What is the Difference Between Axon Hillock and Initial Segment

The key difference between axon hillock and initial segment is that axon hillock is present in the cell body of the neuron while the initial segment is present in the proximal part of the axon of the neuron.

Neurons or nerve cells are responsible for the transmission of information to other cells, muscles, and gland cells. An axon is the portion of the neuron that carries nerve impulses away from the cell body. It is a long, slender projection that conducts electrical impulses known as action potentials away from the neuron. Axons are primary transmission lines in the nervous system. The axonal regions include axon hillock, initial segment, rest of the axon, axon telodendria, and axon terminals. Axon hillock and the initial segment located in the neuron play an important role in conducting nerve impulses.

CONTENTS

1. Overview and Key Difference
2. What is an Axon Hillock  
3. What is an Initial Segment
4. Similarities – Axon Hillock and Initial Segment
5. Axon Hillock vs Initial Segment in Tabular Form
6. Summary – Axon Hillock vs Initial Segment

What is an Axon Hillock?

The axon hillock is a specialized part of the cell body of a neuron that connects to the axon. The axon arises from the cell body at a small elevation called the axon hillock. It has numerous specialized properties that help to generate action potentials. An axon hillock of the axon has approximately 100-200 voltage-gated sodium channels per square micrometer. Axon hillock is usually observed under a light microscope by its appearance and location in a neuron. It is also the last site where the membrane potentials propagate from synaptic inputs in the cell body before they transmit to the axon. An axon hillock also separates membrane domains between the cell body and the axon. This allows the localization of membrane proteins to either the axonal side or towards the cell body.

Axon hillock sums up inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs). As a result, the triggering threshold is exceeded, and an action potential is propagated through the rest of the axon. Triggering takes place due to positive feedback between the highly crowded voltages gated sodium channels present at the axon hillock with a critical density. Once the initial action potential begins at the axon hillock, it propagates down the axon. During depolarization, the presynaptic neurons release excitatory neurotransmitters and bind to postsynaptic dendritic spines. This opens the ligan-gated ion channels, and sodium ions enter the cell. This makes the postsynaptic membrane depolarized, and depolarization travels towards the axon hillock. If this event repeats in a short time, the axon hillock is depolarized sufficiently to open the voltage-gated sodium channels. This in turn initiates an action potential and propagates down the axon.

What is an Initial Segment?

The initial segment is a part of the axon located at the proximal end. It contains a high density of voltage-gated ion channels. It is the site of action potential initiation and contains a high amount of sodium and potassium channels compared to other membrane domains. The initial segment separates the somatodendritic compartment from the axon.

The main functions of an initial segment are clustering and maintaining ion channels in high densities to initiate action potential and to control neuronal polarity through the regulation of differential distribution and trafficking of proteins, organelles, vesicles, and lipids between axonal and somatodendritic compartments. The initial segment is unmyelinated and contains specialized protein complexes. The main scaffolding protein that is responsible for the anchoring of ion channels in the initial segment is Ankyrin G (AnkG). The absence or loss of AnkG causes the disassembly of its structure. AnkG is one of the main proteins involved in the formation of initial segments. The position on the axon and the length of the initial segment show a degree of plasticity that can adjust the neuronal output. A longer initial segment is associated with greater excitability. The initial segment is highly specialized in conducting nerve impulses due to a high concentration of voltage-gated sodium channels. Therefore, action potential also initiates from the initial segment.

What are the Similarities Between Axon Hillock and Initial Segment?

• Axon hillock and initial segment are located on the neuron.
• Both contain voltage-gated sodium channels.
• They conduct impulses.
• Both structures consist of a cytoplasm.
• Both axon hillock and initial segment play a vital role in managing signal impulses.

What is the Difference Between Axon Hillock and Initial Segment?

Axon hillock is present in the cell body of the neuron, while the initial segment is present at the proximal part of the axon of the neuron. Thus, this is the key difference between axon hillock and initial segment. Axon hillock manages total inhibitory and excitatory signals, but the initial segment manages signal conductivity. So, this is the functional difference between axon hillock and initial segment. Moreover, the axon hillock consists of Nissl granules, while the initial segment consists of high-density ion channels.

The below infographic presents the differences between axon hillock and initial segment in tabular form for side-by-side comparison.

Summary – Axon Hillock vs Initial Segment

Axon hillock and initial segment are two parts of the neuron that work in terms of conducting impulses. Axon hillock is present in the cell body of the neuron, and the initial segment is present at the proximal part of the axon of the neuron. Axon hillock manages total inhibitory and excitatory signals, while the initial segment manages signal conductivity. In addition, axon hillock consists of Nissl granules while the initial segment consists of high-density ion channels. Thus, this summarizes the difference between axon hillock and initial segment.

Reference:

1. Huang, Claire Yu-Mei, and Matthew N Rasband. “Axon Initial Segments: Structure, Function, and Disease.” Annals of the New York Academy of Sciences, U.S. National Library of Medicine.
2. PM;, Palay SL;SoteloC;PetersA;Orkand. “The Axon Hillock and the Initial Segment.” The Journal of Cell Biology, U.S. National Library of Medicine.

Image Courtesy:

1. “Axon Hillock” By M.aljar3i – Own work (CC BY-SA 3.0) via Commons Wikimedia