•“Firing” refers to passing a signal along the axon and releasing
chemicals from the terminal buttons.
•Signals arrive at the dendrites by the thousands, and are
oExcitatory - signals stimulate the neurons to fire. (Excitorty
depolarize the neurons) Or
oInhibitory – reduce the likelihood of neurons firering.
(Hyperpolarize the neuron)
•Signals work by polarization.
Depolarization and Hyperpolarization
•Cells fire when they are depolarized past a certain threshold.
Depolarization causes a change in the permeability of the cell
membrane, which opens the gates for sodium, the influx of sodium
causes the inside of the neuron to be slightly more positive charged
than the outside. (This change is the basis of the action potential)
•When a neuron Hyperpolarizes it makes it more difficult for the
sodium to come through, thus it doesn’t fire.
Action Potentials Spread Along the Axon
•An action potential is generated when there are more excitatory
signals that inhibitory signals, thus the total amount of depolarization
surpasses the neurons threshold.
•When the neuron is fires, the depolarization of the cell membrane
moves along the axon, like a wave, which is called propagation
•Like toppling dominos, sodium rushing through its ion channels leas to
further depolarization, causing adjacent sodium channels to open.
•At any point along the axon, the sudden influx of sodium repels
•This continues until a equilibrium is reached, and other forces work to
close the sodium gate (this all happens in about 1/1,000 of a second.
Absolute and Relative Refractory Periods
•The gate stops the flow of sodium, potassium stops leaving (this take 1-
2 milliseconds) in this time, a decreased amount of potassium in the
cell body momentarily creates a state of hyperpolarization.
•This brief period is referred to as ‘absolute refractory period’ – it is
impossible for the neuron to fire, (which keeps from a ripple effect
•Each section of the axon depolarizes, the preceding section enters its
absolute refractory state. (the signal cant travel backwards)
•As potassium stops leaving the neuron, and the cell membrane is
returning to its resting membrane potential, the neuron can fire. (But
only in response to a very strong signal)
• - This period of time is called relative refractory period.