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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
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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
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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
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Graded potentials at synapse:
- Variable amplitude
- Positive or negative (EPSP or IPSP)
- Slow
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Action potentials at axon initial segment
- All-or-none response: size converted to frequency
- Spike frequency – EPSP amplitude
- Fast, stereotyped response
- Threshold for activation
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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
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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
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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
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Potassium channel
- Resting state: activation gate
- Depolarize – Channel activated: activation gate open, K+ rushes in
- Repolarization closes the activation gate
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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
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Voltage-dependent Sodium Channel Transmembrane topology
- S4 – voltage sensor
- Inactivation gate:
- S5 and S6 change shape in response to S1 and open up
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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!!
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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
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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: !
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Hyperkalemia
- Will slightly depolarize cell
- Resuts in hyperexcitability
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Hypokalemia
- Hyperpolarize cell
- Hypoexcitability
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Larger diameter axons
- Conduct faster because are likely to be myelinated – myelination is a greater factor that diameter
- Conduct faster if myelinated or demyelinated
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Myelin
- Reduces membrane capacitance and allows for spike hopping between nodes of Ranvier
- Nodes of Ranvier speed conduction by spike hopping
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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
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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
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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
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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
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Sensory-Evoked Responses
- Nerve cells electrical response to stimulation
- Signal averaging can be used to enhance detection
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Electroretinogram (ERG)
- a-wave: photoreceptor response
- b-wave: ON bipolar cell response
- d-wave: OFF bipolar cell response
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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
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