Neuron signaling- electrical conduction and chemical synapses.txt

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Neuron signaling- electrical conduction and chemical synapses.txt
2010-11-19 09:37:09

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  1. Neuron signaling: electrical conduction and chemical synapses
    • Electrical signals travel from dendrites through the neuron to the synaptic terminal
    • In myelinated axons the action potential only appear at nodes between the myelin sheaths
  2. Graded potentials
    • Are formed at synapses
    • Dissipate quickly so neurons convert graded potentials to action potentials
    • EPSPs and IPSPs (excitatory and inhibitory synaptic potentials): Are graded potentials – are the response of a postsynaptic neuron to the stimulus from a presynaptic neuron
  3. Graded to Action potentials
    • Go from amplitude-dependent graded potential (greater amplitude = greater strength) to action potentials – an all-or-none response, frequency-dependent strength
    • EPSPs can produce action potentials if the graded depolarization is large enough (threshold)
    • Action potential frequency is proportional to graded potential amplitude
  4. Graded potentials at synapse:
    • Variable amplitude
    • Positive or negative (EPSP or IPSP)
    • Slow
  5. Action potentials at axon initial segment
    • All-or-none response: size converted to frequency
    • Spike frequency – EPSP amplitude
    • Fast, stereotyped response
    • Threshold for activation
  6. Effects of stimulus current strength on rate of depolarization and excitation onset
    • APs occur sooner if you have a stronger stimulus
    • Need to reach a certain threshold – but past the threshold - if more Na rushing in, get a faster response and the shorter the duration of stimulus required
  7. The action potential: Fast conduction without decay
    • Threshold: synaptic input
    • Rising phase: Sodium channel activation
    • Decay: Potassium channel activation (delayed) and Sodium channel inactivation
    • Undershoot: Potassium channels – b/c takes a while for K+ ch’s to turn off
  8. Sodium channel
    • Resting state: activation gate closed, inactivation gate open
    • Channel is activated: activation gate opens, but takes a while for inactivation gate to close
    • Channel is inactivated: when the inactivation gate closes after some time
    • Repolarization: slow recovery from inactivation – need for inactivation gate to open back up again
  9. Potassium channel
    • Resting state: activation gate
    • Depolarize – Channel activated: activation gate open, K+ rushes in
    • Repolarization closes the activation gate
  10. Channels: probabilistic opening
    So threshold – when enough probability that enough Na+ ch’s are open and able to depolarize the cell on their own and open more and more sodium channels
  11. Voltage-dependent Sodium Channel Transmembrane topology
    • S4 – voltage sensor
    • Inactivation gate:
    • S5 and S6 change shape in response to S1 and open up
  12. Absolute refractory period and Relative refractory period
    • Absolute refractory period: Na+ channels close and K+ channels open – some Na+ channels are closing down towards the end, but some are still open; K+ channels open
    • Relative refractory period: Na+ channels reset to original position while K+ channels remain open
    • Slowly closing potassium channels keep neuron further away from threshold, reducing excitability
    • Need a stronger stimulus to reach spike threshold during the relative refractory period; therefore, spike frequency depends on stimulus strength
    • From beginning of relative refractory period: the strength of stimulus needed to produce an AP decreases exponentially to threshold value
    • Absolute refractory period: prevents back propagation of the action potential!!
  13. Slow depolarization can raise spike threshold
    • So have to depolarize at a certain rate, otherwise get no or less AP
    • During a slow depolarization some sodium channels open and inactivate before other sodium channels open
  14. Block of voltage-gated sodium channel
    • Local anesthetics
    • Use-dependent (block active Na channels) – thus block nerve conduction
    • Better block of small diameter fibers
    • Many local anesthetics block by enhancing Na channel inactivation: !
  15. Hyperkalemia
    • Will slightly depolarize cell
    • Resuts in hyperexcitability
  16. Hypokalemia
    • Hyperpolarize cell
    • Hypoexcitability
  17. Larger diameter axons
    • Conduct faster because are likely to be myelinated – myelination is a greater factor that diameter
    • Conduct faster if myelinated or demyelinated
  18. Myelin
    • Reduces membrane capacitance and allows for spike hopping between nodes of Ranvier
    • Nodes of Ranvier speed conduction by spike hopping
  19. Demyelination
    • If loss of functional nodes – get a decay of signal over a distance
    • If lose 1-2 nodes can still be above threshold and propagate
    • BUT if lose 3 or more nodes, that can’t recover AP b/c decays below threshold
  20. Nerve conduction velocity
    • Average nerve conduction velocities are in the range of about 45-55 m/s
    • At this rate it would take a signal about 5msec to travel 25 cm
    • Sensory nerve fibers from muscle spindles travel at 120 m/s
    • Measure time from stimulus to nerve response, to twitch response – so can get a conduction velocity
  21. Conduction of compound action potential
    • Distinct action potentials can being at about the same time
    • But as they move down a pathway, can be conducted at different speeds and separate
    • So can produce different responses
  22. Field potentials
    • Synchronized activity by a large number of neurons
    • Neurons oriented in the same direction
    • Can get “addition” or currents and get field a current loop
  23. Sensory-Evoked Responses
    • Nerve cells electrical response to stimulation
    • Signal averaging can be used to enhance detection
  24. Electroretinogram (ERG)
    • a-wave: photoreceptor response
    • b-wave: ON bipolar cell response
    • d-wave: OFF bipolar cell response
  25. BAER (brainstem auditory evoked response)
    • Recorded from the top of the head but originate from structures within the brain
    • They are very small signals, which necessitates the use of signal averaging