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Axon hillock

 
Axon Hillock

 
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Axon hillock
 
 
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"Hillock" redirects here. A hillock is also a small hill
Hill

A hill is a landform that extends above the surrounding terrain, in a limited area. Hills often have a distinct Summit , although in areas with Escarpment a hill may refer to a particular section of scarp slope without a well-defined summit ....
.
The axon hillock is the anatomical part of a neuron
Neuron

Neurons are responsive cell in the nervous system that process and transmit information by electrochemical Signal . They are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves....
 that connects the cell body (the soma
Soma (biology)

The soma, or cyton or perikaryon, is the bulbous end of a neuron, containing the cell nucleus. The word soma is Greek language, meaning "body"; the soma of a neuron is often called the "Cell body"....
) to the axon
Axon

An axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts action potentialaway from the neuron's cell body or soma....
. It is described as the location where the summation of inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs) from numerous synaptic inputs
Synaptic inputs

Synaptic inputs are the area in a neuron where presynaptic neurons synapse. Neurons can have inputs from other neurons or from sensory receptors....
 on the dendrites or cell body occurs.

It is electrophysiologically
Electrophysiology

Electrophysiology is the study of the electrical properties of biological cell s and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart....
 equivalent to the initial segment where the summated membrane potential
Membrane potential

Membrane potential , is the voltage difference between the interior and exterior of a cell. Because the fluid inside and outside a cell is highly conductive, whereas a cell's plasma membrane is highly resistive, the voltage change in moving from a point outside to a point inside occurs largely within the narrow width of the membrane itself...
 reaches the triggering threshold, an action potential
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
 propagates through the rest of the axon (and "backwards" towards the dendrites as seen in neural backpropagation
Neural backpropagation

Neural backpropagation is the phenomenon in which the action potential of a neuron creates a voltage spike both at the end of the axon and back through to the dendritic arbor or dendrites, from which much of the original input current originated....
).






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Encyclopedia


"Hillock" redirects here. A hillock is also a small hill
Hill

A hill is a landform that extends above the surrounding terrain, in a limited area. Hills often have a distinct Summit , although in areas with Escarpment a hill may refer to a particular section of scarp slope without a well-defined summit ....
.
The axon hillock is the anatomical part of a neuron
Neuron

Neurons are responsive cell in the nervous system that process and transmit information by electrochemical Signal . They are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves....
 that connects the cell body (the soma
Soma (biology)

The soma, or cyton or perikaryon, is the bulbous end of a neuron, containing the cell nucleus. The word soma is Greek language, meaning "body"; the soma of a neuron is often called the "Cell body"....
) to the axon
Axon

An axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts action potentialaway from the neuron's cell body or soma....
. It is described as the location where the summation of inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs) from numerous synaptic inputs
Synaptic inputs

Synaptic inputs are the area in a neuron where presynaptic neurons synapse. Neurons can have inputs from other neurons or from sensory receptors....
 on the dendrites or cell body occurs.

It is electrophysiologically
Electrophysiology

Electrophysiology is the study of the electrical properties of biological cell s and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart....
 equivalent to the initial segment where the summated membrane potential
Membrane potential

Membrane potential , is the voltage difference between the interior and exterior of a cell. Because the fluid inside and outside a cell is highly conductive, whereas a cell's plasma membrane is highly resistive, the voltage change in moving from a point outside to a point inside occurs largely within the narrow width of the membrane itself...
 reaches the triggering threshold, an action potential
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
 propagates through the rest of the axon (and "backwards" towards the dendrites as seen in neural backpropagation
Neural backpropagation

Neural backpropagation is the phenomenon in which the action potential of a neuron creates a voltage spike both at the end of the axon and back through to the dendritic arbor or dendrites, from which much of the original input current originated....
). The triggering is due to positive feedback between highly crowded voltage gated sodium channels, which are present at the critical density at the axon hillock (and nodes of ranvier) but not in the soma.

The axon hillock also functions as a tight junction
Tight junction

Tight junctions, or zonula occludens, are the closely associated areas of two cell whose Cell membranes join together forming a virtually impermeable barrier to fluid....
, since it acts as a barrier for lateral diffusion of transmembrane proteins, GPI anchored proteins such as thy1, and lipids embedded in the plasma membrane.

Functionality

When neurotransmitters from the presynaptic neuron attach to the receptor sites on the postsynaptic dendritic spines, the postsynaptic membrane may become depolarized
Depolarization

In biology, depolarization is a decrease in the absolute value of a cell's membrane potential. Thus, changes in membrane voltage in which the membrane potential becomes less positive or less negative are both depolarizations....
 (more positive). This depolarisation will travel towards the axon hillock, diminishing exponentially with time and distance. It, therefore, takes multiple such events, arriving in close temporal order, to have any significant effect on the axon hillock. Since the axon hillock has the highest concentration of ion channels, it is almost always the action potential initiation site. At the axon hillock, the depolarization will activate the voltage gated sodium channels, transporting sodium ions into the negatively charged cell. 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 potassium leak channels), initiating a positive feedback loop (rising phase
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
). At around +40 mV the voltage gated sodium channels begin to close (peak phase
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
) and the voltage gated potassium channels begin to open, moving potassium against its electrochemical gradient and out of the cell (falling phase
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
). The potassium channels exhibit a delayed reaction to the membrane repolarisation, and even after the resting potential is achieved, some potassium continues to flow out, resulting in an intracellular fluid which is more negative than the resting potential, and during which, no action potential can begin (undershoot phase
Action potential

An action potential is a self-regenerating wave of electrochemical activity that allows nerve cells to carry a signal over a distance. It is the primary electrical signal generated by nerve cells, and arises from changes in the permeability of the nerve cell's axonal Cell membranes to specific ions....
). As will be seen shortly, 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 myelinated. A myelin sheath
Myelin

Myelin is an electrically-insulating dielectric material that forms a layer, the myelin sheath. Usually, myelin surrounds only the axon of a neuron....
 ensures that the signal can not escape through the ion or leak channels. There are, nevertheless, gaps in the insulation (nodes of ranvier
Nodes of Ranvier

Nodes of Ranvier are the gaps formed between the myelin sheaths generated by different cells. A myelin sheath is a many-layered coating, largely composed of a fatty substance called myelin, that wraps around the axon of a neuron and very efficiently insulates it....
) 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 ions open and sodium rushes in, triggering a fresh new action potential. The undershoot phase, thus, guarantees that the action potential can never propagate backwards up along the axon.

In other words:

In its resting state, a neuron has a negative charge inside, and is surrounded by a positive charge outside (enforced opposite charges = polarization). In general, when a neurotransmitter attaches to a dendrite receptor, a channel opens and little bit of positive charge from outside enters the neuron (it becomes slightly depolarized). This positive charge travels throughout the cell body/soma (the large "head" part) of the neuron, making the neuron, overall, just a little bit more positive. So, when enough neurotransmitters have attached to enough dendrite (dendritic spine) receptors or when a few neurotransmitters have hit a few receptors a lot, the neuron, overall, becomes quite a bit more positive. The axon hillock is a little bulb at the opposite end of the cell body from the dendrite receptors (a gate between the dendrite + soma and the axon of the neuron), and it measures how positive the cell body has become. When the axon hillock has a certain level of positive charge inside (when it is depolarized past a certain point), channels in the axon hillock open, allowing more positive charge to come into the cell. It travels down the axon (away from the already-positive axon hillock toward the still-negative axon), stimulating the next channel to open, which allows more positive charge to enter, which stimulates the next channel to open, and so on, until you reach the end of the axon, or, the axon terminal. This flowing of charge is known as an "action potential." At the axon terminal, other neurotransmitters are waiting in packages, so when the action potential reaches the terminal, they are released to go find other dendrite receptors to attach to and start the process over again. Within the neuron, once the neurotransmitters have been released, the cell pumps out all the positive charge to return the neuron to its negative state.

More details:

-The positive charge that rushes into the dendrite channels and the axon hillock channels and the axon channels is positively charged sodium (Na)

-In its resting state, the negative inside of the neuron has a charge of -60mV. When it becomes positive, it reaches about +30mV (read more about this in the next section). When the neuron pumps out all the positive charge, it reaches a charge of -80mV, just so the neuron knows it's done sending a signal. It eventually returns to -60mV, where it is maintained until the next action potential.

-Because the dendrite channels are opened by neurotransmitters hitting their receptors, these are known as "receptor gated" channels. Because the axon hillock channels and axon channels are opened by that area reaching a certain positive charge, they are known as "voltage-gated channels"

-To move the action potential down the neuron quickly, and to keep from losing positive charge to the outside environment, the axon is coated with an insulating layer of myelin (a cholesterol derivative). This myelin has gaps in it, called "nodes of Ranvier," where the axon is still exposed to the outside environment. These gaps/nodes are where the sodium continues to rush in

More about the axon hillock!:

-It is known as "plastic," because several things about it can vary from neuron to neuron (or even change within the same neuron).

-The positive point, at which the action potential starts, changes. One axon hillock might start an action potential when it reaches a positive charge of, say, +20, another axon hillock (or the same one, in a different situation) might release an action potential when it reaches a positive charge of, say, +35. This can be altered by hormones that are also communicating with the neuron, or by "second messenger systems," when a neurotransmitter not only opens a channel, but stimulates a protein inside the cell which communicates with another protein which communicates with another protein (this process is called the second messenger system) which communicates with another protein that tells the axon terminal when the action potential should start.

-The many channels (for adding positive charge) in the neuron are made up of different subunits. Different neurons might have use different versions of these subunits, which is known as "alternative splicing." Additionally, one neuron may start out with one set of subunits, but then may degrade those subunits and put in different ones (also alternative splicing). These different subunits may dictate how long the channel is open, or the level of charge at which the channel opens.



External links

- "Slide 3 Spinal cord
Spinal cord

The spinal cord is a long, thin, tubular bundle of neuron and glia that extends from the brain. The brain and spinal cord together make up the central nervous system....
"