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Nerve and action Potentials

Irfan Idris

Physiology DepartmentMedical Faculty, UNHAS

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lassification of Neurons and

Nerves

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Supporting ells

! "here are four types of supporting cells, calledneuroglial #or glial$ cells, in the central nervoussystem

 % oligodendrocytes, &hich form myelin sheathsaround a'ons of the NS(

 % microglia, &hich migrate through the NS andphagocytose foreign and degenerated material

 % astrocytes, &hich help to regulate the e'ternal

environment of neurons in the NS( % ependymal cells, &hich line the ventricles #cavities$

of the )rain and the central canal of the spinal cord

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! "here are t&o types of supporting cells in

the peripheral nervous system*

 % Schwann cells, &hich form myelin sheaths

around peripheral a'ons( and

 % Satellite cells, or ganglionic gliocytes,

&hich support neuron cells )odies &ithin the

ganglia of the PNS+

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Resting Membrane Potential

of Nerves

!  Active "ransport of Sodium and Potassium

Ions "hrough the Mem)rane"he

Sodium-Potassium #Na.-/.$

! 0ea1age of Potassium and Sodium

"hrough the Nerve Mem)rane

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Origin of the Normal Resting

Membrane Potential

! Contribution of the Potassium DiffusionPotential 2ecause of the high ratio ofpotassium ions inside to outside, 34*5, theNernst potential corresponding to this ratio is %

67 millivolts "herefore, if potassium ions&erethe only factor causing the resting potential

! Contribution of Sodium Diffusion !hroughthe Nerve Membrane diffusion of sodium ions

through the /.-Na. lea1 channels+ "he ratio ofsodium ions from inside to outside themem)rane is 8+5

! Contribution of the Na."#. Pump

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Nerve $ction

Potential

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%nitiation of the $ction Potential

! $ Positive"&eedbac' (icious Cycle Opens theSodium Channels % if any event causes enough initial rise in the mem)rane potential

from %68 millivolts to&ard the 9ero level, the rising voltage itselfcauses many voltage-gated sodium channels to )egin opening+"his allo&s rapid inflo& of sodium ions, &hich causes a furtherrise in the mem)rane potential

! !hreshold for %nitiation of the $ction Potential %  A sudden rise in mem)rane potential of 54 to 38 millivolts usually

is re:uired+ "herefore, a sudden increase in the mem)ranepotential in a large nerve fi)er from %68 millivolts up to a)out %;4millivolts usually causes the e'plosive development of an actionpotential+ "his level of %;4 millivolts is said to )e the thresholdfor stimulation

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TRANSMISI SINAPTIK

ELEKTRIK 

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Nerve fi)er types in mammalian nerve+

&iber !ype &unction

&iber Diameter 

)*m+

Conduction(elocity

)ms+

Spi'eDuration

)ms+

$bsoluteRefractoryPeriod )ms+

  $  

< Proprioception( somaticmotor 

5=-=8 >8-5=8

? "ouch, pressure 4-5= 38->8 8+7-8+4 8+7-5

  @ Motor to musclespindles

3-; 54-38

Pain, cold, touch =-4 5=-38

- Preganglionicautonomic

B3 3-54 5+= 5+=

  C  

Dorsal root Pain, temperature,some mechano-reception, refle'

responses

8+7-5+= 8+4-= = =

  Sympathetic Postganglionicsympathetics

8+3-5+3 8+>-=+3 = =

5 A and 2 fi)ers are myelinated( fi)ers are unmyelinated+

 

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Numerical classification sometimes used for sensoryneurons+

Number   Origin  &iber !ype

Ia Muscle spindle, annulospinal ending+ A <

I) Colgi tendon organ+ A <

II Muscle spindle, flo&er-spray ending( touch, pressure+ A ?

III Pain and cold receptors( some touch receptors+ A

I Pain, temperature, and other receptors+ Dorsal root