Excitabilidad en Neuronas (en Inglés, pero muy útil)

7/29/2019 Excitabilidad en Neuronas (en Ingls, pero muy til)

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Functions of nervous system

Sensory neurons CNS Motor neurons

Perception Integration Action


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Structure of a Neuron Cell body

Dendrites

Conduct impulses toward cell body

Axon

Carries electrical impulse away fromcell body

May be covered by Schwann cells

Forms discontinuous myelin sheathalong length of axon

Synapse

Contact points between axon of oneneuron and dendrite of anotherneuron


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Structure of a Chemical Synapse

Synaptic cleft

Signal passes from pre-

synaptic to post-

synaptic cell

neurotransmitter


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Two types of signaling

pathways

Cell surface receptors

Intracellular (nuclear)receptors

Signal Concentration

< 10-8M

High receptor Affinity


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Resting Potential

Resting potential

Donnan Equilibrium

-70mV

Determined by balance

between sodium andpotassium concentrations

Sodium/Potassium pumps

Separate sodium andpotassium channel proteins


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Sodium gradient

Potassium gradient

Electrochemical gradient

Positive

charge

Negative

charge

Resting

Potential


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Sodium potassium dependant ATPase

QuickTime and aPhoto - JPEG decompressor

are needed to see this picture.


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An Action Potential

So what changes during an action

potential?


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Squid Giant axon

t


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o um entersnerve during

AP


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Goldman Equation I

E=Potential across membrane

R=Universal gas constant

T= Absolute temperature oK

z=Charge

F=Faradays constant

At 25oC, RT/ZF = 25

During Resting potential PK=1 (i=400mMo=20)

PNa=0.0013 (i=50 o=440)

PCl= 0.0017 (i=90 o=560)

So what is the Resting potential?

E=RTzF

(PK[K]o)+(PNa[Na]o)+(PCl[Cl]i)

(PK[K]i)+(PNa[Na]i)+(PCl[Cl]o)loge


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Goldman Equation II

During Action Potential

PK=1 (i=400mM o=20)

PNa=100 (i=50 o=440)

PCl= 0.0017 (i=90 o=560)

What is the potential across the membrane?

E=RTzF

(PK[K]o)+(PNa[Na]o)+(PCl[Cl]i)

(PK[K]i)+(PNa[Na]i)+(PCl[Cl]o)loge


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Ion Channels


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Several types of ion channel


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Potassium channel


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Potenti

al


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Sodium Channel Blockers


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Potassium Channel Blockers


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u K d

Nerve conduction velocities

u=velocity

K=constant

d=diameter

True for non-

myelinatedneurons


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Myelin sheath

Cat motor neuron


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Myelinated neuron velocity is

dependent upon size

However, related to inter-node distance,

not diameter

Inter-node distance greater in large

axons

Non-myelinated neuron velocity related

to square root of diameter


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Neurotransmitters

Must be released by

pre-synaptic cell

Must elicit effect on

post-synaptic cell

Agonists should work

the same way

Must be removed after

action


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Deactivation of Acetylcholine

Enzyme.

Acetylcholinesterase


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Chemically gated ion channel


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Acetylcholine gated ion channel Pore formed by

5 homologous

subunits

Ringed by -

helices

(leucines)forming gate

Acetylcholine

binding

changes gate

conformation


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Calcium ions cause vesicle transport


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Axon Hillock

Determines

sensitivity to

stimulation

High number of

ion channels


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Effect of ion channel density on

membrane threshold

To generate an AP, a certain

number of sodium ions must

cross membrane Determined by number of open

sodium ion channels

Ion channels have varying

voltage sensitivities (normal

distribution)

Potential mV#

ofc

hannelsopened


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Threshold dependent upon ion

channel density

Cell One (Red)

1,000 channels per mm2

Must generate enough potential

to open 1/10th

Cell Two (Blue)

10,000 channels per mm2

Must generate enough potential

to open 1/100th

Potential mV#

ofchannelsop

ened


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Axon Hillock

Determines

sensitivity to

stimulation

High number of

ion channels


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Spatial summation

Excitatory input

Integration of


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Integration of

excitatory and

inhibitory inputs

Spatial summation

Excitatory and

Inhibitory input


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Temporal summation

Allows integration of

signals in time


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Nerves define stimulus strength by the number of impulsesgenerated

Increased pre-synaptic input elevates post-synaptic potential athillock

Leads to elevated rate of AP generation down axon

Can be modulated for accommodation

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Excitabilidad en Neuronas (en Inglés, pero muy útil)