SINIR SISTEMI FIZYOLOJISI Yrd.Do

1
SINIR SISTEMI FIZYOLOJISIYrd.Doç.Dr. Ercan
ÖZDEMIR
2
Sinir Sistemi Fonksiyonlari

• Sinir sisteminin üç önemli fonksiyonu vardir
• Duysal
• Reseptörler ile iç ve dis çevrenin denetimi
• Bütünlesme, kaynasma
• Sensoriyal bilgileri toplayip bunlari isleyerek
  uygun cevaplarin olusturulmasi
• Motor
• Olusan bu cevaplarin efektör sinyallerle uygun
  sekilde hedef organlara ulastirilmasi

3
Sinir sistemi bölümleri

• Merkezi sinir sistemi (CNS)
• beyin
• spinal kord

4
Sinir sistemi bölümleri

• Periferik sinir sistemi (PNS)
• Kranial ve spinal sinirler
• Ganglionlar
• Duysal reseptörler
• Alt gruplari
• Somatik
• Otonomik
• Motor komponent
• sempatik
• parasempatik
• Enterik

5
Sinir Hücresi
6
Nöronlar

• Sinir hücresi
• Hücre gövdesi
• Dendritler
• Aksonlar

7
Aksonal Transport
8
Nöroglia
CNS nöroglia
PNS nöroglia

• Nöroglia
• Nöronlardan daha çok sayida
• Nöronlari çok çesitli yönlerden destekler

9
(No Transcript)
10
Istirahat membran potansiyeli MP)

• RMP nöronda 70 mV (range 40 mV to 90 mv).
• Sonuçlari
• Zarin her iki yanindaki iyon konsantrasyonlari
  esit sekilde dagilmaz.
• Bunu sodyum ve potasyum pompalari saglar

11
Dereceli potansiyeller

• Dereceli potansiyel
• Membran potansiyelinde lokal degisiklikler olur
• Stimuluslara cevaplar farkliliklar gösterir.
• Atesleme esik sini-
• rina yaklasilmasi depolarizasyon seklinde,
• Esik degerden uzaklasilmasi ise hiperpolarizasyon
  seklinde kendini gösterir.

12
Aksiyon Potansiyel

• Membranin geçirgenligi artar ve iyonlarin akimi
  saglanir
• Membranda voltaj degisikligi olur
• Elektriksel sinyaller aksonlar boyunca yayilir
• Nöronlar arasinda voltaj farki artar
• Belirli nöronlar için bu süreç aynidir.

13
Aksiyon potansiyel

• 2 fazi vardir
• Depolarizasyon
• graded potentials move toward firing threshold
• if reach threshold voltage regulated sodium
  channels open
• reversal of membrane permeability
• Repolarizasyon
• sodium channels close
• potassium channels open

14
Aksonal iletim

• Unmyelinated fibres
• continuous conduction
• Myelinated fibres
• saltatory conduction
• High density of voltage gated channels at Nodes
  of Ranvier
• Larger diameter axons propagate impulses faster
• Stimulus intensity encoded by
• frequency of impulse generation
• number of sensory neurons activated

15
Clinical Note

• Multiple Sclerosis
• Caused by progressive destruction of myelin
  sheaths of CNS neurons
• Usually appears between ages of 20 40
• Twice as common in females as males
• Auto-immune disease
• Immune system spearheads attack
• Myelin sheaths deteriorate to scleroses (hardened
  scars or plaques)
• Slows and short-circuits propagation of nerve
  impulses
• Cause of disease unclear
• Genetic and environmental components
• Exposure to herpes virus?
• No cure
• Managed with beta-interferon
• Reduces viral replication

16
Synapses

• Synapse - functional junction between neurons or
  neuron and effector
• Structure and function change with learning
• Changes may allow signals to be transmitted or
  blocked
• In neuron neuron synapses
• presynaptic neuron
• post-synaptic neuron

17
Synapses

• Electrical synapse
• ions flow directly from one cell to another
  through gap junctions
• fast communication
• synchronisation

18
Synapses

• Chemical synapse
• presynaptic neuron releases neurotransmitter
• elicits postsynaptic potential in postsynaptic
  neuron
• Excitatory (EPSP)
• depolarises postsynaptic membrane bringing closer
  to firing threshold.
• Inhibitory (IPSP)
• hyperpolarises postsynaptic membrane moving
  further from firing threshold
• Postsynaptic neuron integrates excitatory and
  inhibitory inputs and responds accordingly
• Spatial summation
• Temporal summation

19
Neural circuits

• Divergence
• Single presynaptic neuron synapses with several
  postsynaptic neurons
• Example sensory signals spread in diverging
  circuits to several regions of the brain
• Convergence
• Several presynaptic neurons synpase with single
  postsynaptic neuron
• Example single motor neuron synapsing with
  skeletal muscle fibre receives input from several
  pathways originating in different brain regions

20
Neural circuits

• Reverberating circuit
• Once presynaptic cell stimulated causes
  postsynaptic cell to transmit a series of
  impulses
• Example coordinated muscular activity
• Parallel after-discharge circuit
• Single presynaptic neuron synapses with multiple
  neurons which synapse with single postsynaptic
  cell
• results in final neuron exhibiting multiple
  postsynaptic potentials
• Example may be involved in precise activities
  (eg mathematical calculations)

21
Regeneration and repair of nervous tissue

• Neruons exhibit plasticity
• New dendrites
• New proteins
• New synaptic contacts
• Limited capacity to regenerate
• PNS
• Damage to dendrites and myelinated axons possible
  if
• cell body intact
• Schwann cells (myelin producing) remain active
• CNS
• Little or no repair of damage to neurons

22
Central Nervous System

• Neurogenesis
• Birth of new neurons from undifferentiated stem
  cells occurs in hippocampus (area of brain
  involved in learning)
• Nearly complete lack of neurogenesis in other
  parts of CNS, due to
• Inhibitory influences from neuroglia
  (particularly oligodendrocytes)
• Absence of growth promoting signals that were
  present during fetal development
• CNS injury
• Injury of brain or spinal cord usually permanent
• Following axonal damage nearby astrocytes
  proliferate rapidly forming scar tissue
• Physical barrier to regeneration

23
Peripheral Nervous System

• Axons and dendrites of PNS may repair if
• Associated with a neurolemma
• most PNS cell processes covered with a neurolemma
• Cell body intact
• Schwann cells functional
• Form neurolemma
• Scar tissue does not form too rapidly

24
Peripheral Nervous System

• 24-28 hours after injury to neuron
• Nissl bodies (clusters of rough ER) break up into
  granular masses (chromatolysis)
• 72-90 hours post-injury
• Part of axon distal to injury undergoes Wallerian
  degeneration
• axon swells and breaks up into fragments
• myelin sheath deteriorates
• Macrophages then phagocytose debris
• Later on
• Synthesis of RNA and protein accelerates
• Schwann cells undergo mitosis and form
  regeneration tube across injured area
• Guides growth of new axon
• Eventually forms new myelin sheath