SİNİR SİSTEMİ FİZYOLOJİSİ Yrd.Doç.Dr. Ercan ÖZDEMİR

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

Sinir sistemi bölümleri Merkezi sinir sistemi (CNS) beyin spinal kord

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

Sinir Hücresi

Nöronlar Sinir hücresi Hücre gövdesi Dendritler Aksonlar

Aksonal Transport

Nöroglia Nöroglia Nöronlardan daha çok sayıda CNS nöroglia PNS Nöronları çok çeşitli yönlerden destekler Astrocytes - maintain chemical environment for action potential generation - help form blood-brain barrier Oligodendrocytes - produce myelin sheath in CNS Microglia - CNS phagocytes Ependymal cells - form cerebrospinal fluid Schwann cells - produce myelin sheath in PNS - form neurolemma Satellite cells - maintain chemical environment in ganglia

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

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

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.

Aksiyon potansiyel 2 fazı vardır: Depolarizasyon Repolarizasyon 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

Aksonal iletim Unmyelinated fibres Myelinated 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

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

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

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

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 Spatial summation - results from build up of neurotransmitter released simultaneously by numerous synaptic end bulbs Temporal summation - results from build up of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession

Neural circuits Divergence Convergence 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 Spatial summation - results from build up of neurotransmitter released simultaneously by numerous synaptic end bulbs Temporal summation - results from build up of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession

Neural circuits Reverberating circuit Parallel after-discharge 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) Spatial summation - results from build up of neurotransmitter released simultaneously by numerous synaptic end bulbs Temporal summation - results from build up of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession

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 Spatial summation - results from build up of neurotransmitter released simultaneously by numerous synaptic end bulbs Temporal summation - results from build up of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession

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 Spatial summation - results from build up of neurotransmitter released simultaneously by numerous synaptic end bulbs Temporal summation - results from build up of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession

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 Spatial summation - results from build up of neurotransmitter released simultaneously by numerous synaptic end bulbs Temporal summation - results from build up of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession

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 Spatial summation - results from build up of neurotransmitter released simultaneously by numerous synaptic end bulbs Temporal summation - results from build up of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession