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The nerve impulse - Coggle Diagram
The nerve impulse
resting potential
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when at rest, the membrane maintains an electrical gradient, also known as polarization (a difference in electrical charge between the inside and outside of the cell)
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sodium-potassium pump
repeatedly transports three sodium ions out of the cell, while drawing two potassium ions into it - this requires energy, active transport
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this only works because of the selective permeability of the membrane - however some potassium ions in the neuron slowly leak out, carrying a positive charge with them
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potassium ion is positively charged, ant the inside of a cell is negatively charged so the electrical gradient tends to pull potassium in
however, potassium is more concertrated inside the cell, so the concentration gradient tends to drive it out
a cell has negative ions too - chloride ions are negatively charged, they are mainly outside the cell - chloride does have a net flow when the membrane's polarization changes
what is it good for? - prepares the neuron to respond rapidly (just like a poised bow, its ready to fire)
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action potential
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all-or-none law
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signaling different messages: the axon cannot send bigger or faster action potentials, all it can change is the timing (like flashing a light with varying speed, or like morse code)
myelin sheath
to increase the speed of an impulse traveling from one place to another, vertebrate axons evolved a special mechanism: sheaths of myelin - an insulating material composed of fats and proteins
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refractory period
when the sodium gates snap shut, the cell is in a refractory pediod: it resists the production of further action potentials
this is called the absolute refractory period, in which the membrane cannot produce another action potential, regardless of the stimulation
the second part is the relative refractory period, when a stronger-than-usual stimulus is necessary to initiate an action potential
this is the reason the electrical charge of an action potential cannot flow in the opposite direction (normally) - in and of itself, it would travel both ways, but the refractory period prevents this
local neurons
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because they don't have axons, they do not follow the all-or-none law - when it receives info from other neurons, it has a graded potential - varies in magnitude in proportion to the intensity of the stimulus
anesthesia
local anesthetic drugs (novocain, xylocaine) attach to the sodium channels of the membrane, preventing sodium ions from entering - receptors are screaming "pain" but the axons cannot transmit the message to the brain
