Action potential/nerve impulse

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Action potential/nerve impulse:
The action potential or nerve impulse is a rapid and reversible depolarisation signal – it is more dramatic than local potentials. Produced by voltage regulated ion gates (local potentials produced by ligand regulated gates).

Characteristics:
• it is an ‘all-or-none’ phenomenon – peak voltage is the same regardless of strength of stimulus (local potential is proportional to strength of stimulus)
• signal maintains same strength regardless of distance (non-decremental)
• self propagating

Generation of action potential:
initiated at ‘initial segment’ of the axon (has lowest threshold potential)
initial depolarisation appears as a rising local potential
when local potential reaches a threshold (usually –55mV), voltage regulated gates open  cation potential generated (neuron “fires”)
at threshold, Na+ and K+ gates open (Na+ quicker then K+, so initial effect due to Na+)
as Na+ enters cell, more Na+ gates open (positive feedback cycle)
when membrane potential passes 0mV  Na+ gates inactivated; flow of Na+ eventually stops and potential is about +35mV
membrane now positive on inside and negative on inside (opposite of resting membrane potential)
K+ now leave cells as they are repelled by positive charge of intracellular fluid  repolarises membrane
K+ stay open longer than Na+ gates  greater flow of K+ than Na+  voltage drops a 1-2mV less than original resting membrane potential  ‘hyperpolarisation’ or positive after potential (astrocytes gradually restore original resting membrane potential)

For the nerve impulse to be propagated, the action potential provides a depolarisation stimulus for triggering the action potential in the next section of the membrane – this will only occur if a threshold is reached.

Stronger stimuli do not cause a greater action potential  but, cause action potentials to be generated more frequently.

Refractory period:
• neuron cannot respond to another stimulus as it is generating its action potential or for a short period after – this resistance to restimulation is referred to as the refractory period
• absolute refractory period – no stimulus of any strength will trigger a new action potential; lasts from start of action potential until membrane returns to resting potential
• relative refractory period – action potential can be triggered, but must be a strong stimulus; lasts until hyperpolarisation ends (K+ gates are still open)

Velocity of conduction:
• varies widely
• largely depends on two factors:
1) Axon diameter (generally, the larger the diameter  faster conduction of impulse)
2) Myelin sheath (faster in myelinated fibres)

Group A fibres:
• larger diametre and thicker myelin sheaths
• conduction velocity of 15 to 150m/s
• mostly somatic sensory and motor fibres to/from skin, muscles and joint
Group B fibres:
• intermediate diameter and lightly myelinated
• conduction velocity of 3-15m/s
Group C fibres:
small diametre and not myelinated
conduction velocity of <1m/s

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