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SİNİR SİSTEMİ FİZYOLOJİSİ Yrd.Doç.Dr. Ercan ÖZDEMİR.

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1 SİNİR SİSTEMİ FİZYOLOJİSİ Yrd.Doç.Dr. Ercan ÖZDEMİR

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

3 Sinir sistemi bölümleri n Merkezi sinir sistemi (CNS) – beyin – spinal kord

4 Sinir sistemi bölümleri n Periferik sinir sistemi (PNS): – Kranial ve spinal sinirler – Ganglionlar – Duysal reseptörler n Alt grupları: – Somatik – Otonomik n Motor komponent: – sempatik – parasempatik – Enterik

5 Sinir Hücresi

6 Nöronlar n Sinir hücresi – Hücre gövdesi – Dendritler – Aksonlar

7 Aksonal Transport

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

9

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

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

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

13 Aksiyon potansiyel n 2 fazı vardır: – Depolarizasyon n graded potentials move toward firing threshold n if reach threshold voltage regulated sodium channels open n reversal of membrane permeability – Repolarizasyon n sodium channels close n potassium channels open

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

15 Clinical Note n Multiple Sclerosis – Caused by progressive destruction of myelin sheaths of CNS neurons – Usually appears between ages of 20 – 40 n Twice as common in females as males – Auto-immune disease n 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 n Genetic and environmental components – Exposure to herpes virus? – No cure n Managed with beta-interferon – Reduces viral replication

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

17 Synapses n Electrical synapse – ions flow directly from one cell to another through gap junctions n fast communication n synchronisation

18 Synapses n Chemical synapse – presynaptic neuron releases neurotransmitter n 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 n Spatial summation n Temporal summation

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

20 Neural circuits n Reverberating circuit – Once presynaptic cell stimulated causes postsynaptic cell to transmit a series of impulses n Example: coordinated muscular activity n Parallel after-discharge circuit – Single presynaptic neuron synapses with multiple neurons which synapse with single postsynaptic cell n 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 n Neruons exhibit plasticity: – New dendrites – New proteins – New synaptic contacts n Limited capacity to regenerate – PNS n Damage to dendrites and myelinated axons possible if: – cell body intact – Schwann cells (myelin producing) remain active n CNS – Little or no repair of damage to neurons

22 Central Nervous System n 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: n Inhibitory influences from neuroglia (particularly oligodendrocytes) n Absence of growth promoting signals that were present during fetal development n CNS injury – Injury of brain or spinal cord usually permanent n Following axonal damage nearby astrocytes proliferate rapidly forming scar tissue – Physical barrier to regeneration

23 Peripheral Nervous System n Axons and dendrites of PNS may repair if: – Associated with a neurolemma n most PNS cell processes covered with a neurolemma – Cell body intact – Schwann cells functional n Form neurolemma – Scar tissue does not form too rapidly

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


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