NEURON LEC1 copy.pptx AND SYPMTOS AND THIER FUNCTIONS
- 1. STRUCTURE, FUNCTION AND PHYSIOLOGICAL PROPERTIES OF NEURON MUHAMMAD AYAZ
- 2. Overview of the nervous system Nervous system Peripheral Somatic Autonomic Sympathetic Parasympathetic Central Brain Spinal cord
- 3. Cells of Nervous system • Two types of cells – Neurons: • functional unit of nervous system capable of transmitting electrical signals to and from the brain and spinal cord. • Excitable; capable of generating action potential – Glial cells: • Constitute 90% of the cells in Nervous system • Provide structural and metabolic support to the neurons
- 4. Neurons Three components • Cell body: – Contains nucleus and cellular organelles – Main functional part. – Cells can divide. – Can act as a • Dendrite(s): – Branches from the cell body. – Receive signals through synapse. • Axon or nerve fiber: – Sends electrical signals from the neuron. Branches of axon are called collaterals. – Signals are sent in the form of electrical potential – Axon hillock; site where axon begins and action potential initiates – Axon terminal; site where electrical potential arrives to release neurotransmitter • Pre- and post-synaptic cells
- 5. Localization of ion channels • Leak channels: – Always open – Responsible for resting membrane potential • Ligand gated channels: – Most dense in the dendrites and cell body – Receives message from neurotransmitters. • Voltage gated channels: – On the axon hillock (Na+ and K+ ion channels), axon fiber (Na+ and K+ ion channels), and axon terminal (Ca+2 channels) – Receive action potential
- 6. Structural classification of neurons Olfactory and ophthalmic Ganglia and PNS Most neurons of CNS
- 7. Functional classification of neurons
- 8. Glial cells • 90% of all cells in the nervous system • Provides structural support • Chemical and neurological support to the neurons • Intracellular communication • 5 types of glial cells – Astrocytes – Ependymal cells – Microglia – Oligodendrocytes- forms myelin sheath around the CNS neurons. One cell provide sheath for multiple neurons – Schwann cells; forms myelin sheath around PNS neurons. One cell provide sheath for one neuron
- 9. • Substantially reduce the leakage of ions due to multiple sheathing • Nodes of Ranvier; – Gaps in the myelin sheath – It has voltage gated sodium and potassium channels – Allows movement of ions across membrane to transmit action potential
- 10. Physiology of nerve fibers
- 11. Nerve fiber • Threadlike extension of a nerve cell, consisting of axon and myelin sheath. • They run from cell body of neuron to the receptors.
- 13. PROPERTIES OF NERVE FIBERS Respond to changes surrounding them Conduct nerve impulses along their length Detect the changes From receptors to CNS. Convert the change into NERVE IMPULSE From CNS to effector organs. Excitability Conductivity
- 14. Basic Physics Of Membrane Potentials • Membrane potentials are caused by diffusion. • Diffusion potential is caused by concentration difference of ions across the two sides of membrane. • Inside concentration of potassium is greater than outside. • So potassium will diffuse to the exterior of the membrane. • The potential difference for potassium ions is -94 mV
- 15. • In case of sodium ions, greater amounts of sodium ions are present outside the nerve membrane. • Sodium ions will diffuse to the interior of the nerve fiber. • The potential difference for sodium ions is +61mV.
- 17. The Nernst Potential • The amount of diffusion potential needed to oppose the net diffusion of a particular ion across a membrane is called the Nernst potential. • The magnitude of Nernst potential is determined by ratio of concentrations of that specific ion on the two sides of the membrane. • EMF = _+61 log conc. Insideconc. outside
- 18. Resting Membrane Potential • Potential difference across cell membrane under resting condition is called RMP. • Contributed by; 1. K+ D.P -94 mV 2. Na+ D.P +61 mV 3. Na+ K+ pump -4 mV • RMP of large nerve fibers is -90mV
- 19. Na+ - K+ Pump • This pump continuously pumps Na+ ions outside the cell. K+ ions inside the cell. • This pump moves 3 sodium ions to the outside for every 2 potassium ions. • This causes a –ive potential inside the cell membrane.
- 20. • This pumps creates large concentration gradients for sodium and potassium ions across resting nerve membrane. Na+ (outside): 142 mEq/L Na+ (inside): 14 mEq/L K+ (outside): 4 mEq/L K+ (inside): 140 mEq/L
- 21. Leak channels • These are protein channels through which sodium and potassium ions can leak. • Therefore these are called Na/K leak channels. • These channels are more permeable to potassium as compared to sodium.
- 23. Action potential • These are rapid changes in the membrane potential that spread rapidly along the nerve fiber membrane. • Each A.P begins with a sudden change from a normal resting -ive membrane potential to a +ive potential and then ends with an almost equally rapid change back to the –ive potential.
- 24. Stages of Action Potential 1. Resting stage: This is the RMP before A.P begins. The membrane is polarized during this stage because of -90mV negative membrane potential that is present.
- 25. 2. Depolarization stage: During this stage, the membrane become highly permeable to sodium ions & large amounts of Na ions move inside the axon. The polarized state of nerve will neutralize by influx of +ive ions, with the potential rising in +ive direction. This is called depolarization.
- 26. • In large nerves, great influx of Na ions will actually “overshoot” beyond the zero level and become positive. • In smaller fibers potential does not overshoot to the +ive state.
- 27. 3. Repolarization stage: Within a few milliseconds, the sodium channels begins to close and potassium channels open in large numbers. Rapid efflux of potassium ions to the exterior of nerve will restore the normal –ive membrane potential. This is called repolarization.
- 28. Overshoot
- 29. Plateau in action potential • Seen in heart muscle fiber. • Results from fast Na channels and slow Ca-Na channels and partly from K channels.
- 30. Voltage-Gated Na+ channels • Na+ channels have two gates. Activation Gates near outside of the channel Inactivation Gates near inside of the channel • During resting stage of A.P the activation gate is closed, thus preventing entry of any Na ions through these channels.
- 31. Activation of Na+ Channels • During depolarization stage, when the membrane potential reaches somewhere between -70 to -50 mV a conformational change occurs in activation gate, flipping it open. • This is the activated state of the channel and allows for Na+ ions influx.
- 32. Inactivation of Na+ Channels • At the same time when activation gate opens, the inactivation gate closes. • But closing of inactivation gate is a slow process so closing occurs few milliseconds after the opening of activation gate. • With the closing of this gate, no more Na ions will be allowed to move inside.
- 35. Voltage-Gated K+ Channels • During resting stage, the gate of K+ channel is closed thus preventing K+ ions to diffuse out. • When the membrane potential rises from -90 mV towards zero, conformational change in gate will cause it to open. • As this is a slow process, the K channels open almost at the same time when Na channels are beginning to close.
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