The axon hillock is a specialized part of the cell body (or

soma Soma may refer to: Businesses and brands * SOMA (architects), a New York–based firm of architects * Soma (company), a company that designs eco-friendly water filtration systems * SOMA Fabrications, a builder of bicycle frames and other bicycle ...

) of a

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. N ...

that connects to the

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action po ...

. It can be identified using

light microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...

from its appearance and location in a neuron and from its sparse distribution of Nissl substance. The axon hillock is the last site in the soma where membrane potentials propagated from

synaptic inputs In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...

are summated before being transmitted to the axon. For many years, it was believed that the axon hillock was the usual site of initiation of

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

s—the

trigger zone In neuroscience and neurology, a trigger zone is an area in the body, or of a cell, in which a specific type of stimulation triggers a specific type of response. The term was first used in this context around 1914 by Hugh T. Patrick, who was writin ...

. It is now thought that the earliest site of action potential initiation is at the

axonal initial segment An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...

: just between the peak of the axon hillock and the initial (unmyelinated) segment of the

. However, the positive point, at which the action potential starts, varies between cells. It can also be altered by hormonal stimulation of the neuron, or by

second messenger Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules—the first messengers. (Intercellular signals, a non-local form or cell signaling, encompassing both first me ...

effects of neurotransmitters. The axon hillock also delineates separate membrane domains between the cell body and axon. This allows for localization of

membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane ...

s to either the axonal or somal side of the cell.

Structure

The axon hillock and initial segment have a number of specialized properties that make them capable of action potential generation, including adjacency to the axon and a much higher density of voltage-gated ion channels than is found in the rest of the cell body. In dorsal root ganglion cells, the cell body is thought to have approximately 1

voltage-gated sodium channel Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the channel ...

per square micrometre, while the axon hillock and initial segment of the

have about ~100–200

s per square micrometre; in comparison, the

nodes of Ranvier In neuroscience and anatomy, nodes of Ranvier ( ), also known as myelin-sheath gaps, occur along a myelinated axon where the axolemma is exposed to the extracellular space. Nodes of Ranvier are uninsulated and highly enriched in ion channels, al ...

along the axon are thought to have ~1000–2000 such channels per square micrometre. This clustering of voltage-gated ion channels is a consequence of plasma-membrane and cytoskeletal associating proteins such as

ankyrin Ankyrins are a family of proteins that mediate the attachment of integral membrane proteins to the spectrin-actin based membrane cytoskeleton. Ankyrins have binding sites for the beta subunit of spectrin and at least 12 families of integral mem ...

. In electrophysiological models, the axon hillock is included with the

initial segment In mathematics, an upper set (also called an upward closed set, an upset, or an isotone set in ''X'') of a partially ordered set (X, \leq) is a subset S \subseteq X with the following property: if ''s'' is in ''S'' and if ''x'' in ''X'' is larger ...

of the axon where membrane potentials propagated from

to the dendrites or cell body are summed.

Function

Both inhibitory postsynaptic potentials ( IPSPs) and excitatory postsynaptic potentials ( EPSPs) are summed in the axon hillock and once a triggering threshold is exceeded, an

propagates through the rest of the axon (and "backwards" towards the dendrites as seen in

neural backpropagation Neural backpropagation is the phenomenon in which, after the action potential of a neuron creates a voltage spike down the axon (normal propagation), another impulse is generated from the Soma (biology), soma and propagates towards the Apical den ...

). The triggering is due to

positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the ...

between highly crowded

s, which are present at the critical density at the axon hillock (and nodes of ranvier) but not in the soma. In its resting state, a neuron is polarized, with its inside at about −70 mV relative to its surroundings. When an

excitatory neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neurot ...

is released by the

presynaptic neuron Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous sys ...

and binds to the postsynaptic dendritic spines,

ligand-gated ion channels Ligand-gated ion channels (LICs, LGIC), also commonly referred to as ionotropic receptors, are a group of transmembrane ion-channel proteins which open to allow ions such as Na+, K+, Ca2+, and/or Cl− to pass through the membrane in res ...

open, allowing sodium ions to enter the cell. This may make the postsynaptic membrane depolarized (less negative). This

depolarization In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is ess ...

will travel towards the axon hillock, diminishing exponentially with time and distance. If several such events occur in a short time, the axon hillock may become sufficiently depolarized for the voltage-gated

sodium channels Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the chann ...

to open. This initiates an action potential that then propagates down the axon. As sodium enters the cell, the cell membrane potential becomes more positive, which activates even more sodium channels in the membrane. The sodium influx eventually overtakes the potassium efflux (via the

two-pore-domain potassium channel The two-pore-domain or tandem pore domain potassium channels are a family of 15 members that form what is known as leak channels which possess GHK current equation, Goldman-Hodgkin-Katz (open) rectifier, rectification. These channels are regulated ...

s or ''leak channels'', initiating a positive feedback loop (rising phase). At around +40 mV, the voltage-gated sodium channels begin to close (peak phase) and the voltage-gated potassium channels begin to open, moving potassium down its electrochemical gradient and out of the cell (falling phase). The potassium channels exhibit a delayed reaction to the membrane repolarisation, and, even after the

resting potential A relatively static membrane potential which is usually referred to as the ground value for trans-membrane voltage. The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as opp ...

is achieved, some potassium continues to flow out, resulting in an intracellular fluid that is more negative than the resting potential, and during which no action potential can begin (undershoot phase/ refractory period). This undershoot phase ensures that the action potential propagates down the axon and not back up it. Once this initial action potential is initiated, principally at the axon hillock, it propagates down the length of the axon. Under normal conditions, the action potential would attenuate very quickly due to the porous nature of the cell membrane. To ensure faster and more efficient propagation of action potentials, the axon is

myelin Myelin is a lipid-rich material that surrounds nerve cell axons (the nervous system's "wires") to insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the axon. The myelinated axon can be ...

ated. Myelin, a derivative of cholesterol, acts as an insulating sheath and ensures that the signal cannot escape through the ion or leak channels. There are, nevertheless, gaps in the insulation (

), which boost the signal strength. As the action potential reaches a node of Ranvier, it depolarises the cell membrane. As the cell membrane is depolarised, the voltage-gated sodium channels open and sodium rushes in, triggering a fresh new action potential.

References

External links

* - "Slide 3

Spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

" {{DEFAULTSORT:Axon Hillock Neurohistology