some, but not all, neurons use a myelin sheath to impove propagation in the axon.

40

signal amplification of Na

densly packed axonal membrane with Na channels amplify signal (positive feedback)
ensures directional propagation because the refractory period of Na ensures that AP propagates fowards and not backwards

41

effects of myelination

utilizes the Nodes of Ranvir (packed with Na channels) in order to boost the AP in order to travel further

42

saltatory conduction

occurs at Nodes of Ranvir...gives a jumping appearance to signal flow down the axon

43

afferent pathways

carry signals from the periphery toward the central parts of the nervous system

44

efferent pathways

carry signals from central parts of the nervous system out toward the periphery (muscles/organs)