Neuron signaling- electrical conduction and chemical synapses.txt

• 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

• 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

• 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

• Variable amplitude
• Positive or negative (EPSP or IPSP)
• Slow

• All-or-none response: size converted to frequency
• Spike frequency – EPSP amplitude
• Fast, stereotyped response
• Threshold for activation

• 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

• 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

• 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

• Resting state: activation gate
• Depolarize – Channel activated: activation gate open, K+ rushes in
• Repolarization closes the activation gate

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

• S4 – voltage sensor
• Inactivation gate:
• S5 and S6 change shape in response to S1 and open up

• 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!!

• 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

• 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: !

• Will slightly depolarize cell
• Resuts in hyperexcitability

• Hyperpolarize cell
• Hypoexcitability

• Conduct faster because are likely to be myelinated – myelination is a greater factor that diameter
• Conduct faster if myelinated or demyelinated

• Reduces membrane capacitance and allows for spike hopping between nodes of Ranvier
• Nodes of Ranvier speed conduction by spike hopping

• 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

• 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

• 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

• 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

• Nerve cells electrical response to stimulation
• Signal averaging can be used to enhance detection

• a-wave: photoreceptor response
• b-wave: ON bipolar cell response
• d-wave: OFF bipolar cell 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