AKSIYON POTANSIYEL

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AKSIYON POTANSIYEL
Dr. Ayse DEMIRKAZIK
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Organizmada iyonlarin dagilimi

• Hücre disi ortam Na ve Cl- iyonlarindan
• Hücre içi ortam K iyonlarindan zengindir
• Ayrica hücre içinde asla hücreyi terk edemeyen
  negatif yüklü protein anyonlar vardir
• Normal hücre zari Ka Na a 100 kat daha
  geçirgendir

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Nernst (denge) potansiyeli

• Iyonlarin konsantrasyon gradyanlarini elektriksel
  olarak ifade eder
• Difüzyon potansiyeli ile konsantrasyon farkinin
  iliskisine dayanir
• Membranin iki tarafindaki potansiyel farki, bir
  iyonun membrandan bir yönde net difüzyonunu
  önleyecek düzeyde ise, bu potansiyele o iyon için
  Nernst potansiyeli denir

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Na, K ve Cl- için Nernst

• Hücre içi 90mV olmak üzere sadece sodyumun net
  difüzyonunun durmasi için içerideki potansiyelin
  61.5mV olmasi gerekir
• Yani membranla en büyük problemi sodyum
  yasamaktadir

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Temel potansiyel mantigi

• K ve Clun Nernst denge potansiyelleri istirahat
  potansiyeline çok yakindir
• Bu nedenle membrani rahatlikla geçebilirler

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Denge potansiyelleri

• Memeli spinal motor nöronuna ait degerler

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Membran potansiyelinin siniri

• Membrandan uzak yerlerde yükler tamamen birbirine
  esittir
• Elektriksel potansiyel farki tamamen membranin
  iki tarafinda ortaya çikar

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Membran potansiyelinin olusmasi için

• Membranin iki tarafinda farkli konsantrasyonlarda
  iyonlarin bulunmasi
• Membranin iyonlara seçici geçirgenlik göstermesi
  (K/Na100)
• Her bir iyonun elektriksel yükünün çesidi

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Gibbs-Donnan Dengesi

• Membranin bir tarafinda membrandan geçemeyen iyon
  bulundugu zaman, geçebilen iyonlarin dagilimini
  etkiler

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Na-K pompasi

• Hücrelerde devamli bir Na-K sizmasi söz
  konusudur.
• Bu devam ederse hücre siser ve ölür.
• Pompa enerji harcayarak 3Na-2K prensibi ile
  çalisir.

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Membran dinlenim potansiyeli

• Iyonlarin farkli dagilimi
• Membranin seçici geçirgenligi
• Donnan dengesi
• Na-K pompasinin özelligi sayesinde bir dengeye
  ulasir

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sonuç

• Membrandan uzak bölgelerde tamamen bir iyonik
  denge mevcuttur
• Membranin her iki tarafinda konsantrasyonlari
  farkli iyonlar vardir
• Membran seçici geçirgendir ve geçirgenlikleri
  farklidir
• Bazi hücre içi anyonlar membrani geçemezler
• Na-K pompasi diye bir sey vardir

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Aksiyon potansiyeli

• Laboratuar ortaminda elektriksel olan uyari
• Organizmada
• Elektriksel,
• Hormonal,
• Mekanik ve
• Kimyasal
• uyaranlarla olur

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Neurons (Nerve Cells)
Figure 11.4b
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Istirahat halinin bozulmasi

• Iç kisim disa göre daha negatif (-70/-90mV)
• Yeterli büyüklükte uyaran
• Kritik degere kadar depolarizasyon
• Aksiyon potansiyeli

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Membran ve Sodyum

• Yeterli büyüklükte uyaran
• Membranin Na iyonlarina karsi geçirgenliginde
  voltaja bagli artis (aktivasyon)
• Voltaja bagli inaktivasyon

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Esik deger yoksa AP de yokhep ya da hiç

• Hücre membranini esik degere kadar depolarize
  edemeyen uyaranlar AP olusturamaz
• Esik ya da esik üstü uyaranlar da daima ayni
  genlik ve sekle sahip AP olusturur

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Aslinda ne oldu?

• Istirahat halinde membran temel olarak K
  iyonlarina karsi yüksek geçirgenlik gösterir
• Bu nedenle istirahat potansiyeli K denge
  potansiyeline yakindir
• Esik degere kadar depolarize edildiginde
• Naa karsi olan membran geçirgenliginde
  milisaniyelerle sinirli bir artis ortaya çikar
• Membran Naun denge potansiyeline dogru çekilir
  yani membran pozitif degerlere dogru hizla
  depolarize olur

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Geriye dönüs- Repolarizasyon

• Depolarizasyon ile denge durumundan uzaklasan K,
  membrani tekrar kendi dinlenim potansiyeline
  dogru tasimak için
• Hücre disina akar
• Bu sayede membran tekrar K membrani karakterini
  kazanir

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AP dönemleri

• Dinlenim (istirahat) dönemi
• Depolarizasyon dönemi
• Repolarizasyon dönemi
• Hiperpolarizasyon dönemi

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Hodgkin çemberi

• feed back bir olaydir
• Esik degere ulastiktan sonra hizla içeri giren
  Na, voltaji daha da yukarilara çeker
• Voltaj yükseldikçe daha fazla Na içeri akar, daha
  fazla Naun içeri akmasi voltaji daha da
  yukarilara tasir

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Hodgkin çemberi
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Kanallarin zamanlamalari
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Herkese lazim kanallar

• Na-K sizma kanallari
• Voltaj kapili Na kanallari
• Voltaj kapili K kanallari
• Voltaj kapili Ca-Na kanallari

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Aksiyon potansiyeline katkilar

• Pek çok hücre Na-K pompasina benzer bir Na-Ca
  pompasina sahiptir.
• Pompa, kalsiyumu hücre içinden disina ve/veya
  endoplazmik retikuluma pompalar.

• Bu pompa sayesinde hücre içi ile disi arasinda
  disarda fazla olmak üzere 10 000 katlik bir
  gradiyent yaratilir.
• Bu sayede hücre içi kalsiyumu 10-7 molarda
  tutulurken, hücre disinda 10-3 molarda tutulur.

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Bir gariplik var!!!

• Membran potansiyeli istirahat durumuna geri
  döndügünde her sey normal degildir.
• Na-K iyonlarinin yerleri terstir.
• YASASIN POMPA!!!

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Yeniden uyarilma-Reeksitasyon

• Mutlak refrakter periyod
• Voltaj kapili Na kanallarinin yapisi ile ilgili
  bir durumdur
• APden sonra yeniden açilabilmeleri için mutlaka
  baslangiç konumlarina dönmeleri gerekir
• Relatif refrakter periyod
• Esik üstü uyaranlarla AP meydana gelebilir

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Saniyenin dilimlerinde...

• Çok kisa süre içinde,
• Çok hizli bir sekilde,
• Iletisim kurmanin,
• bilgi alma,
• yorumlama ve
• gerekli cevaplarin iletmesinin en iyi yolu...

Aksiyon potansiyeli Iyi fikir!!!
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Operation of a Gated Channel
Figure 11.6a
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Operation of a Voltage-Gated Channel
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Gated Channels

• When gated channels are open
• Ions move quickly across the membrane
• Movement is along their electrochemical gradients
• An electrical current is created
• Voltage changes across the membrane

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Electrochemical Gradient

• Ions flow along their chemical gradient when they
  move from an area of high concentration to an
  area of low concentration
• Ions flow along their electrical gradient when
  they move toward an area of opposite charge
• Electrochemical gradient the electrical and
  chemical gradients taken together

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Resting Membrane Potential (Vr)

• The potential difference (70 mV) across the
  membrane of a resting neuron
• It is generated by different concentrations of
  Na, K, Cl?, and protein anions (A?)
• Ionic differences are the consequence of
• Differential permeability of the neurilemma to
  Na and K
• Operation of the sodium-potassium pump

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Resting Membrane Potential (Vr)
Figure 11.8
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Membrane Potentials Signals

• Used to integrate, send, and receive information
• Membrane potential changes are produced by
• Changes in membrane permeability to ions
• Alterations of ion concentrations across the
  membrane
• Types of signals graded potentials and action
  potentials

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Changes in Membrane Potential

• Changes are caused by three events
• Depolarization the inside of the membrane
  becomes less negative
• Repolarization the membrane returns to its
  resting membrane potential
• Hyperpolarization the inside of the membrane
  becomes more negative than the resting potential

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Changes in Membrane Potential
Figure 11.9
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Graded Potentials

• Short-lived, local changes in membrane potential
• Decrease in intensity with distance
• Their magnitude varies directly with the strength
  of the stimulus
• Sufficiently strong graded potentials can
  initiate action potentials

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Graded Potentials
Figure 11.10
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Graded Potentials

• Voltage changes in graded potentials are
  decremental
• Current is quickly dissipated due to the leaky
  plasma membrane
• Can only travel over short distances

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Conduction of Action Potentials
Figure 8-14a Conduction of action potentials
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Conduction of Action Potentials
Figure 8-14b Conduction of action potentials
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Conduction of Action Potentials
Figure 8-14c Conduction of action potentials
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Speed of Conduction

• Larger diameter faster conduction
• Myelinated axon faster conduction
• Salutatory conduction
• Disease damage to myelin
• Chemicals that block channels
• Alteration of ECF ion concentrations

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Speed of Conduction
Figure 8-16b Axon diameter and speed of
conduction
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Speed of Conduction
Figure 8-17 Saltatory conduction
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Graded Potentials
Figure 11.11
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Action Potentials (APs)

• A brief reversal of membrane potential with a
  total amplitude of 100 mV
• Action potentials are only generated by muscle
  cells and neurons
• They do not decrease in strength over distance
• They are the principal means of neural
  communication
• An action potential in the axon of a neuron is a
  nerve impulse

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Action Potential Resting State

• Na and K channels are closed
• Leakage accounts for small movements of Na and
  K
• Each Na channel has two voltage-regulated gates
• Activation gates closed in the resting state
• Inactivation gates open in the resting state

Figure 11.12.1
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Action Potential Depolarization Phase

• Na permeability increases membrane potential
  reverses
• Na gates are opened K gates are closed
• Threshold a critical level of depolarization
  (-55 to -50 mV)
• At threshold, depolarization becomes
  self-generating

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Action Potential Repolarization Phase

• Sodium inactivation gates close
• Membrane permeability to Na declines to resting
  levels
• As sodium gates close, voltage-sensitive K gates
  open
• K exits the cell and internal negativity of
  the resting neuron is restored

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Action Potential Hyperpolarization

• Potassium gates remain open, causing an excessive
  efflux of K
• This efflux causes hyperpolarization of the
  membrane (undershoot)
• The neuron is insensitive to stimulus and
  depolarization during this time

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Action Potential Role of the Sodium-Potassium
Pump

• Repolarization
• Restores the resting electrical conditions of the
  neuron
• Does not restore the resting ionic conditions
• Ionic redistribution back to resting conditions
  is restored by the sodium-potassium pump

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Phases of the Action Potential

• 1 resting state
• 2 depolarization phase
• 3 repolarization phase
• 4 hperpolarization

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Propagation of an Action Potential (Time 0ms)

• Na influx causes a patch of the axonal membrane
  to depolarize
• Positive ions in the axoplasm move toward the
  polarized (negative) portion of the membrane
• Sodium gates are shown as closing, open, or closed

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Propagation of an Action Potential (Time 0ms)
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Propagation of an Action Potential (Time 1ms)

• Ions of the extracellular fluid move toward the
  area of greatest negative charge
• A current is created that depolarizes the
  adjacent membrane in a forward direction
• The impulse propagates away from its point of
  origin

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Propagation of an Action Potential (Time 1ms)
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Propagation of an Action Potential (Time 2ms)

• The action potential moves away from the stimulus
• Where sodium gates are closing, potassium gates
  are open and create a current flow

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Propagation of an Action Potential (Time 2ms)
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Threshold and Action Potentials

• Threshold membrane is depolarized by 15 to 20
  mV
• Established by the total amount of current
  flowing through the membrane
• Weak (subthreshold) stimuli are not relayed into
  action potentials
• Strong (threshold) stimuli are relayed into
  action potentials
• All-or-none phenomenon action potentials either
  happen completely, or not at all

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Coding for Stimulus Intensity

• All action potentials are alike and are
  independent of stimulus intensity
• Strong stimuli can generate an action potential
  more often than weaker stimuli
• The CNS determines stimulus intensity by the
  frequency of impulse transmission

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Coding for Stimulus Intensity

• Upward arrows stimulus applied
• Downward arrows stimulus stopped

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Coding for Stimulus Intensity

• Length of arrows strength of stimulus
• Action potentials vertical lines

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Absolute Refractory Period

• Time from the opening of the Na activation gates
  until the closing of inactivation gates
• The absolute refractory period
• Prevents the neuron from generating an action
  potential
• Ensures that each action potential is separate
• Enforces one-way transmission of nerve impulses

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Absolute Refractory Period
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Relative Refractory Period

• The interval following the absolute refractory
  period when
• Sodium gates are closed
• Potassium gates are open
• Repolarization is occurring
• The threshold level is elevated, allowing strong
  stimuli to increase the frequency of action
  potential events

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Conduction Velocities of Axons

• Conduction velocities vary widely among neurons
• Rate of impulse propagation is determined by
• Axon diameter the larger the diameter, the
  faster the impulse
• Presence of a myelin sheath myelination
  dramatically increases impulse speed

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Saltatory Conduction

• Current passes through a myelinated axon only at
  the nodes of Ranvier
• Voltage-gated Na channels are concentrated at
  these nodes
• Action potentials are triggered only at the nodes
  and jump from one node to the next
• Much faster than conduction along unmyelinated
  axons

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Saltatory Conduction
Figure 11.16
73
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• Shunting and short-circuiting of nerve impulses
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