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Axon hillock n., plural: axon hillocks <ˈæksɒn ˈhɪlək> Definition: tapering region between a cabinet body and also axon of a neuron


Axon Hillock Definition

What is axon hillock? If friend are familiar with the different parts of the neuron, the axon hillock is the conical forecast of the neuron the connects the cell body (or soma) through the axon. In various other words, the an ar where the axon is associated to the soma is at the axon hillock. So, this raises this question: Is the axon hillock component of the axon? Some references think about the axon hillock to be the originating point or the initial segment of the axon. Others watch it together the base that the axon. Others still speak to this segment the trigger zone that neuron because the neuronal activity potential is triggered and generated here.

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If you space not familiar with a neuron and also its parts, let’s have a short overview the the structure of a neuron.


Structure the Neuron

Neurons, or the nerve cells, room the distinguished cells of the nervous mechanism responsible for distributing and receiving “signals” (impulses). Structurally, neuronal cell are unique from the remainder of the body cells. Each neuronal cell has actually the complying with parts: cabinet body, axon, and also dendrites. Let’s recognize each of this neuronal parts.


Axon hillock (definition, anatomy): the neuronal estimate that transmits the neuronal signals from the neuronal cell human body to different body cell (Note: axons bring outward-bound signals) while the dendrites space the root-like projections that receive the signal from the other cells the the body and also relay them come the nervous system (Note: dendrites bring inward-bound signals).

Axon hillock (definition, medicine): an axon nerve fiber that is a lengthy projection of a neuron that carries the outbound neuronal cabinet signals together opposed come dendrites, which space the brief protrusions native the neuronal cabinet body that brings in the inbound signals to the neuron.

Etymology: “axon”, indigenous Greek “áxōn”, meaning “axle” or “axis”; hillock, from center English “hillok”, “hil”, an interpretation “hill”.


Let’s understand the difference in between axon and also dendrites (Table 1)

Table 1: Axon vs. Dendrites

Axon Dendrites
One axon every neuron Multiple dendrites every neuron
Axon transmits the outgoing signals Dendrites transmit the inbound or the just arrived signals
Relatively, axon are long ~sevral meter in length Relatively smaller sized in length
Axons own a uniform thickness throughout your length, and also the branching occurs only terminally A very branched framework that have the right to be checked out throughout its length
Terminal end of the axons type synaptic knobs The tapering ends in dendrites; absence of synaptic knobs
Neurofibrils kind the cytoskeletal framework of the axon. However, Nissl’s granules room absent Presence that neurofibrils and Nissl’s granules
Axons carry the signals far from the neuronal cell body Dendrites bring the signals in the direction of the neuronal cell body
Axon location- between neuronal cell body or soma and also the axon terminal Dendrites location- lot of projections transparent the perimeter of the neuronal cell human body or soma.

Axons are usually covered through a myelin sheath in stimulate to maintain the signal during its transmission to different parts the the body. This axons are referred to as myelinated axons. However, part axons might be unmyelinated as well.

Depending top top the myelination the the nerve fibers, they have the right to be classified right into three types:

team A nerve fibers- myelinated axons team B nerve fibers- myelinated axons team C nerve fibers- unlike form A and also B axons, type C axons room unmyelinated

Axons conduct a specific kind of electrical impulse, recognized as the activity potential. These activity potentials room outward-bound, i.e., from the neuronal cell human body to the terminal finish of the axons.

Sequential abrupt transforms in the electric potential or the voltage across the plasma membrane are the activity potential. Nonstimulated or relaxing axonal activity potential is (−)60 mV. Take keep in mind that the negative sign suggests a relatively an unfavorable potential the the plasma membrane’s inner side to the outer side of the plasma membrane. This activity potential of the neuronal cell is equivalent to the non-neuronal cells. This action potential can rise as much as (+)50mV, which means that the inner activity potential is positive relative to the plasma membrane’s outer side. Thus, a net adjust of ~110mV can occur throughout the depolarization of the axons.

This depolarization is followed by repolarization, i.e., returning to the non-stimulated or the resting action potential. The succession of depolarization and repolarization that the axonal membrane results in the conduction or the transmission of the signals/action potential in ~ a rate of 100 m/sec native the neuronal cell body to the various cells of the body.

The action potential moves along the axon towards the terminal component of the axon wherein in ~ the chemical synapse it incites the release of neurotransmitters for more propagation of the signals.

The cell human body is the core of the neurons the contain the hereditary information, upholds the neuronal structure, and also provides the power for the neuronal activities. The nucleus and also the other cellular organelles (like absorbent reticulum, Golgi apparatus, mitochondria, ribosomes, secretory vesicles, etc.) are present inside the cell body of the neurons. (Figure 1)

The bridge between the cabinet body and also the axon is well-known as the axon hillock. The generation that the action potential occurs at the axon hillock. Many of the neurons possess one crucial axon and also multiple dendrites. The terminal suggest of the axon is the presynaptic terminal (also described as terminal bouton). These terminal buttons connect with various other neuronal cell to kind a synapse. Thus, signal from one neuronal cell are transmitted to the various other neuronal cabinet via this synapses.

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Figure 1: (A) framework of neurons with labeled parts: axon, dendrites, cell body, and axon hillock. Photo Credit: M.aljar3i, CC BY-SA 3.0. (B) added parts space shown. Image Credit: 2. Nerve and Muscle cells – bem.fi.

Now that we learned the basic parts that the neuron, let’s take it a closer look in ~ the axon hillock.

Axon Hillock Structure

Axon hillock is the neuronal region in the cell body wherein the summation of the excitatory & inhibitory task occurs. The neuronal activity potential is usually created at the axon hillock. It is a cone-shaped area in the neuronal cell body from whereby the axon originates. (Figure 1-A)

The initial region that lies between the axon hillock and the myelin sheath’s starting point is dubbed the axon early stage segment or AIS (Figure 2-A). The early stage segment is the site for the initiation the the activity potential.Characteristically, the axon hillock does no contain ribosomesand many of the various other cell organelles. However, that does containcytoskeletal elementsormicrotubular structuresand organelles that relocate along the axon. (Figure 2-B) The neurofilaments in the axon hillock form a cluster, which is described asfascicles.

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Figure 2: (A) place of Axon hillock and Axon early segment or AIS in a neuron. (B) interior microtubular cytoskeletal framework of axon hillock. Credit: Excitable Tissue: Nerve – Ganong’s evaluation of medical Physiology, 24th version – Doctorlib.info

The terminal part of the axon and also its collaterals (i.e. Collaterals room the branches that the central axon) kind small branches or telodendria that kind button-shaped tips well-known as terminal boutons (also referred to as nerve endings or synaptic knobs) (Figure 3).

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Figure 3: Axon hillock and also terminal boutons. Source: Maria Victoria Gonzaga of biology Online.

Another distinguishing feature of the axon hillock region in the neuron is the visibility of a high density of voltage-gated sodium ion channels. The axon hillock region possesses practically 100-200/m2 the voltage-gated salt ion channels, which are thought to be because of cytoskeletal and plasma-membrane-associated proteins, ankyrin.