Propagation of the Action Potential
structure of the neuron
- soma: body of the nerve cell
- contains nucleus, most of cellular synthetic machinery
- can receive communication from other nerve cells via synapses directly on membrane of the soma
- dendrites: small diameter process specialized to receive communication from other neurons via synapses
- can be extensively branched, receiving inputs from thousands of other cells
- activity at synapses changes local potential
- pulse travels to soma and axon (often with attenuation) to contribute to whether an AP will fire
- axon hillock: initial segment of the axon
- axon: neuronal process specialized to conduct action potentials
- axon terminal: terminal arborization of the axon, forming synapses with other neurons and influencing excitability
- neurotransmitters: chemical transmitters frequently used in synapse
- synthesis: cell body
- transport: ordered system of kinesin, tubulin (microtubules), and myosin, actin (microfilaments)
- energy: ATP hydrolysis
mechanism of propagation
- local currents mediate propagation
- depolarization: influx of (+) ions causes local current that flows in either direction from the site of depolarization
- decrementation: loss in Vm magnitude with increasing distance from the site of depolarization
- fates of current
- propagation: flow further down the core of the axon
- capacitance: flow into the membrane to charge the capacitance
- leak conductance: flow through leak channels and loss
- only propagation is maintained with increasing distance
- quantitative analysis
- length constant: distance over which Vm has fallen to 1/e of its original value
- length constant (λ): 
- Rm: membrane resistance, representing inverse of leak conductance
- Ri: current-carrying capability of ions in the axoplasm
- current takes the path of least resistance
- large Rm (less leak conductance): increased propagation
- large Ri: decreased propagation due to increased flow through leak channels
- time constant: time over which Vm has changed to 1/e of its original value
- time constant (τ): 
- Rm: membrane resistance
- Cm: membrane capacitance
- time constant determines how fast adjacent patches of membrane are charged (and discharged) by local current
- determinants of propagation
- voltage-gated Na+ and K+ channels
- local currents generated by AP
- passive properties of the adjacent, quiescent membrane
propagation is highly reliable
- safety factor: margin of reliability for conduction of an action potential
- propagation of an AP is accomplished by regenerative depolarizations of successive membrane segments
- local currents are large, usually several-fold greater than required to bring a membrane to threshold
- temporarily refractory membrane: temporarily inactive region of membrane left by a propagating AP
- action potentials will move in either direction from the point of origin, but will not move back
- charges are still moving backwards
- Na+ channels are still inactivated in the reverse direction, so charges do not have an effect
- typically, AP arises at axon hillock and propagates towards axon terminals
factors that affect propagation velocity
- fiber diameter: increasing diameter causes increased λ
- Ri (resistance to longitudinal flow)
- inversely proportional to axonal cross-sectional area (thus to radius2)
- rapidly reduced with increasing axon diameter
- Rm (membrane resistance)
- inversely proportional to membrane surface area (thus to radius)
- reduced with increasing axon diameter
- combined effect: Ri falls faster relative to Rm, causing increased λ
- myelination
- myelin: lipid used to insulate some neuronal cells at regular intervals along the axon
- CNS: produced by oligodendrocytes
- PNS: produced by Schwann cells
- node of Ranvier: small segments of axonal membrane in myelinated neurons that are unmyelinated
- saltatory conduction: “leap” conduction from one node to the next
- consequences of myelination
- increased Rm: results from fewer channels (and leak channels) in myelinated portions of membrane
- decreased Cm: results from increased distance between the two “plates” of the capacitor
- result: currents are carried, with little decrement, from one node to the next
- relative velocities
- unmyelinated: ~0.5 m/s
- myelinated: ~50 m/s
conduction abnormalities
- conduction block: failure to propagate the action potential
- multiple sclerosis: demyelinating disease
- channel number under myelinated regions of axons
- relatively rich in K+ channels
- relatively poor in Na+ channels
- exposed membrane causes current shunting (loss) through membrane
- reduces current available to depolarize axon membrane at the next node
- prolongs the time required to reach threshold of action potential
- reduces conduction velocity, safety factor
- in severe cases, can cause conduction block
- additional causes of conduction abnormalities
- deficits in trafficking
- deficits in the function or regulation of ion channels
- electrolyte imbalance
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