Axon hillock

:"Hillock" redirects here. A hillock is also a small hill."The axon hillock is the anatomical part of a neuron that connects the cell body (the soma) to the axon. It is described as the location where the summation of inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs) from numerous synaptic inputs on the dendrites or cell body occurs.

It is electrophysiologically equivalent to the "initial segment" where the summated membrane potential reaches the triggering threshold, an action potential propagates through the rest of the axon (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 axon hillock (and nodes of ranvier) but not in the soma.

The axon hillock also functions as a tight junction, since it acts as a barrier for lateral diffusion of transmembrane proteins, GPI anchored proteins such as thy1, and lipids embedded in the plasma membrane.

Functionality

When neurotransmitters from the presynaptic neuron attach to the receptor sites on the postsynaptic dendritic spines, the postsynaptic membrane may become depolarized (more positive). This depolarisation will travel towards the axon hillock, diminishing exponentially with time and distance. It, therefore, takes multiple such events, arriving in close temporal order, to have any significant effect on the axon hillock. Since the axon hillock has the highest concentration of ion channels, it is almost always the action potential initiation site. At the axon hillock, the depolarization will activate the voltage gated sodium channels, transporting sodium ions into the negatively charged cell. 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 potassium 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 against 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 which is more negative than the resting potential, and during which, no action potential can begin (undershoot phase). As will be seen shortly, this undershoot phase ensures that the action potential propagates down the axon and not back up it. Once this initial action potential is initiated, principally at the axon hillock, it propagates down the length of the axon. 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 axon is myelinated. A myelin sheath ensures that the signal can not 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 ions open and sodium rushes in, triggering a fresh new action potential. The undershoot phase, thus, guarantees that the action potential can never propagate backwards up along the axon.

References

External links

* - "Slide 3 Spinal cord"