Axon Hillock: Structure, Function & Diseases

The axon hillock represents the site of origin of the axon. This is where the action potential is formed, which is transmitted through the axon to the presynaptic terminal. The action potential forms in the axon hillock from the sum of individual specific stimuli and must reach a specific threshold value for stimulus transmission.

What is the axon hillock?

The axon hillock serves as the starting point for action potential transmission. It represents the central control center for postsynaptic stimuli. In this process, the action potential is first built up by summing the individual postsynaptic signals recorded by the dendrites of the nerve cell. When this potential reaches a certain threshold, it is transmitted via the axons to the presynaptic terminal or retrogradely back to the dendrites via the soma. Stimuli, which in sum do not reach the threshold value, are excluded from impulse transmission and no longer serve perception. The axon hillock does not yet belong to the actual axon, but represents its starting point. Because it is free of the so-called Nissl clods, it can be easily recognized in the context of Nissl staining by a lighter-appearing coloration.

Anatomy and structure

Within the neuron, the axon hillock is found between the soma (cell body) and axon. Although it is not yet part of the axon proper, it is considered to be its origin. It also contains no ergastoplasm (Nissl substance) and can therefore be very easily recognized by its lighter appearing Nissl staining. The axon hillock is located directly at the actual cell body (perikaryon). The connecting axon is surrounded by lipid-rich cells that electrically insulate it from the environment. These cells are composed of lipid-rich myelin and are called Schwann cells. So-called Ranvier’s lacing rings interrupt these Schwann cells in regular sections. Due to their different voltages, the Ranvier’s lacing rings cause the conduction of the excitation. At the end of the axon, the electrical stimuli continue to the presynaptic terminals. There, the electrical stimulus is converted into a chemical signal. In the process, neurotransmitters are released into the synaptic cleft. Subsequently, these neurotransmitters bind again to special receptors located on the dendrites of the next neuron. The ion channels at the dendrite are then opened. This results in a change in voltage, which causes the electrical impulse to be transmitted through the cell body to the next axon hillock. From there, the entire process repeats again.

Function and tasks

The axon hillock has the function of receiving incoming electrical signals and summing them to form the action potential. In this process, it is considered the central summation site of excitatory and inhibitory postsynaptic potentials. When the threshold value for the action potential is reached, it is conducted anew via the axon to the presynaptic terminal or via the soma back to the dendrites. In principle, potential summation occurs at every point in the cell. However, the membranes of dendrites and cell body are less excitable than the nerve fibers (axons). Therefore, action potentials are preferentially triggered at the origin of the nerve fibers. There, there is a high density of sodium ion channels that determine whether local synaptic potentials are combined into a relayed excitation. In this sense, the axon hillock plays a crucial role in signal selection. Initially, the stimuli are not directed. From the axon hillock, the action potentials are transmitted directionally across the nerve fibers from neuron to neuron. Without this control center, the body would be exposed to a stimulus overload that it would no longer be able to cope with. Important signals could no longer be distinguished from unimportant stimuli. Thus, if a stimulus acts more intensively on the organism, more potential differences are formed than with less intense stimuli. As a consequence, the threshold potential by potential summation is also reached faster and more often for the stronger signals in the axon hillock than for the weaker ones.

Diseases

The processes in the axon hillock are also broadly related to the disorders of stimulus transmission. Often, the causes of these disorders are not known. Only rarely is the control center of nerve conduction itself likely to be their starting point.However, since all electrical impulses are always conducted via the axon hillock, it is necessarily an integral part of these malfunctions. Depending on the intensity of incoming electrical excitations, action potentials are formed there for further conduction when the threshold value is reached. An oversupply of stimuli can already be responsible for the formation of too many action potentials and thus lead to an overload of stimulus processing. Frequently, there are disturbances at the synapses in the conversion of electrical impulses to chemical signals and vice versa. Causes include missing or excess neurotransmitters, disturbances in their binding to receptors, or intoxication with neurotransmitter-like substances. As a result, either too much or too little stimulus is transmitted. The resulting diseases are manifested by a variety of symptoms. When stimulus transmission is increased, general symptoms may include nervousness, restlessness, increased urge to move, attention deficit disorder, and many others. An example of this condition is the clinical picture of ADHD. If too little stimuli are transmitted, depression often results. If there is a local increase in the transmission of stimuli, such diseases as epilepsy or Tourette’s syndrome can develop. Malfunctions in other organs, such as cardiac arrhythmias, can also be caused by conduction disorders. The causes of these disorders are mainly to be found at the synapses. The axon hillock plays a role only as a switching center.

Typical and common nerve disorders

  • Nerve pain
  • Nerve inflammation
  • Polyneuropathy
  • Epilepsy