The axon hillock is a specialized part of the cell body (or soma) of a neuron that connects to the axon. As a result, 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.
Famous quotes containing the word hillock:
“In every landscape, the point of astonishment is the meeting of the sky and the earth, and that is seen from the first hillock as well as from the top of the Alleghanies. The stars at night stoop down over the brownest, homeliest common, with all the spiritual magnificence which they shed on the Campagna, or on the marble deserts of Egypt.”
—Ralph Waldo Emerson (1803–1882)