Neuron

∞ generated and posted on 2016.02.07 ∞

Neurons are the means by which animals transmit signals from specific locations to specific locations within the body.

Neurons carry action potentials either from cell to cell or instead from signals received by receptors towards the central nervous system; neurons synapse with other cells including, especially, with other neurons.

This page contains the following terms: Membrane potential, Excitable cell, Neuron, Action potential, Synapse, Summation, Threshold potential, Cell body, Axon Hillock, Axon, Nerve, Dendrite, Neurotransmitter, Glial cells, Myelin sheath, Myelination, Schwann cells, Oligodendrocytes, Nodes of Ranvier, Saltatory conduction

The above video is a good start to introducing what material this page covers.

The above video is part 2 of that good start to introducing what material this page covers.

And this is video number 3…



Membrane potential

Voltage that exists going from outside to inside of an organelle or cell.
A voltage is a physically separated difference in electrical charge, here seen as a difference in the quantity of negatively charged ions and positively charged ions (anions and cations, respectively) that are present on the inside of a cell versus the outside, as separated by a plasma membrane. Unless anions and cations are balanced between the inside and outside of a cell, then the cell possess a voltage across its plasma membrane and therefor a membrane potential. This membrane potential is actively maintained by animal cells via the action of sodium-potassium pumps and the loss of this membrane potential is described as depolarization (versus polarization which is a gain in membrane potential). Waves of depolarization are described as action potentials and cells that can display action potentials are described as excitable cells.


Links to terms of possible interest: Cytosol, Electrode, Microelectrode, Negative charge, Plasma membrane, Positive charge, Potentiometer, Salt solution, Voltage

Nice overview of membrane potentials and resting potentials as well as the basics of the sodium-potassium pump.

The above video also provides a nice overview of membrane potentials are all about.



Excitable cell

Entity capable of propagating action potentials.
Among excitable cells are neurons as well as muscle cells. These cells maintain a membrane potential that is both capable of depolarizing and over which that depolarization can be propagated from one location on the cell's surface to another. This excitability can also be characterized as a cell possessing an "off" state during which a non-depolarized membrane potential (i.e., resting potential) is sustained or alternatively an "on" state that coincides with depolarization along with propagation of the associated action potential. This "on" state coincides with the excitable cell being "excited".

The above video is a chalk talk where I introduce the concept of an action potential as considered from the perspective of movement of ions across membranes.



Neuron

Type of animal cell capable particularly of transmitting electrochemical information from one end to another.
Neurons, or simply nerve cells, are excitable, a.k.a., able to propagate an action potential, meaning that they can change their state from what essentially is an off position to what instead is an on position. Further, this change doesn't happen across the entire cell at once but instead propagates from a point of excitement to adjacent parts of the cell, eventually (though very quickly!) propagating from one "side" of the cell to the other. In addition, nerve cells tend to be quite long, meaning that this signal propagation can travel substantial distances, i.e., up to meters, carrying the information that excitement has occurred (that is, switch from an off to an on state) from one location in the body to another. Where these signals come from as well as where they go to can be quite specific, which allows for the transmission of more information than just "on", but instead such that "on" has specific meanings to recipient cells and tissues. Lastly, multiple cells can provide signals to multiple other cells, resulting in the building up of the complex circuitry that makes up reflex arcs as well as, in much more complicated form, brains.


Links to terms of possible interest: Action potential, Axon, Cell body, Dendrite, Myelin sheath, Nodes of Ranvier, Nucleus, Neuron, Postsynaptic, Presynaptic, Schwann cell, Soma, Synapsis, Synaptic terminals

This video gets fairly deeply into nerve cell terminology, but nonetheless provides a fairly well done and quick overview of what neurons are all about.

The above video provides a nice introduction to neurons, their anatomy, functioning, and basic diversity.

The above video is a chalk-talk introduction to what neurons/nerve cells are all about.

The above video on how neurons function is pretty awesome.

Introduction to neurons and other basic cellular structures of the nervous system. Note that at 3:44 "intracellular" is used where, I believe, "intercellular" should have been. Also, the narrator around 5:30 speaks of neurotransmitters being synthesized via ribosomes, which for the most part is not correct. Also (alas) mention of interneurons as glial cells around 9:00 is, I believe, not correct, as interneurons and glial cells are distinctly different types of cells, that is, glial cells are not neurons.



Action potential

Propagatable depolarization of the plasma membrane of an excitable animal cell.
The depolarization of an action potential is across the plasma membrane such as of a nerve cell though also of muscle cells. Action potentials propagate from wherever they are occurring on the plasma membrane away from that point (or points) and therefore away from wherever they had already immediately occurred. That is, action potentials both move forward – along or over plasma membranes – and cannot move backwards, where forward is away from where the action potential currently is occurring or where the action potential instead immediately previously occurred.

What action potentials consist of, further, is the moment of ions, specifically sodium ions and potassium ions (Na+ and K+, respectively), across this plasma membrane. This results in depolarization which is a change in what literally is the voltage, that is, the electrical charge across that membrane, towards zero (that is, towards no voltage).

An action potential is initiated in a specific location on the surface of an excitable cell with reductions in voltage towards zero occurring via the movement across the plasma membrane of sodium ions and potassium ions. This movement causes adjacent areas on the plasma membrane that have not yet experienced this ion movement to allow such ion movement and therefore subsequent localized voltage movement also towards zero. The result is a propagation of the depolarization signal in one direction, particularly from one end of the neuron to another. When the signal reaches its terminus, at a synapse, then neurotransmitters are stimulated to be released such as to initiate the excitation/depolarization of a subsequent, postsynaptic cell.


Links to terms of possible interest: Action potential, Depolarization, Excitable cell, Potassium channel, Potassium ion, Resting potential, Resting voltage, Sodium channel, Sodium ion, Voltage, Voltage-gated channel

The above video does a good job of illustrating both what an action potential is and why it needs to have the properties that it has in order to allow for proper neuron functioning.

The above video provides a really nice illustration of action potential propagation.

In the above video the contribution of voltage-gated channels to the formation of action potentials is emphasized.

The above video describes the propagation of an action potential along an axon.

Action potential discussion that seems to do a good job of describing the physiological basis of the refractory period.



Synapse

Gap between nerve cells over which signal is propagated via the release, diffusion, and reception of neurotransmitters.
Action potentials do not usually propagate continuously from nerve cell to nerve cell but instead terminate at what are known as axon terminals. At this point the release of compounds known as neurotransmitters is stimulated, by the action potential, into a narrow gap between nerve cells—this specifically is between axon terminals and either a dendrite or cell body of the downstream, that is, postsynaptic nerve cell. The neurotransmitters then diffuse across the synapse where they interact with specific receptors found on the surface of the downstream cell.

These interactions either stimulate the downstream cell or instead inhibit that cell. If sufficient stimulation, i.e., a threshold potential is received by the downstream neuron, across sufficient numbers of synapses (and particularly in excess of inhibition signals), then an action potential in that recipient cell will be initiated. Thus, signals from one nerve cell to another are propagated across synapses. Though those signals are not necessarily stimulating, they do provide information, in the form of chemical compounds, to the recipient cell, which then is either stimulated to an "on" state (action potential) or, instead, is not stimulated to an "on" state.


Links to terms of possible interest: Axon terminal, Dendrite, Downstream, Mitochondrion, Nerve impulse, Neurotransmitter, Postsynaptic neuron, Presynaptic neuron, Receptor, Synapse, Synaptic gap, Synaptic knob, Synaptic vesicle

The above video provides a good, short introduction to neuronal synapses.

The above video also provides a good, short introduction to neuronal synapses; it has less fancy graphics but nonetheless is more informative.

The above video is nicely done and illustrated but also is a little narrow in its focus, and totally avoids the concepts of summation.

The above video nicely introduces key terminology and serves as a good segue to the following discussion of summation.



Summation

Combining by postsynaptic neurons of presynaptic signals, potentially towards establishment of an action potential.
For neurons in many cases a single impulse from a single upstream, i.e., presynaptic neuron is insufficient to result in an action potential (that is, to switch the neuron from being in an "off" state to instead temporarily being in an "on" state). Instead, it is usually necessary for multiple presynaptic neurons to together stimulate the postsynaptic neuron to result in the formation of sufficient depolarization that an action potential results (a.k.a., to achieve a threshold potential). That summing together of the stimulation associated with multiple presynaptic neurons (from axon terminals and as propagated across synapses) is called summation.

Thus, the activity of multiple neurons can be integrated by downstream, that is, postsynaptic neurons such that a decision is made either to remain in the "off" state or instead to proceed to the "on" state. Keep in mind that the presynaptic neurons also may have been subject to such either-or decisions, and the consequent result is that highly complex neural processes and resulting behaviors are possible. The substantial distinction between the simplistic view of neurons stimulating neurons and the more realistic view of neuronal connections as information processing entities is found, in other words, in the process of summation. In addition, keep in mind that not all upstream signals are stimulated towards achievement of a threshold potential but, depending on the neuron and the connection, instead can inhibit the summation of signals towards a threshold potential (that is, can inhibit summation towards an "on" state).


Links to terms of possible interest: Axon, Cell body, Dendrite, EPSP, Excitatory synapse, Inhibitory synapse, IPSP, Membrane potential, Postsynaptic, Presynaptic neuron, Synapse

The above video provides a nice summary of summation.

The above video continues the previous summary of summation.

The above video considers temporal summation, and as with the above videos, does so from the perspective of the postsynaptic neuron.

The above video is a very fast, very cute description of how a presynaptic neuron can bias its message so that it is more likely to give rise to an action potential.



Threshold potential

Sufficient membrane depolarization of the plasma membrane of an excitable cell to trigger extensive lateral propagation of that depolarization.
A cell's membrane potential can be locally reduced (depolarized) by various stimuli including post synaptically (downstream from a synapsing cell), where such reductions represent movement towards a threshold potential. The degree of depolarization achieved, however, may be insufficient to trigger the propagation of an action potential and indeed multiple individual depolarization events over relatively small areas on a neuron's surface, in a process known as summation, may be required to trigger action potential propagation. That amount of depolarization that in fact will trigger action potential propagation is described as a threshold potential. Neurons thus do not switch to an "on" state unless a threshold potential, such as over the course of summation, is achieved.


Links to terms of possible interest: Action potential, Depolarization, Excitable membrane, Membrane potential, Subthreshold stimulus, Suprathreshold stimulus, Threshold potential, Threshold stimulus

The above video only briefly mentions threshold potential but nevertheless does a good job of placing the threshold potential within the overall context of the action potential.



Cell body

The portion of neurons other than the axon or dendrites.
The cell body is the standard "cell" portion of a neuron, that is, the part where the cell nucleus is located. It can be thought of as that portion of neurons that supplies much of the support for the cell as a whole, allowing the axon and dendrites therefore to be dedicated mostly to the conduction of nerve impulses (i.e., the propagation of action potentials). The cell body, or soma or perikaryon as it is also known, also contains the bulk of cytoplasm associated with nerve cells, or at least that portion of these cells where substantial amounts of cytoplasm is present relative to amounts of plasma membrane.


Links to terms of possible interest: Axon, Axon hillock, Axon terminal, Cell body, Dendrite, Myelin sheath, Neuron, Nodes of Ranvier, Schwann cell, Synapse

The above video doesn't really focus on the cell body but does discuss it within the context of the neuron as a whole.



Axon Hillock

Point of connection between neuron cell body and axon.
Action potentials, following summation and establishment of a threshold potential in association with the cell body are either at the point, or immediately adjacent to the point of initiation of the action potential in the associated axon. In association with this function, the axon hillock possesses substantially more voltage-gated channels than does the cell body of the neuron. It is in fact particularly at the axon hillock that the summation process is manifest, either leading to or not leading to a threshold potential. This summation in any case is the product of dendrite stimulation along with stimulation in the associated cell body. You might imagine the cell body and associated dendrites as experiencing a swarm of signals that occasionally together are of sufficiently high levels of depolarization that an "on-off" switch, that is the axon hillock, is switched to the on state, thereby initiating an action potential down the associated axon.


Links to terms of possible interest: Axon, Axon hillock, Axon terminal, Axosomatic synapse, Cell body, Dendrite, Depolarization, Excitatory synapse, Inhibitory synapse, Myelin sheath, Postsynaptic neuron, Presynaptic neuron, Summation, Synaptic terminal, Threshold potential, Trigger zone

The above video really is simply another overview of the structure of neurons with a tiny bit of mention of function, but at 3:23 there a quick positional and to some degree structural description of the axon hillock, a.k.a., the initial segment.



Axon

Neuronal extensions that carry signals away from the cell bodies of neurons.
A nerve cell will possess only a single axon, though that axon can be branched at its terminus (and therefore may stimulate multiple sources). The axon serves as a long – up to meters long – conduit ending with axon terminals and starting with the dendrites associated with the same cell. You can think of the neuron as consisting of potential start points that are followed by a "wire" and then one or more points where signals propagate to. The start points are the dendrites and/or the cell body, the "wire" is the axon itself, and the signals propagate to the axon terminals, and from there to synapses as found with other cells. The axon is also both the portion of nerve cells that typically is the longest (often by substantial amounts) and which often possesses a myelin sheath. Continuing the illustration of a start, a wire, and then one or more targets, the myelin sheath serves as an insulator of the axon "wire".


Links to terms of possible interest: Action potential, Axon, Axon terminal, Dendrite, Membrane potential, Neuron, Synapse

The sound could be better but the above video provides an impressively broad overview of what axons are all about.



Nerve

Long bundles of axons.
Though not consisting of whole cells, since axons make up an only a portion of nerve cells, nerves nonetheless can be viewed as bundled as well as sheathed nerve cells that link together disparate locations of bodies, particularly peripheral body aspects to the spinal cord or brain stem. As nerves consist of multiple, bundled axons, they are substantially larger in diameter (thicker) than are individual axons. Importantly, because of the existence of nerves, the peripheral nervous system consists substantially of bundled together neuronal processes rather than just individual axons snaking about the body. Furthermore, individual nerves are both typical, going from person to person, and possess names. Indeed, in diagrams indicating the reach of the peripheral nervous system it is nerves, typically drawn as lines and rather than individual neurons, that are indicated.


Links to terms of possible interest: Axon, Endoneurium, Epineurium, Fascicle, Myelination, Nerve, Nerve fascicle, Perineurium, Schwann cell, Spinal nerve,



Dendrite

Neuronal extensions that carry signals away from synapses.
Dendrites are both shorter and (much) more numerous than axons. What they are responsible for is transferring nerve impulses from other cells, including multiple other cells, such as, in particular, other neurons. Indeed, multiple nerve cells often synapse with individual other neuronal as well as non-neuronal cells. The potential for what is known as an action potential to be initiated in these other cells is dependent on whether dendrites on the receiving or downstream cell are activated (as well as whether dendrites instead are suppressed by upstream, that is, presynaptic nerve impulses).

In short, it is the axons synapsing with a given cell, along with the degree to which the dendrites of these downstream cells are activated, that determines whether the downstream cell will display an action potential. In a very real sense, all over the place within our central nervous system exist special-purpose, low-power computers that are set up specifically for the sake of integrating information and then providing, as appropriate, specific responses. That integration is a product to a large degree with the association of neurons with other neurons and this occurs through the synaptic association of axon terminals with dendrites as well as axon terminals with postsynaptic cell bodies.


Links to terms of possible interest: Axon, Axon terminal, Dendrite, Dendritic spine, Postsynaptic spine, Presynaptic terminals, Synapse, Synaptic cleft, Synaptic vesicle

The above video talks briefly about dendrites within the context of the overall structure of neurons.



Neurotransmitter

Chemical that diffuses across synapses to signal postsynaptic cells.
Neurotransmitters are released by nerve cells at axon terminals and into the clefts of synapses. Their purpose is to continue the propagation of the information associated with an action potential from one cell (the upstream, that is, presynaptic cell) to a second cell (the downstream, i.e., postsynaptic cell). The neurotransmitters are received either by dendrites or instead directly by cell bodies. If the neurotransmitters are stimulating in their effect then their impact can be summed together to potentially give rise to an action potential in the downstream cell. Alternatively, the neurotransmitter can be inhibiting of an action potential, leading to a temporary lowering of the ability of the downstream cell – that is, the downstream neuron – to reach a threshold potential and thereby display an action potential. The release of neurotransmitters across synaptic clefts thus provides the actual, chemical means by which neurons influence the behavior of other cells, either contributing to the turning "on" of those cells or instead contributing to an inhibition of the turning "on" of those cells.


Links to terms of possible interest: Acetylcholine, Adrenaline, Closed channel, Dopamine, Endorphins, Glutamate, Ligand-gated channel, Nerve impulse, Neurotransmitter, Noradrenaline, Open channel, Serotonin, Synaptic vesicle

The above video provides nice explanation of what neurotransmitters are all about though is graphically simplistic.

The above video is a bit superficial but it has pretty graphics and does make some good points.



Glial cells

Non-neuron, nervous-system support cells.
While neurons are responsible for transmitting information, specifically signals of "on" from one location in the body to specific other locations in the body, the glial cells, a.k.a., neuroglia, are responsible for assuring that neurons can do this job. Glial cells do this by supplying numerous functions to neurons that neurons are either unable or less able to do for themselves. This includes nourishing neurons, holding neurons in place, and protecting neurons. In addition, glial cells are responsible for supplying the insulating myelin found in association with and surrounding the axons of neurons.


Links to terms of possible interest: Astrocyte, Axon terminal, Cell body, Central nervous system, Dendrite, Ependymal cells, Glial cell, Microglia, Oligodendrocyte, Peripheral nervous system, Satellite cells, Schwann cells

The above video provides a nice introduction to glial cells.

The above video provides neat, quasi three-dimensional animations of some of the central nervous system neuroglial cells. Note, though, that oligodendrocytes do not actually "secrete" myelin and the "harmful substances" engulfed by microglial cells are relatively large and complex things such as bacterial invaders.

The above video talks about gray matter, white matter, and especially glial cells (which are key to differentiating gray matter from white matter).



Myelin sheath

Glial-cell based insulator of the axons of neurons.
The myelin sheath consists of cells – such as what are known as Schwann cells in the peripheral nervous system – that have wrapped themselves multiple times around the axon on nerve cells. In the central nervous system these cells are described instead as oligodendrocytes. What these cells accomplish is to locally prevent, along the surface of nerve cells, the depolarization of the neuron plasma membrane. The result is a combination of increased efficiency with which action potentials are propagated, in terms of body energy expenditure, and also increased rates of action potential propagation. Neurons as a consequence of this myelin sheath work both more effectively and less expensively (again, in terms of energy usage). The loss of myelin sheaths is called demyelination and results in loss of effective nerve function, such as is associated with the neurodegenerative disease, multiple sclerosis.


Myelination

Glial-cell based insulation of the axons of neurons.
Neurons are not "born" with a myelin sheath, and we indeed we are not born with our neurons fully myelinated. Instead, this myelination occurs relatively gradually, in us, with a similarly gradual increase in neuron functionality and thereby the functioning of our nervous systems. As such, babies are not nearly as coordinated and capable as adults and not just because babies are young and inexperienced but also because the central command center of their bodies, their nervous system, is not nearly as myelinated as it will become in the course of their maturation towards and then into adulthood. Furthermore, absent full myelination or given demyelination then these capabilities that come within the maturation of our bodies, and associated maturation of our nervous systems, will be either absent or less profound.


Links to terms of possible interest: Astrocyte, Axon, Myelin, Myelin sheath, Myelination, Oligodendrocyte, Schwann cell

The above video describes the process of myelination in the peripheral nervous system and then the importance of myelination in terms of the speed of nerve impulse conduction, that is, via saltatory conduction.



Schwann cells

Myelin-supplying cells of the peripheral nervous system.
The Schwann cells are a form of neuroglia that serve as counterparts to the oligodendrocytes of the central nervous system.


Links to terms of possible interest: Axon, Dendrite, Myelin, Neurilemmal sheath, Neurofibrils, Nodes of Ranvier, Schwann cell



Oligodendrocytes

Myelin-supplying cells of the central nervous system.
The oligodendrocytes are a form of neuroglia which serve as counterparts to the Schwann cells of the peripheral nervous system.


Links to terms of possible interest: Axon, Central nervous system, Myelination, Nodes of Ranvier, Oligodendrocyte, Soma

Ultimately the above video doesn't quite say as much about oligodendrocytes as I would prefer it would, it is cleverly done is not not worth watching.



Nodes of Ranvier

Gaps between myelinating cells found along axons.
The nodes of Ranvier, or simply myelin sheath gaps, are tiny, uninsulated regions of axon plasma membrane over which action potentials can be propagated. Since such propagation is limited to these gaps, this makes the number of ions involved in this propagation (depolarization) relatively slight, which increases the efficiency of nerve conduction. Since nerve impulses can pass between nodes of Ranvier faster than they can along an equivalent length of uninsulated neuron, the nodes of Ranvier along with associated myelin sheaths result in faster nerve conduction as well. The jumping of action potentials from node to node is referred to as saltatory action potential propagation, or simply saltatory conduction. Contrasting the nodes of Ranvier are the internodes, which are the axon regions that instead are myelin insulated.


Links to terms of possible interest: Action potential, Axon, Cell body, Myelin sheath, Nodes of Ranvier, Saltatory conduction, Schwann cell



Saltatory conduction

Action potential jumping along axons from nodes of Ranvier to adjacent nodes of Ranvier.
Action potentials can form on axons only in unmyelinated regions, which correspond to nodes of Ranvier. The propagation of an action potential along an axon consequently does not involve a wave of action potentials through myelinated regions of axons. Instead the presence of an action potential in an adjacent node of Ranvier is detected, triggering an action potential in the downstream node of Ranvier. This action potential jumping between nodes of Ranvier is both energetically efficient, in comparison to given the alternative lack of myelination situation, and is faster than what can be achieved also given an absence of myelination.

The above video talks about myelin nodes of Ranvier saltatory conduction etc., but also provides a nice segue into talking about white matter more generally, as we will cover when considering the central nervous system.


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