What is an Axon Hillock?

Neurons, the cells that make up the nervous system, are composed of four parts: the cell body, the dendrites, the axon, and the terminal buttons. The axon hillock is situated between the cell body and the axon. Electrical impulses from the dendrites and cell body are summed at this point, which then can send an action potential down the axon.

The body of a neuron has dendrites sticking out of all sides. On one end, it has a long tail, called an axon, which ends in the terminal buttons. The cell body contains the nucleus and other cell parts necessary to keep the neuron alive. The dendrites, which look like tiny trees jutting away from the cell body, receive messages from other neurons. The axon hillock acts as a bridge between the axon and the cell body. The axon, a long tube covered in a myelin sheath, carries an electrical signal away from the cell body toward the terminal buttons, which can communicate with other neurons.

Neurons send signals through chemical transmissions between cells using special chemicals called neurotransmitters. The dendrites and cell body of the neuron receive these signals and change them from chemical signals to electrical impulses. There are two types of signals a neuron can receive: an excitatory post-synaptic potential (EPSP) or an inhibitory post-synaptic potential (IPSP). Excitatory signals depolarize the neuron, meaning they make the cell more positively charged. Inhibitory signals do the opposite, polarizing or negatively charging the cell.

A neuron typically receives several of both types of signals. These signals are then added up at the axon hillock. If the excitatory signal is strong enough, the axon will create an action potential, an electrical impulse that travels down the axon. The axon hillock will do both temporal summation, adding electrical impulses that happen over a short period of time, and spatial summation, adding the signals that occur across the cell body.

The action potential, which begins at the axon hillock, is an electrical impulse that travels down the axon toward the terminal buttons. The action potential operates on an “all or none” principle, meaning the axon hillock either creates an action potential or does nothing. The action potential ends at the terminal buttons. It then stimulates them to send a chemical signal using neurotransmitters. The terminal buttons use these chemicals to communicate with other neurons.