Physiology of neurotransmission


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Nervous system forms an interconnecting fibers of communication network.
In the ‘hard-wiring’ of the nerves, the signals travel in the form of a flow of electrical current called nerve impulses.
The stimulus-response reactions afford internal constancy in the face of environmental changes.

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Physiology of neurotransmission

  1. 1.  Overview of the nervous system. Neuron doctrine Neuron – discovery, structure, description and properties. Nerve impulse – definition, all or none property, velocity and refractory period Neurotransmission-history, mechanism and properties.
  2. 2.  Nervous system forms an interconnecting fibers of communication network. In the „hard-wiring‟ of the nerves, the signals travel in the form of a flow of electrical current called nerve impulses. Irritability is the universal property of life which means the capacity of organisms to respond to changes in the environment called stimuli. The specific reaction elicited by a stimulus is termed a response. The stimulus-response reactions afford internal constancy in the face of environmental changes.
  3. 3. Central Nervous System (CNS) Peripheral Nervous nervous systemIntegrating System(PNS) systems Autonomic Endocrine Nervous system System(ANS)
  4. 4. Neuron Doctrine wasestablished by Cajal (1852-1934)• The neuron is the structural and functional unit of the nervous system;• Neurons are individual cells, which are not continuous.• The neuron has three parts: dendrites, soma (cell body) and axon• Conduction takes place in the direction from dendrites to soma, to the end of the axon
  5. 5. Camillo Golgi (1843-1926) invented aspecific staining technique for neurons. Cajalin 1888 identified the networks of nerve cells.Golgi and Cajal received the Nobel prize in1906 for Medicine and Physiology.• Wilhelm His in 1886 showed that the dendrites and axons grow out progressively from the immature neurons in the brains of embryos.• Henri Forel (1848-1931) in 1886,observed that when the cell body dies or an axon is cut, degeneration of the neuron stop at the junction to another neuron, thus giving evidence that they are separate.
  6. 6.  A neuron consists of a cell body and two kinds of processes, the dendrites and the axon. The cell body has neuroplasm, a nucleus, nissl bodies, neurofibrils and a cell membrane. The dendrites carry impulses towards the cell body. The axon carries impulses away from the cell body. The axon originates from axon hillock of the cell body. The axon is surrounded by two coverings: myelin sheath and Schwann sheath. These two coverings are interrupted at intervals by nodes of Ranvier. The fine branches at the end of axon are called axon terminals.
  7. 7. 1. Excitability –stimulated by suitable stimuli- mechanical, thermal, chemical, electrical.2. Conductivity – impulse is conducted similar to cable conduction and digital in character.3. All or none law – the stimulus should be in adequate threshold strength.4. Refractory period – when the nerve fiber is once excited, it will not respond to a second stimulus for a brief period.5. Indefatigability – nerve is normally not fatigued.6. Adaptation –the nerve quickly adapts itself.7. Accommodation – slowly applied stimulus is accommodated.
  8. 8. Nerves/Environment organism neuron Stimulus Impulse Response
  9. 9. Nerve impulse• A nerve impulse is the sum total of physical and chemical events associated with the transmission of a signal along an axon.• A wave of physiological activity- primarily an electrical phenomenon.
  10. 10.  Stimulus is defined as a sudden change in the environment which is strong enough to cause a response in the living organism. Mechanical Physical Chemicals Electrical• Sharp • heat • acids • Application pressure • cold • bases of electricity
  11. 11.  Stimulation can be affected by strength and duration. The weaker the stimulus, the longer it will have to be applied to produce a response. The nerve takes lesser time to respond for a stronger stimulus.
  12. 12. All or none law indicates the relation betweenthe stimulus and response.A stimulus, if it is capable of causing aresponse, causes a maximum response. If it isbelow the capacity, it will not cause anyresponse.The lowest strength of stimulus required to giverise to an action potential is the thresholdstimulus. A stimulus which is less than the threshold failsto induce any response-sub-threshold stimulus.A stimulus which is greater than optimum issupra threshold stimulus.
  13. 13. Johannes Muller Helmholtz showed that The velocity varies believed that nerve velocity of conduction from 100 m per sec inimpulse travelled at a was 100m per sec. large fibers to 0.5 m speed of light- (about 10 times faster per sec in small non- 1,86,000 miles per than a man can run) myelinated fibers. sec.
  14. 14. Once a impulse has passed over anypart of the neuron, for a short time itis unable to conduct any otherstimulus.This brief period of non-conductivityis called refractory period. Under good physiological conditions, the nerve fiber is indefatigable.
  15. 15. Bioelectricity• L. Galvani in 1786 discovered the presence of electrical current in nerves and muscles.Electrophysiology• Du Bois Raymond in 1848 concluded that impulse transmission was electrical by a wave of relative negativityMembrane potential hypothesis• Hodgkin and Huxley in 1939 demonstrated the electrical and chemical processes involved in bioelectricity.
  16. 16.  In order to confirm the generation of electricity, one microelectrode is placed on the outer surface of the nerve membrane and the other placed inside the nerve cell. When the terminals are connected to a galvanometer, the needle show a deflection indicating the flow of electrical current(-70mV). In a resting neuron the electrical potential ranges form 20 to 100 mV.
  17. 17.  According to the membrane potential hypothesis of Bernstein (1902), the differential concentration of ions between the inside and outside of the nerve cell is the basis of resting potential. The inner side of the nerve contain large –vely charged non-diffusible protein ions and smaller diffusible K+ and Cl- ions. Na+ ions are more concentrated on outside of the nerve cell. Radio-isotopic studies showed that potassium and sodium ions readily diffuse through the nerve cell membrane.
  18. 18.  In a state of physiological rest, the inner side of a neuron is negative to outside. This difference in electrical charge is called resting potential. Biological factors that govern resting potential The resting potential is maintained as long as the cell is alive and active. The resting potential depends upon the selectivity and variable permeability of cell membranes. The resting potential depends upon unequal distribution of ions.
  19. 19.  Active transport Concentration gradient Membrane permeability Electrostatic attraction
  20. 20.  This Na+ - K+ pump maintains unequal concentration of ions in the nerve fibers. This pump changes the electrical character of the nerve fibers. ATP is used as an energy for the process.
  21. 21.  The active transport establishes the concentration gradient. The extrusion of sodium ions is linked with active uptake of potassium ions. The rates of diffusion of ions depend upon gradients and membrane permeability.
  22. 22.  “at equilibrium, the product of the concentration of the diffusible ions inside the membrane equals the product of the concentration of the diffusible ions outside.” (Donnan 1928).
  23. 23.  All the plasma membranes are selectively permeable to ions. This membrane selectively allows inward diffusion of K+ and prevents inward diffusion of Na+ ions.
  24. 24.  The inner side of the nerve cell is negative to the outside. There is a growing attraction between ions of the same charges. The parallel diffusion of ions restore the resting potential.
  25. 25.  When a nerve is stimulated, Na+ ions suddenly move into the cell and causing a positive potential. The influx of Na + ions reaches its peak in 100msec.Sodium permeability causes depolarization. The inward diffusion of Na + ions halted near the peak of action potential. The diffusion K+ ions restores the membrane potential –called repolarization.
  26. 26. Stimulus Increased Increased Sodium Potassium Sodium Potassium influx effluxconductance conductanceMembrane Membrane Membranepermeability depolarization repolarization
  27. 27.  By using radioactive sodium (Na 24) and potassium (K42) Hodgkin and Keynes (1955) showed that inward flux of sodium was increased 20 times and outward flux of potassium 3-4 times after stimulation of nerve.
  28. 28.  The conduction of nerve impulse causes two phases – biphasic action potential. The conduction of nerve impulse in a myelinated fiber is so rapid because the action potential skips from node to node – saltatory conduction. The magnitude of nerve impulse transmission occurs without decrement – Non-decremental conduction. The velocity of conduction in a myelinated nerve is directly proportional to its diameter. The introduction of myelinization speeded up conduction velocity, reduced body weight and consumption of energy.
  29. 29.  Dr.B.Victor is a highly experienced professor, recently retired from the reputed educational institution- St. Xavier‟ s College, Palayamkottai, India-627001. He was the dean of sciences and assistant controller of examinations. He has more than 32 years of teaching and research experience. He has taught a diversity of UG and PG courses and guided 12 PhDs. Send your comments to :