Both inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs) are summed in the<unk>  and once a triggering threshold is exceeded, an action potential propagates through the rest of the<unk>  (and "backwards" towards the dendrites as seen in neural backpropagation). The triggering is due to positive feedback between highly crowded voltage-gated sodium channels, which are present at the critical density at the<unk>  (and nodes of ranvier) but not in the soma. In its resting state, a neuron is polarized, with its inside at about −70 mV relative to its surroundings. When an excitatory neurotransmitter is released by the presynaptic neuron and binds to the postsynaptic dendritic spines, ligand-gated ion channels open, allowing sodium ions to enter the cell. This may make the postsynaptic membrane depolarized (less negative). This depolarization will travel towards the<unk> , diminishing exponentially with time and distance. If several such events occur in a short time, the<unk>  may become sufficiently depolarized for the voltage-gated sodium channels to open. This initiates an action potential that then propagates down the<unk> . As sodium enters the cell, the cell membrane potential becomes more positive, which activates even more sodium channels in the membrane. The sodium influx eventually overtakes the potassium efflux (via the two-pore-domain potassium channels or leak channels, initiating a positive feedback loop (rising phase). At around +40 mV, the voltage-gated sodium channels begin to close (peak phase) and the voltage-gated potassium channels begin to open, moving potassium down its electrochemical gradient and out of the cell (falling phase). The potassium channels exhibit a delayed reaction to the membrane repolarisation, and, even after the resting potential is achieved, some potassium continues to flow out, resulting in an intracellular fluid that is more negative than the resting potential, and during which no action potential can begin (undershoot phase/refractory period). This undershoot phase ensures that the action potential propagates down the<unk>  and not back up it. Once this initial action potential is initiated, principally at the<unk> , it propagates down the length of the<unk> . Under normal conditions, the action potential would attenuate very quickly due to the porous nature of the cell membrane. To ensure faster and more efficient propagation of action potentials, the<unk>  is myelinated. Myelin, a derivative of cholesterol, acts as an insulating sheath and ensures that the signal cannot escape through the ion or leak channels. There are, nevertheless, gaps in the insulation (nodes of ranvier), which boost the signal strength. As the action potential reaches a node of Ranvier, it depolarises the cell membrane. As the cell membrane is depolarised, the voltage-gated sodium channels open and sodium rushes in, triggering a fresh new action potential.
Axon hillock
Pocock boundary
Ulnar claw
Lines of Zahn
Collateral eminence
Ossification center
Dorsal nexus
Epiphyseal plate