Axon and dendritic transport. Axon and axonal transport (fast and slow, anterograde and retrograde)

Group A fibers alpha

(diameter -13-22 microns, speed - 60-120 m / s, duration of PD - 0.4-0.5 ms)

1). efferent fibers that conduct

excitation to skeletal muscles from alpha motor neurons

2) afferent fibers that conduct excitation from muscle receptors in the central nervous system

Group A beta fibers

(diameter - 8-13- microns, speed - 40-70 m / s, duration of PD - 0.4-0.6 ms)

1. Afferent fibers that conduct

stimulation from touch receptors and tendon receptors in the CNS

Group A fiber gamma

(diameter - 4-8 microns, speed - 15-40 m / s, duration of PD - 0.5 -0.7 ms)

1) efferent fibers to muscle spindles from gamma motor neurons

2). afferent fibers that conduct

stimulation from touch and pressure receptors in the CNS

Group B fibers

(diameter - 1-3 microns, speed -3-14 m / s, duration of PD - 1.2 ms)

These are the preganglionic fibers of the autonomic nervous system.

Group C fibers

(diameter - 0.5-1.0 microns, speed -0.5-2.0 m / s, duration of PD - 2.0 ms)

1.postganglionic fibers of the ANS

2. afferent fibers that conduct excitation from pain, pressure and heat receptors in the central nervous system

axon transport. Fast axon transport. Slow axon transport.

Axon transport is the movement of substances along the axon. Proteins synthesized in the cell body, synaptic mediator substances and low molecular weight compounds move along the axon together with cell organelles, in particular mitochondria. For most substances and organelles, transport in the opposite direction was also found. Viruses and toxins can enter the axon at its periphery and move along it. Axon transport is an active process. Distinguish

fast axon transport and slow axon transport.

Slow axon transport is the transport of large molecules, in this case, apparently, the transport mechanism itself is not slower, but the transported substances from time to time enter cellular compartments that are not involved in transport. So, mitochondria sometimes move at the speed of fast transport, then stop or change direction of movement, resulting in slow transport.

The rate of fast axon transport is 410 mm/day. Such a rate is found in all neurons of warm-blooded animals, regardless of the type of molecules carried.

In many cases, the transport of organelles in the cell depends on microtubules. Microtubules in the axon are relatively stable compared to other cells. This is probably due to the high content of MAPs, which are able to stabilize microtubules. In addition, this is facilitated by the formation of microtubule bundles with the help of various associated proteins.

There are two main types of transport: direct (anterograde) - from the cell body along the processes to their periphery and reverse (retrograde) - along the processes of the neuron to the cell body

In the neuron, as in other cells of the body, the processes of disintegration of molecules, organelles, and other components of the cell are constantly taking place. They need to be constantly updated. Neuroplasmic transport is important for ensuring the electrical and non-electrical functions of the neuron, for the implementation of feedback between the processes and the body of the neuron. When nerves are damaged, regeneration of damaged areas and restoration of innervation of organs is necessary.

A variety of substances are transported along the processes of a neuron at different speeds, in different directions, and using different transport mechanisms. There are two main types of transport: direct (anterograde), from the cell body along the processes to their periphery, and reverse (retrograde), along the processes of the neuron to the cell body (Table 1).

Five groups of "motor" proteins, closely associated with the cytoskeletal network, are involved in the implementation of transport processes in the neuron. They include proteins such as kinesins, deneins and myosins.

Five groups of the so-called are involved in the implementation of transport processes in the neuron. "motor" molecules (Fig. xx).

Mechanisms of axon and dendritic transport

Direct axonal transport is carried out by motor molecules associated with the cytoskeletal system and the plasma membrane. The motor part of the kinesin or denein molecules binds to the microtubule, while its tail part binds to the transported material, to the axonal membrane, or to adjacent elements of the cytoskeleton. A number of auxiliary proteins (adapters) associated with kinesin or denein also take part in ensuring transport along the processes. All processes go with a significant expenditure of energy.

Reverse (retrograde) transport.

In axons, the main mechanism of reverse transport is the system of denein and myosin motor proteins. The morphological substrate of this transport is: in the axon - multivesicular bodies and signal endosomes, in dendrites - multivesicular and multilamellar bodies.

In dendrites, reverse transport is carried out by molecular complexes of not only denein, but also kinesin. This is due to the fact that (as mentioned earlier) in the proximal parts of the dendrites, the microtubules are oriented in mutually opposite directions, and only kinesin complexes transport molecules and organelles to the “+” end of the microtubules. As in the case of direct transport, different components and substances are transported retrogradely in different neurons at different rates, and apparently in different ways.

The smooth endoplasmic reticulum plays an important role in the transport processes in the neuron. It is shown that a continuous branched network of cisterns of the smooth reticulum extends along the entire length of the processes of the neuron. The terminal branches of this network penetrate into the presynaptic sites of the synapses, where synaptic vesicles are laced from them. It is through its tanks that many mediators and neuromodulators, neurosecrets, enzymes of their synthesis and decay, calcium ions and other components of axotok are quickly transported. The molecular mechanisms of this type of transport are not yet clear.

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axon transport online, axon transport minsk
Axon transport- this is the movement along the axon of a nerve cell of various biological material.

Axonal processes of neurons are responsible for the transmission of the action potential from the body of the neuron to the synapse. Also, the axon is a path along which the necessary biological materials are transported between the body of the neuron and the synapse, which is necessary for the functioning of the nerve cell. Membrane organelles (mitochondria), various vesicles, signaling molecules, growth factors, protein complexes, cytoskeletal components, and even Na+- and K+-channels are transported along the axon from the area of ​​synthesis in the body of the neuron. The end points of this transport are certain areas of the axon and synaptic plaque. in turn, neurotrophic signals are transported from the synapse region to the cell body. This acts as a feedback that informs about the state of the target's innervation.

The length of the axon of the human peripheral nervous system can exceed 1 m, and may be longer in large animals. The thickness of a large human motor neuron is 15 microns, which, with a length of 1 m, gives a volume of ~ 0.2 mm³, which is almost 10,000 times the volume of a liver cell. This makes neurons dependent on the efficient and coordinated physical transport of substances and organelles along the axons.

The lengths and diameters of axons, as well as the amount of material transported along them, certainly indicate the possibility of failures and errors in the transport system. Many neurodegenerative diseases are directly related to disturbances in the functioning of this system.

• 1 Key features of the axon transport system
• 2 Classification of axon transport
• 3 See also
• 4 Literature

Key Features of the Axon Transport System

Simplified, axon transport can be represented as a system consisting of several elements. it includes cargo, motor proteins that carry out transport, cytoskeleton filaments, or "rails" along which the "motors" are able to move. Also needed are linker proteins that connect motor proteins to their cargo or other cellular structures, and auxiliary molecules that start and regulate transport.

Classification of axon transport

Cytoskeletal proteins are delivered from the cell body, moving along the axon at a rate of 1 to 5 mm per day. This is slow axon transport (transport similar to it is also present in dendrites). Many enzymes and other proteins in the cytosol are also carried by this type of transport.

Non-cytosolic materials that are needed at the synapse, such as secreted proteins and membrane-bound molecules, move along the axon at a much faster rate. These substances are transported from their site of synthesis, the endoplasmic reticulum, to the Golgi apparatus, which is often located at the base of the axon. Then, these molecules, packed in membrane vesicles, are transported along the microtubule rails by fast axonal transport at a speed of up to 400 mm per day. Thus, mitochondria, various proteins, including neuropeptides (neurotransmitters of a peptide nature), non-peptide neurotransmitters are transported along the axon.

The transport of materials from the body of the neuron to the synapse is called anterograde, and in the opposite direction - retrograde.

Transport along the axon over long distances occurs with the participation of microtubules. Microtubules in the axon have their inherent polarity and are oriented with a rapidly growing (plus-) end to the synapse, and a slowly growing (minus-) end to the body of the neuron. Axon transport motor proteins belong to the kinesin and dynein superfamilies.

Kinesins are primarily plus-terminal motor proteins that transport cargo such as synaptic vesicle precursors and membrane organelles. This transport goes towards the synapse (anterograde). Cytoplasmic dyneins are minus-terminal motor proteins that transport neurotrophic signals, endosomes, and other cargo retrograde to the neuron body. Retrograde transport is not exclusive to dyneins: several kinesins moving in a retrograde direction have been found.

see also

• Wallerian degeneration
• Kinesin
• Dinein
• DISC1

Literature

1. Duncan J.E., Goldstein L.S. The genetics of axonal transport and axonal transport disorders. // PLoS Genet. 2006 Sep 29;2(9):e124. PLoS Genetic, PMID 17009871.

Histology: Nervous tissue Neurons (Gray matter) Pericarion Axon (Axon hillock, Axon terminal, Axoplasm, Axolemma, Neurofilaments) Growth cone Wallerian degeneration Dendrite (Nissl substance, Dendritic spine, Apical dendrite, Basal dendrite) Dendritic plasticity Dendritic action potential Types: Bipolar neurons Unipolar neurons Pseudo-unipolar neurons Multipolar neurons Pyramidal neuron Star neuron Purkinje cell Granular cell Interneuron Renshaw cell afferent nerve/ sensory neuron GSA GVA SSA SVA Nerve fibers (Muscle spindles (Ia), Tendon spindle (Ib), II or Aβ fibers, III or Aδ fibers, IV or C fibers) Efferent nerve/ motor neuron GSE GVE SVE Upper motor neuron Lower motor neuron (α motor neurons, γ motor neurons) Synapse Chemical synapse Neuromuscular synapse Ephaps (Electrical synapse) Neuropile Synaptic vesicle sensory receptor Meisner corpuscle Merkel corpuscle Pacini corpuscle Ruffini corpuscle Neuromuscular spindle Free nerve ending Olfactory neuron Photoreceptor cells Hair cells Taste bud neuroglia Astrocytes (Radial glia) Oligodendrocytes Ependymal cells (Tanicites) Microglia myelin (White matter) CNS: Oligodendrocytes PNS: Schwann cells (Neurolemma · Node of Ranvier/Internodal segment · Myelin notch) Connective tissue Epineurium Perineurium Endoneurium Bundles of nerve fibers Meninges: hard, arachnoid, soft

axon transport minsk, axon transport online, axon transport ternopil, axon transport

Axon Transport Information About

6. Transport in cell vesicles.
7. Transport by the formation and destruction of organelles. Microfilaments.


10. Regulation of cellular functions. Regulatory effects on the cell membrane. membrane potential.
11. Extracellular regulatory substances. synaptic mediators. Local chemical agents (histamine, growth factor, hormones, antigens).
12. Intracellular communication with the participation of second mediators. Calcium.
13. Cyclic adenosine monophosphate, cAMP. cAMP in the regulation of cell function.
14. Inositol phosphate "IF3". Inositol triphosphate. Diacylglycerol.

Processes of intracellular transport can be most clearly demonstrated in axon of a nerve cell. axon transport discussed here in detail to illustrate events that probably occur in a similar way in most cells. The axon, which is only a few microns in diameter, can reach a length of one meter or more, and the movement of proteins by diffusion from the nucleus to the distal end of the axon would take years. It has long been known that when any section of the axon undergoes constriction, the proximal portion of the axon expands. It looks like the centrifugal flow is blocked in the axon. Such a flow of fast axon transport can be demonstrated by the movement of radioactive markers, as in the experiment shown in Fig. 1.14. Radiolabeled leucine was injected into the dorsal root ganglion, and then, from the 2nd to the 10th hour, the radioactivity was measured in the sciatic nerve at a distance of 166 mm from the neuron bodies. For 10 hours, the peak of radioactivity at the injection site did not change significantly. But the wave of radioactivity propagated along the axon at a constant speed of about 34 mm per 2 hours, or 410 mm/day. It has been shown that in all neurons of homoiothermic animals, fast axon transport occurs at the same rate, and there are no noticeable differences between thin, unmyelinated fibers and the thickest axons, as well as between motor and sensory fibers. The type of radioactive marker also does not affect fast axon transport speed; markers can be a variety of radioactive molecules, such as various amino acids included in the proteins of the neuron body. If we analyze the peripheral part of the nerve to determine the nature of the carriers of the radioactivity transported here, then such carriers are found mainly in the protein fraction, but also in the composition of mediators and free amino acids. Knowing that the properties of these substances are different and especially the sizes of their molecules are different, we can explain the constant rate of transport only by the common for all of them transport mechanism.

Rice. 1.14. An experiment demonstrating rapid axon transport in sensory fibers of the sciatic nerve of a cat. Tritium-labeled leucine is injected into the dorsal root ganglion and radioactivity is measured in the ganglion and sensory fibers 2, 4, 6, 8, and 10 hours after administration (lower panel). The abscissa shows the distance from the ganglion to the areas of the sciatic nerve where the measurement is made. On the y-axis, only for the upper and lower curves, radioactivity (imp./min) is plotted on a logarithmic scale. The "wave" of increased radioactivity (arrows) moves at a speed of 410 mm / day (according to )

The fast axon transport described above is anterograde, i.e., directed away from the cell body. It has been shown that some substances move from the periphery to the cell body using retrograde transport. For example, acetylcholinesterase is transported in this direction at a rate two times lower than the rate of fast axonal transport. A marker often used in neuroanatomy, horseradish peroxidase, also moves retrogradely. Retrograde transport probably plays an important role in the regulation of protein synthesis in the cell body. A few days after axon transection, chromatolysis is observed in the cell body, which indicates a violation of protein synthesis. The time required for chromatolysis correlates with the duration of retrograde transport from the site of axon transection to the cell body. Such a result also suggests an explanation for this violation - the transmission from the periphery of the “signal substance” that regulates protein synthesis is disrupted.

Obviously, the main means of transportation used for rapid axonal transport are vesicles (vesicles) and organelles such as mitochondria containing the substances to be transported. The movement of the largest vesicles or mitochondria can be observed using a microscope in vivo. Such particles make short, quick movements in one direction, stop, often move slightly backwards or to the side, stop again, and then make a dash in the main direction. 410 mm/day correspond to an average anterograde velocity of approximately 5 μm/s; the speed of each individual movement should therefore be much higher, and if we take into account the size of organelles, filaments and microtubules, then these movements are really very fast. Rapid axon transport requires a significant concentration of ATP. Poisons such as microtubule-destroying colchicine also block fast axonal transport. It follows from this that in the transport process we are considering, vesicles and organelles move along microtubules and actin filaments; this movement is provided by small aggregates of dynein and myosin molecules acting as shown in fig. 1.13, using the energy of ATP.

Rapid axon transport may also be involved in pathological processes. Some neurotropic viruses (for example, herpes or polio viruses) penetrate the axon at the periphery and move with the help of retrograde transport to the neuron body, where they multiply and exert their toxic effect. Tetanus toxin- a protein produced by bacteria that enter the body when the skin is damaged, is captured by nerve endings and transported to the body of the neuron, where it causes characteristic muscle spasms.

Cases of toxic effects on the axon transport itself are known, for example, exposure to the industrial solvent acrylamide. In addition, it is believed that the pathogenesis of avitaminosis " take-take" And alcoholic polyneuropathy includes disruption of fast axon transport.

Rice. 1.13. Non-muscle myosin complex at a certain orientation, it can bind to actin filaments of different polarity and, using the energy of ATP, shift them relative to each other

Apart from fast axon transport in the cell exists and quite intense slow axon transport. Tubulin moves along the axon at a rate of about 1 mm/day, while actin moves faster, up to 5 mm/day. Other proteins also migrate with these components of the cytoskeleton; for example, enzymes appear to be associated with actin or tubulin. The rates of movement of tubulin and actin are roughly consistent with the growth rate found for the mechanism described earlier when molecules are incorporated into the active end of a microtubule or microfilament. Therefore, this mechanism may underlie slow axonal transport. The rate of slow axon transport also approximately corresponds to the rate of axon growth, which, apparently, indicates the limitations imposed by the structure of the cytoskeleton on the second process.

Concluding this section, it should be emphasized that cells are by no means static structures, as they appear, for example, in electron microscopic photographs. The plasma membrane and especially the organelles are in constant rapid movement and constant restructuring; that is the only reason they are able to function. Further, these are not simple chambers in which chemical reactions take place, but highly organized conglomerates of membranes and fibers, in which reactions proceed in an optimally organized sequence.

Of particular interest, from the point of view of the physiology of the central nervous system, is the process of intracellular transport, transmission of information, signal in the axon of a nerve cell. The axon of a nerve cell is only a few microns in diameter. At the same time, the length of the axon in some cases reaches 1 m. How is the constant and high speed of transport along the axon ensured?

For this, a special axon transport mechanism is used, which is divided into fast and slow.

First, keep in mind that the fast transport mechanism is anterograde, i.e. directed from the cell body to the axon.

Secondly, the main “vehicles” for fast axonal transport are vesicles (vesicles) and some structural cell formations (for example, mitochondria), which contain substances intended for transportation. Such particles make short fast movements, which corresponds to approximately 5 µm s(-1). Rapid axon transport requires a significant concentration of ATP energy.

Thirdly, slow axon transport moves individual elements of the cytoskeleton: tubulin and actin. For example, tubulin, as an element of the cytoskeleton, moves along the axon at a speed of about 1 mm day(-1). The rate of slow axon transport is approximately equal to the rate of axon growth.

Of great importance for understanding the physiology of the CNS are the processes of regulation of effects on the cell membrane. The main mechanism of such regulation is a change in the membrane potential. Changes in the membrane potential are carried out due to the influence of neighboring cells or changes in the extracellular concentration of ions.

The most significant regulator of membrane potential is the extracellular substance in interaction with specific receptors on the plasma membrane. These extracellular substances include synaptic mediators that transmit information between nerve cells.

synaptic neurotransmitters are small molecules released from nerve endings at the synapse. When they reach the plasma membrane of another cell, they trigger electrical signals or other regulatory mechanisms (Fig. 6).

Rice. 6. Scheme of release of neurotransmitters and processes occurring in the synapse

In addition, individual chemical agents (histamine, prostaglandin) move freely in the extracellular space, which are quickly destroyed, but have a local effect: they cause a short-term contraction of smooth muscle cells, increase the permeability of the vascular endothelium, cause itching, etc. Certain chemical agents promote nerve growth factors. In particular, for the growth and survival of sympathetic neurons.

In fact, there are two information transmission systems in the body: nervous and hormonal (for details, see unit 2).