Axon hillock

Axon hillock
red labeled is pointing directly at the axon hillock.
Details
Identifiers
Latin	Colliculus axonis
Code	TH H2.00.06.1.00006
Anatomical terminology []

The axon hillock is a specialized part of the cell body (or soma) of a neuron that connects to the axon.

The axon hillock is the last site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to the axon. For many years, it had been believed that the axon hillock was the usual site of action potential initiation. It is now thought that the earliest site of action potential initiation is found just adjacent, in the initial (unmyelinated) segment of the axon. However, the positive point, at which the action potential starts, varies between cells. It can also be altered by hormonal stimulation of the neuron, or by second messenger effects of neurotransmitters.

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.

The axon hillock has a number of specialized properties that make it capable of action potential generation, including adjacency to the axon and a much higher density of voltage-gated ion channels than is found in the rest of the cell body. In dorsal root ganglion cells, the cell body is thought to have approximately 1 voltage-gated sodium channel per square micrometre, while the axon hillock and initial segment of the axon have about ~100–200 voltage-gated sodium channels per square micrometre; in comparison, the nodes of Ranvier along the axon are thought to have ~1000–2000 such channels per square micrometre. This clustering of voltage-gated ion channels is a consequence of plasma-membrane and cytoskeletal associating proteins such as ankyrin.