Dendrites receive signals from neighboring neurons. Only if the signal passes the threshold an action potential is propagated. If the depolarization is of sufficient strength voltage-gated Na+ channels are opened which propagate the depolarization along the axon. The speed which the signal is propagated depends on the width of the axon (larger=faster) and whether or not the axon has a myelin sheath (myelin sheath=faster; CNS: Oligodendrocyte, PNS: Schwann Cell). Once the depolarization reaches the axon terminal the AP opens transmit signals to other neuron dendrites or tissues (like a radio transmitter)
RESTING STATE
The resting potential is a Dynamic Equilibrium established by various channels and pumps. The concentration of various ions and charged molecules, determine the neurons resting potential. It is typically cited as -70 mV, but can vary. Specifically the Na+/K+ ATPase pump establishes a Na+/K+ gradient; for every 2 K+ pumped in 3 Na+ get pumped out giving the extracellular space a relatively positive charge. Next the cell contains negatively charged molecules such as proteins not found in high concentrations outside the cell, giving the cell a relatively negative charge. Chloride, because of it's negative charge is repelled from going into the cell at rest, however it is important in the hyper polarization stage of an action potential.
THRESHOLD
An Action Potential is an all or nothing event. Neuronal signals in the from of neurotransmitters open Ligand-gated ion channels (Ionotropic receptors) for specific ions. Excitatory Postsynaptic Potential (EPSP) open channels which depolarize (make more positive), while Inhibitory Postsynaptic Potential (IPSP) open channels which hyperpolarize (make more negative). The IPSP and EPSP are summed, and only when the critical threshold level of ~-55 mV to -50mV is reached will an action potential fire.