Neurophys.... Membrane Potential

8/12/2019 Neurophys.... Membrane Potential

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Outline

Resting membrane potentials

Action potential


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RMP

Definition

It is a steady state potential which exists acrossthe cell membrane

It is always hyperpolarized


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RMP

SA node -60 mv

Atrium -90mv

Ventricles -90 mv

Skeletal muscles -70mv

Smooth muscle -60mv

Peripheral nerves -70 mv


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RMP

Determinants of RMP?

Electrolyte

Na/K ATPase pump Intracellular protein

Gibbs Domann

Concentration gradient


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RMP

Electrolytes / concentration gradients

ICF ECF

Na 10-20 135-145

K 150 3-5

Cl 10-20 110

100x


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RMP

The membrane is more permeable to Na and K ,so these ions tend to leak across the membranedown according the concentration gradient

The membrane is 100 times more permeable to K So, there is more K is lost from the cell than Na

enters to the cells

Therefore, net result-larger amount of positive

charges has left the cell than entering it.

Thus, that is why cell are semipermeable


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Na/K ATPase pump

It pumps more Na out from the cell than K ispumped in

( 3Na out: 2K in)

It maintains the RMP


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Intracellular protein

There are negatively charged intracellularly

So RMP is negative


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Gibbs- Donnan effect


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The Donnan Effect

Which fluid compartments are high in protein?

intracellular fluid plasma

Which fluid compartment is low in protein? interstitial fluid

Fig. 1-27 Ganong


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The Donnan Effect

Proteins are impermeant polyanions.

Proteins are impermeant to the membrane.

due both to size and charge

Most proteins have a net negative charge.

This negative charge is balanced by cations, typically

monovalent cations. K+for intracellular fluid

Na+for plasma

Fig. 12.11


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The Donnan Effect

When Donnan and Gibbs analyzed the situation

mathematically, they showed that in and outcould not be in electrochemical and osmoticequilibrium at the same time.

(see Ganong)

As a result of the osmotic imbalance, water will

flow into a compartment (cytosol, blood plasma)

that contains more protein. The portion of the osmotic pressure that is due to the

effects of protein is called colloidal osmotic pressure.


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The problem:Because of theDonnan effect, cellswill swell and burst.

The solution:

For animal cells, thesodium pump solves

the problem. Na+pumped out

Cl-follows Na+

H2O follows NaCl

Alberts et al.,Molecular Biology of the Cell

Animal cells must

have sodium pumps.


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Nernst Equation

at equilibriumthese opposing energiesmust be equal

therefore by rearrangement, viz. Calculate one ion

o

im

K

K

zF

RTE

ln

E

K

KK

i

o

615. log


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GOLDMAN CONSTANT-FIELD EQUATION

V = RT In PK+[K+]o + PNa+[Na

+]o + PCl- [Cl-]i

F PK+[K+]i + PNa+[Na

+]i + PCl- [Cl-]o

The magnitude of the membrane potentialat any given timedepends, of course, upon the distribution of Na+, K+ and Cl-and the permeability of the membraneto each of these ions.

It calculate 3 ions


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THE ACTION POTENTIAL

An action potential is the sequence ofchanges in the membrane potential thatoccurs during the transmission of anelectrical signal along the cell membrane,as measured at a specific point on a nerve

or muscle cell membrane.

depolarization

repolarization

~ 1 ms

Fig. 12-13

Fig. 2-6Ganong


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The Action Potential

skeletal muscle fibers

axons of neurons

Fig. 12.13

Fig. 12.12Fig. 11.1


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Phase 1 whn a nerve is stimulated, it initiates

the ligand-gated Na channels, n there will beinflux of Na ions initiating depolarization

Phase 2 whn the depolarization reaches thethreshold potential ard -55mV, it opens up thevoltage gated fast Na channels

Phase 3- massive influx of Na ions

Phase 4 the potential overshoots the

isopotential line to ard + 30mV ( positive spikeAP)


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Phase 5 Na channels closed, K channelsopen , and there will be K efflux

Repolarization

Phase 6 due to continuation of K efflux ,there will be hyperpolarization


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Action Potentials

Characteristics:

1. Stereotypical size and shape2. Propagation to adjacent sites

3. All-or-none response


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Action Potentials:

Characteristics1. Stereotypical size and shape

Each action potential for any cell type looks identical.

It depolarizes to the same potential.

It repolarizes back to the same resting potential.


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-70 mV

0 mV

1.0 2.0

Action Potential

Resting membrane potential

Time (milliseconds)

Voltage


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Action Potentials :

Characteristics2. Propagation to adjacent sites

The action potential moves along the axon in a wave of

changing membrane potential. After an action potential, the membrane is in a

refractory state so the propagation is in one directiononly.

This action potential propagates from one end of thenerve to the other.


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A

B

C

-+

-+

-+

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

Activeregion

Propagation of action potentials(in unmyelinated fibres)


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Myelin sheath

Action potential

Node of Ranvier

-+ -

+ +-

+- +

- -+

Saltatory conduction in myelinated fibres


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2

4

6

8 myelinated

unmyelinated

Effect of myelination on speed of propagation

Fibre diameter (m)

Conductionvelocity(m

/s)

1 2 3 4


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Action Potentials :

Characteristics3. All-or-none response

If an excitable cell is depolarized to threshold in a

normalmanner, then the occurrence of an actionpotential is inevitable.

On the other hand, if the membrane is not depolarizedto threshold, no action potential can occur.


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-100

-50

0

+50

Threshold potential

Resting membrane potential

Increasing subthreshlod stimuli

Membranepotential(mV)

Time|----------------|

1 ms

All-or-none action potential


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Compound action potentials

Peripheral nerves contain a mixture of fibres

Classified according to

Function

Diameter

Conduction velocity


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Table of compound action potential

Fibre Function Diameter

(m)

Conduction

velocity

(m/s)

A

Skeletal motor, joint position

Touch, pressure

Muscle spindle motor

Pain, temperature touch

10-20

5-10

3-6

2-5

60-120

40-7-

15-30

10-30

B Preganglionic autonomic 1-3 3-15

C Pain 0.5-1 0.5-2


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Voltage

(V)

Time (ms)

Compound action potential

A

B

C


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The electrical signals generated by the action

potentials can be sensed using skin or needleelectrodes placed near the muscle. This is thebasis of electromyography.


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Electroencephalography

Is recording of spontaneous activity of thebrain

Generated by pyramidal cells in cortical layers1, 2 and 5

Its a summation of excitatory and inhibitory

postsynapticpotentials in pathways running

from thalamic nuclei to cortex.


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Eeg

4 types of waveform :


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Occurs Hz Voltage(microv)

beta Fully awakeMaybe also produced by volatile

and IV anaesPrecentral cortex

15-25 20

alpha TransitionalAwake,relaxed with eyes closedOccipital cortex

10-15 20-50

theta SleepOccurs in children, elderlyMay indicates dysfx of drugs

1-5 20-50

delta Occurs during sleep during stage3 and 4 of NREM

0.5 1 >50


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Application of EEG

In tumor and epilepsy shows high voltagespike waves

Cerebral infarcation low volatge and low

frequency Thiopentone, propofol, etomidate, and

volatile agents produce initial fast frontalbeta, then frontal alpha spikes with highampitude , then 1-3 HZ delta burstsuppression with high amplitude , then noisoelectric EEG


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Application of EEG

Ketamine frontal rhythmic delta with highamplitude, then intermittent, polymorphicdelta with large amplitude interspersed withlow amplitude beta. NO EEG silence

BZD- no burst suppression and no isoelectricEEG

Opiods dose related low frequency andhigh amplitude, no isoelectric EEG


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Sleep

Is a state of unconsiousness from which aperson can be aroused by a sensorystimulation

Involves decrease activity of RAS(reticularactivation system)

2 stages :

NREM or slow-wave sleep(4 stages)

REM( tonic and phasic)


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Stages of sleep


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Influence of anaesthesia and

surgery during sleep NREM anaesthetic stages REM- post op complication

Why???? In NREM, mainly is by parasympathetic In REM, esp during the phasic phase is mainly by

sympathetic and is predominant during post op Day1

In REM, thr wil be impaired resp fx

Maldistribution of ventilation , impaired V/Q matchingwhich lead to arterial hypoxemia

Thus, MI, cerebral impairment and hypoxemia iscommon post op Day 1


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Depth of anaesthesia for

awareness Awareness Inadvertent return to conciousness during a supposed

GA with or without recall

Techniques that monitored depth ofanaesthesia: MAC

BIS(bispectral index)

Vital sign Entropy

TCI/TIVA ( plasma concentration drug)

IFT(isolated forearm tech)


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BIS

It monitors record raw EEG which altersanaesthesia and after processing produces adimensionless number from 0 -100 which is

an index of probability that the pt is aware

Put at the frontal lobe


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Limitation of BIS

Site of operation

Cant do in children

Organic brain lesion

Psy problem

Drugs like etomidate and ketamine

Costly Artifacts by surgery

diathermy/cauterization/vibration

Neurophys.... Membrane Potential

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