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Synapses

Synapses. A. Neuromuscular Junction (typical ACh synapse). 1. arrival of action potential at terminal bulb triggers opening of voltage-gated Ca ++ channels. Synapses. 2. Ca ++ influx  phosphorylation. a. vesicle liberated from presynaptic actin network.

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Synapses

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  1. Synapses A. Neuromuscular Junction (typical ACh synapse) 1. arrival of action potential at terminal bulb triggers opening of voltage-gated Ca++ channels

  2. Synapses 2. Ca++ influx  phosphorylation a. vesicle liberated from presynaptic actin network b. vesicle binds to presynaptic membrane

  3. Synapses examples of phosphorylated proteins: N-ethylmaleimide-sensitive fusion protein (NSF)

  4. Synapses soluble NSF attachment proteins (SNAPs) SNAP receptors (SNAREs)

  5. Synapses vesicle recycled via endocytosis

  6. Synapses some neurotransmitter leaks out of cleft some inactivated (acetylcholinesterase)

  7. Neuromuscular Junction remainder binds to postsynaptic receptor on motor end plate each vesicle contains enough ACh to trigger miniature end-plate potential (mepp)

  8. Neuromuscular Junction mepps can sum to generate end-plate potential (epp) epps can sum to threshold to generate action potential adjacent to the motor end plate

  9. Neuromuscular Junction Disorders Curare binds to AChR paralysis

  10. Neuromuscular Junction Disorders Botulinum toxin prevents release of ACh paralysis cleaves SNARE proteins

  11. Neuromuscular Junction Disorders Myasthenia gravis antibody generated against AChR weakness worsens progressively

  12. Neuromuscular Junction Disorders Neuromyotonia antibody generated against presynaptic K+ channels axon terminal constantly depolarized (cramping)

  13. Potential Transmission A. Electrotonic 1. graded 2. receptor (generator) potentials

  14. Potential Transmission a.  stimulus, then  ∆ Vm b. electrical signal spreads from source of stimulus c. problem: no voltage-gated channels here d. signal decay “passive electrotonic transmission”

  15. Potential Transmission A. Electrotonic 3. good for only short distances 4. might reach axon hillock - that’s where voltage-gated channels are - where action potentials may be triggered

  16. Potential Transmission B. Action potential 1. propagation without decrement 2. to axon terminal

  17. Synaptic Transmission C. Alternation of graded and action potentials

  18. Intraneuron Transmission A. All neurons have electrotonic conduction (passive) B. Cable properties 1. determine conduction down the axon process 2. some cytoplasmic resistance to longitudinal flow 3. high resistance of membrane to current “but membrane is leaky”

  19. Intraneuron Transmission C. Propagation of action potentials 1. ∆ Vm much larger than threshold - safety factor

  20. Intraneuron Transmission C. Propagation of action potentials 2. spreads to nearby areas - depends on cable properties - inactive membrane depolarized by electrotonically conducted current

  21. Intraneuron Transmission C. Propagation of action potentials - K+ efflux behind region of Na+ influx

  22. Intraneuron Transmission C. Propagation of action potentials 3. unidirectional a. refractory period b. K+ channels still open

  23. Intraneuron Transmission C. Propagation of action potentials 4. speed a. relates to axon diameter and presence of myelin b.  axon diameter,  speed of conduction

  24. Intraneuron Transmission D. Saltatory conduction 1. myelination a.  Rm ,  Cm b. the more layering, the greater the resistance between ICF and ECF

  25. Intraneuron Transmission D. Saltatory conduction c. charge flows more easily down the axon than across the membrane

  26. Intraneuron Transmission D. Saltatory conduction 2. nodes of Ranvier a. internodes (between Schwann cells or oligodendrocytes) b. only exit for current c. only location along axon where APs are generated

  27. Neuronal Integration A. Motor neurons as example 1. thousands of excitatory and inhibitory terminals on dendrites and soma - density often highest around hillock - proximity often confers preference

  28. Neuronal Integration A. Motor neurons as example 2. control frequency of firing of motor neuron - only excitatory stimuli can cause behavior change

  29. Neuronal Integration A. Motor neurons as example 3. these terminals are weak - multiple stimuli needed to trigger AP - prevents spontaneous activation of motor neurons

  30. Neuronal Integration B. Spatial summation 1. inputs from several synapses summed to simultaneously change Vm 2. often a battle between EPSPs and IPSPs

  31. Neuronal Integration C. Temporal summation 1. second potential follows close after first 2. “piggybacks” 3. amplifies potential 4. spatial and temporal often together

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