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Chapter 4: Action Potential (Action Potential (Threshold: critical level…
Chapter 4: Action Potential
Passive and active membrane potentials
passive: leak channels
active: gated channels
membrane is polarized at rest (more negative inside)
hyper polarization: membrane potential < resting potential
depolarization: membrane potential > resting potential
Action Potential
Threshold: critical level of depolarization that must be reached to trigger action potential
"all or none": once membrane potential crosses THRESHOLD value for AP
change in resting membrane potential can occur
naturally from ADEQUATE STIMULUS to a sensory receptor
naturally in response to CHEMICAL NEUROTRANSMITTERS
experimentally from CURRENT INJECTION
light
firing FREQUENCY reflects MAGNITUDE of depolarizing current
conveys info over distances
neural code: frequency and pattern of action potentials
action potential: spike, nerve impulse, discharge
Properties: rising phase, overshoot, falling phase, undershoot
Action Potential in Theory (ideal neuron)
membrane potential is DEPOLARIZED to THRESHOLD
Na+ channels open, transient increase in gNa
Na+ influx depolarizes neuron
opens more v-gated Na+ channels (positive feedback)
increase in gNa is brief, AP duration is short
K+ channels open, increase in gK during falling phase, repolarizes membrane back to Vrest (restore negative potential)
Action potential, in reality
rising phase
overshoot: Vm approaches ENa
falling phase: K+ channels finally open (delayed relative to Na+), K+ rushes OUT of cell
undershoot: Vm approaches EK as channels remain open
Absolute refractory period: Na+ channels are INACTIVATED until membrane goes negative enough to DEINACTIVATE channels
Relative refractory period: membrane stays HYPERPOLARIZED until K+ channels close, more depolarization required to bring Vm to threshold
Clamps
current clamp: inject current, measure voltage
voltage clamp: "clamp" voltage, measure current
Patch-clamp: manipulates voltage, measures current through INDIVIDUAL ION CHANNELS
Separating Na+ and K+ currents
TTX blocks Na+ currents
TEA blocks K+ currents
Voltage-gated Na+ channel: large protein with 4 domains, each has 6 transmembrane alpha helices, domains connected by intracellular loops
voltage-dependent activation of Na channel
membrane depolarization twists S4 region; opens gate
Closed channel
Opens upon depolarization, Na+ flows in (activation gate)
Inactivation gate, ball of protein swings up and blocks the pore (ball and chain)
deinactivation, ball swings away and pore closes by movement of transmembrane domains
ball: positively charged protein
chain: flexible amino acid
Unlike passively propagates signals, an action potential can TRAVEL a great distance WITHOUT losing amplitude
Factors influencing speed
increased axon diameter: reduces internal resistance
few ions in contact with membrane
little current leaks out
more current down axon
faster propagation
myelination: increases membrane resistance
v-gated channels are concentrated at the NODES OF RANVIER
at the Node of Ranvier: action potential JUMPS from node to node (saltatory conduction)
Spike initiation zone: high DENSITY of v-gated Na+ channels