SİNİR SİSTEMİ FİZYOLOJİSİ Yrd.Doç.Dr. Ercan ÖZDEMİR

1. SİNİR SİSTEMİ FİZYOLOJİSİYrd.Doç.Dr. Ercan ÖZDEMİR

2. Sinir Sistemi Fonksiyonları • Sinir sisteminin üç önemli fonksiyonu vardır: • Duysal • Reseptörler ile iç ve dış çevrenin denetimi • Bütünleşme, kaynaşma • Sensoriyal bilgileri toplayıp bunları işleyerek uygun cevapların oluşturulması • Motor • Oluşan bu cevapların efektör sinyallerle uygun şekilde hedef organlara ulaştırılması

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 grupları: • Somatik • Otonomik • Motor komponent: • sempatik • parasempatik • Enterik

5. SinirHücresi

6. Sinir hücresi Hücre gövdesi Dendritler Aksonlar Nöronlar

7. Aksonal Transport

8. Nöroglia Nöronlardan daha çok sayıda Nöronları çok çeşitli yönlerden destekler CNS nöroglia Nöroglia PNS nöroglia

10. İstirahat membran potansiyeli MP) • RMP nöronda –70 mV (range –40 mV to –90 mv). • Sonuçları: • Zarın her iki yanındaki iyon konsantrasyonları eşit şekilde dağılmaz. • Bunu sodyum ve potasyum pompaları sağlar

11. Dereceli potansiyeller • Dereceli potansiyel • Membran potansiyelinde lokal değişiklikler olur • Stimuluslara cevaplar farklılıklar gösterir. • Ateşleme eşik sını- • rına yaklaşılması depolarizasyon şeklinde, • Eşik değerden uzaklaşılması ise hiperpolarizasyon şeklinde kendini gösterir.

12. Aksiyon Potansiyel • Membranın geçirgenliği artar ve iyonların akımı sağlanır • Membranda voltaj değişikliği olur • Elektriksel sinyaller aksonlar boyunca yayılır • Nöronlar arasında voltaj farkı artar • Belirli nöronlar için bu süreç aynıdır.

13. Aksiyon potansiyel • 2 fazı vardır: • 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