Axonheuvel

Axonheuvel, also known as the axon hillock, is the conical region of a neuron where the axon joins the soma. It is typically located at the proximal end of the axon and serves as the site where electrical signals are integrated and often where action potentials are initiated.

The axon hillock contains a high density of voltage-gated sodium channels and other voltage-dependent channels organized into the axon initial segment (AIS). This specialization makes the hillock highly excitable, so when integrated synaptic inputs depolarize the membrane to a threshold, an action potential is generated and propagates along the axon. The hillock integrates excitatory and inhibitory postsynaptic potentials arriving from the dendrites and soma, contributing to the neuron's output.

Structural and molecular features support this function. The AIS is a distinct subcellular domain located a short distance from the soma, maintained by a cytoskeletal scaffold that includes ankyrin-G and betaIV-spectrin. This scaffold anchors ion channels and cytoskeletal components, helping to stabilize membrane properties and ensure rapid initiation of action potentials. The exact length and position of the axon hillock and AIS can vary between neuron types and developmental stages.

Clinical and research relevance: alterations in AIS structure or function can influence neuronal excitability and have been associated with neurological and psychiatric conditions in some contexts. The axon hillock remains central to understanding how neurons convert integrated synaptic input into an all-or-none electrical impulse.