Conduction along nerve synapse 2013

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  • 1. - Conduction of excitation- Conduction of excitation along the nervealong the nerve - Neuromuscular synapse- Neuromuscular synapse
  • 2. Laws for stimulation of excitableLaws for stimulation of excitable tissuestissues 1.1. The force law (or “all or none”)The force law (or “all or none”) The stimulus has to have a certain minimum strength to produce excitation - thresholdthreshold - a sub-threshold stimulus will not elicit a response - a supra-threshold will elicit the state of excitation
  • 3. 3 Laws for stimulation of excitableLaws for stimulation of excitable tissuestissues 2.2. The time law (the force-time law)The time law (the force-time law) The stimulus has to act over a minimum period of time  The strength-duration curve for stimulus of an excitable tissue
  • 4. 3.3. The law of force gradient (the accommodation law)The law of force gradient (the accommodation law) 4 Laws for stimulation of excitableLaws for stimulation of excitable tissuestissues The stimulus has to act suddenly Neural accommodation or neuronal accommodation occurs  when a neuron or muscle cell is depolarized by slowly rising  current (ramp depolarization) in vitro.  During neuronal accommodation the slowly rising depolarization  drives the activation and inactivation, as well as the potassium  gates simultaneously and never evokes action potential.
  • 5. 1. Depolarization of the membrane under the cathode. 2. Hyperpolarization of the membrane under the anode 5 The action of direct current on anThe action of direct current on an excitable tissues. Position of the polar lawexcitable tissues. Position of the polar law _ + _ + __ _ _ _ + + + + + + + + + + + + + + + + ++ + + + + + + +++++++ _ _ _ _ _ _ _ _ _ _ _ _ _ __ _ _ _ _ _ __ __ ++ cathode anode
  • 6. Types of changes in membrane potentialTypes of changes in membrane potential
  • 7. 7 1 - Cell body 2 - Dendrites 3 - Axon 4 - Axon hillock 5 - Collateral axon 6 - Myelinated region 7 - Node of Ranvier  8 - Terminal branches 9 - Synaptic knobs The structure of neuronThe structure of neuron 1 4 5 3 2 2 6 7 8 9 •myelinated fibersmyelinated fibers •unmyelinated fibersunmyelinated fibers At the nodes of Ranvier  the axon membrane is  not covered by myelin  and is in direct contact  with the extracellular  fluid.
  • 8. The structure of myelinated fibersThe structure of myelinated fibers and unmyelinated fibersand unmyelinated fibers Schwann cell rotates around the axon many  times, laying down multiple layers of Schwann  cell membrane containing the lipid substance  sphingomyelin. This substance is an  excellent electrical insulator. 1. Schwann cells (in peryferal NS) 2. Oligodendrocytes (in CNS) 8
  • 9. Factors that determine theFactors that determine the conduction velocity ofconduction velocity of axonsaxons 1. the resistance of the axon to the flow of electrical current along its length (its internal electrical resistanceits internal electrical resistance) 2.2. the electrical resistance of the axon membranethe electrical resistance of the axon membrane 3.3. the electrical capacitance of the axon membranethe electrical capacitance of the axon membrane. 9 • the electricalthe electrical resistanceresistance of the axon membrane isof the axon membrane is higherhigher than for unmyelinated fibersthan for unmyelinated fibers • the membranethe membrane capacitancecapacitance isis lowerlower than that ofthan that of unmyelinated fibers.unmyelinated fibers. Large axons have the thickest myelin and greatest internodal distance. As a result, largelarge myelinated axonsmyelinated axons have the highesthave the highest conduction velocity.conduction velocity.
  • 10. Propagation of Action Potentials in NervePropagation of Action Potentials in Nerve FiberFiber 10
  • 11. Propagation of Action Potentials in NervePropagation of Action Potentials in Nerve FiberFiber myelinated fibers (saltatory) unmyelinated fibers (continuous) 11
  • 12. Classification of nerve fibersClassification of nerve fibers 12 In the general classification the fibers are divided into types A, B and C, and the type A fibers are further subdivided into α, β, γ, δ fibers.
  • 13. Laws of conduction of stimulationLaws of conduction of stimulation along nerve:along nerve: 1.1. Two-way conduction of stimulation in nerve.Two-way conduction of stimulation in nerve. (During the brining on the threshold or suprathreshold stimulus on the nerve the stimulation will spread in both sides.) 2.2. Isolated conduction of stimulation along a nerveIsolated conduction of stimulation along a nerve trunk.trunk. Action potential spreads only along each fiber and doesn’t pass to another. This is accounted that, electrical resistance along fiber lower (axoplasm and intercellular fluid are good guides for electrical current), then between next situating fibers, which divide by cell membrane. 3.3. The law of physiological and anatomical continuity ofThe law of physiological and anatomical continuity of a nervea nerve.. Conduction of stimulation along the nerve impossible after it’s cutting - the anatomic continuity will disturbe. The disturbing at conduction by way of using of various chemical (local anaesthetic – novocain, hypoxia, inflammation, cooling)). 13
  • 14. Two-wayTwo-way conduction ofconduction of stimulation instimulation in nervenerve During the brining on the threshold or suprathreshold stimulus on the nerve the stimulation will spread in both sides
  • 15. A compound action potential recorded atA compound action potential recorded at different points along an intact nervedifferent points along an intact nerve 15 Each wave reflects the activity of a group of fibers with a similar conduction velocity.
  • 16. Neuromuscular transmissionNeuromuscular transmission • The process of transmitting a signal from a motor nerve (motoneuronmotoneuron) to a skeletal muscle to cause it to contract is called neuromuscular transmissionneuromuscular transmission. • The motoneuronmotoneuron and the muscle fibers it controls form a motor unit as an action potential in the main axon of a motoneuron causes the contraction of all the muscleall the muscle fibers to which it is connectedfibers to which it is connected. • The region of contact between a motor axon and a muscle fiber is called the neuromuscular junctionthe neuromuscular junction or motor end-plate (neuromuscularmotor end-plate (neuromuscular synapssynaps)). 16
  • 18. • SynapsesSynapses connect nerve cells to other nerve cells (also applies for certain muscle cells) as well as to sensory and effector cells (muscle and glandular cells). Synaptic TransmissionSynaptic Transmission Structure of synapse: •presynaptic membrane,presynaptic membrane, •postsynaptic membrane,postsynaptic membrane, •synaptic cleftsynaptic cleft Electrical synapses are direct, ion-conducting cell–cell junctions through channels (connexons) in the region of gap junctions. Chemical synapses utilize neurotransmitters for the transmission of information and provide not only simple 1:1 connections, but also serve as switching elements for the nervous system.
  • 19. ChemicalChemical synapsessynapses 19
  • 20. The neuromuscular junctionThe neuromuscular junction 1. First, an action potential in the motor axon invades the nerve terminal and depolarizes the membranedepolarizes the membrane. 2. This depolarization opens voltage-gated calciumopens voltage-gated calcium channelschannels and calcium ions enter the nerve terminalcalcium ions enter the nerve terminal by diffusion down their electrochemical gradient. 3. This leads to a rise in intracellular free calciumrise in intracellular free calcium which triggers the fusion of docked synapticthe fusion of docked synaptic vesicles with the plasma membranevesicles with the plasma membrane. 4. The AChACh contained within the synaptic vesicles is released into the synaptic cleftinto the synaptic cleft. 20
  • 21. TheThe neuromuscularneuromuscular junctionjunction 5. ACh molecules diffuse across the cleft and bind to thebind to the nicotinic receptorsnicotinic receptors on the postsynaptic membrane. 6. When the receptors bind ACh they cause nonselective cation channels to open and this depolarizes the muscle membrane in the end-plate region. This depolarization is called the end-plate potentialend-plate potential, or epp. 7. Finally, when the epp has reached thresholdthe epp has reached threshold, the muscle membrane generates an action potentialmuscle membrane generates an action potential that propagates along the length of the fiber. This action potential triggers the contraction of the musclethe contraction of the muscle fiberfiber. 21
  • 22. 22
  • 23. Ca + ions Ионы кальция 23
  • 24. 24
  • 25. ACETYCHOLINE Ion channel Potassium ion Sodium ion Cholinoreceptor 25
  • 26. 26
  • 27. The end-plate 27
  • 28. Cholinaestera se 28
  • 29. 29
  • 30. Axon transportAxon transport • Axonal transport (or Axoplasmic) is a cellular process responsible for movement of mitochondria, lipids, synaptic vesicles, proteins, and other cell parts (i.e. organelles) to and from a neuron's cell body, through the cytoplasm of its axon (the axoplasm). • Transport is bi-directional – anterograde • fast (motor: kinesins complex proteins) • slow – retrograde • fast (motor: dynactin complex proteins) • slow – some neurotropic viruses such as poliomyelitis, herpes, and rabies and neurotoxins that enter peripheral nerve endings and ascend to infect the cell body via retrograde transport.
  • 31. Axon transportAxon transport 31 Vesicular cargoes move relatively fast (50– 400 mm/day) whereas transport of soluble (cytosolic) and cytoskeletal proteins takes much longer (moving at less than 8 mm/day).