An axon typically develops side branches called axon collaterals, so that one neuron can send information to several others. These collaterals, just like the roots of a tree, split into smaller extensions called terminal branches. Each of these has a synaptic terminal on the tip. Neurons communicate through synapses - contact points between the axon terminals on one side and dendrites or cell bodies on the other.
Here, in a nanometre-wide gap, electrical signals coming via the axon are converted into chemical signals through the release of neurotransmitters, and then promptly converted back into electricity as information moves from neuron to neuron. Myelin acts as a form of insulation for axons, helping to send their signals over long distances.
For this reason, myelin is mostly found in neurons that connect different brain regions, rather than in the neurons whose axons remain in the local region. Branch specific myelination could result in increased, branch specific conduction velocity, although Koestinger et al. A curious observation related to myelination pertains to the stria of Gennari, the myelinated band of axons in layer 4B of primate area V1. Since this consists of intrinsic collaterals, the common explanation, that myelination is a means of increasing conduction velocity over long distances, is not immediately applicable.
Local collaterals do not need? Could it be that the myelination relates to other factors, such as plasticity-related or state-related changes in axon diameter? The intrinsic collateralization is exceptionally extensive, measured as 8.
Varying diameters presumably indicate differences in the degree of myelination and, by inference, in conduction velocity. Figure 1. A typical, spatially extended proximal axonal arborization of a Meynert cell red asterisk in primary visual cortex of a macaque monkey. There are three major intrinsic collaterals labeled as br. All together, the intrinsic collaterals span 5. A further, extrinsic collateral br. Portions of the individual collaterals and of the main axon thick arrow could not be followed, as is indicated by dashed lines.
The low magnification inset A , at left provides a schematic overview of the general configuration. All branches have numerous small synaptic clusters, one of which is illustrated in B. Note diminished diameter between the main axon arrow and the terminal arborization. Extrinsic axons C are of variable diameters one large diameter axon at arrowhead. D Extrinsic terminations in area MT include some large diameter axons.
Modified from Figures 1, 9 in Rockland and Knutson and Figures 6f, 7b from Rockland with permission. An important component of collateralization is that the daughter branches are often not uniform, but especially at branch points, vary in diameter Figure 2.
Variability in diameter together with other parameters will impact on excitability, conduction velocity, and other aspects of signal propagation. Other impacting parameters include width of myelin and intermodal length, and density and distribution of ion channels reviewed in Debanne et al. These would have effects on neural response properties at the microcircuitry level.
Differential recruitment of postsynaptic populations or network recombinations could be factors in state transitions or modulation. Figure 2. A typically branched segment of axon in the white matter macaque monkey.
The segment originates from a neuron in parietal cortex and is seen here in the vicinity of ventral temporal cortex. Panel A is lower magnification of B. Note double bifurcations, where the first daughter branch solid arrow is conspicuously thinner and unmyelinated? In the second, slightly more distal bifurcation hollow arrow , the daughter branches appear about equal in diameter, but both are thinner than the main axon.
Reproduced from Zhong and Rockland with permission. C Schematic of a neuron blue and its extended branching topology foreshortened for the sake of convenient formatting. An action potential AP can follow circuitous paths A—C to multiple targets.
Reliability of propagation depends both on active electrical properties of the axon and its geometry, including membrane inhomogeneties such as swellings and incompatible branch diameters.
Below: schematic to illustrate reliable propagation A , with optimal impedance matching between the mother and daughter branches , and slowed or failed propagation B , where the daughter branch has an enlarged diameter; C , where there is an interposed membrane swelling.
Reproduced from Huguenard with permission. The axon geometry, active electrical properties, and membrane inhomogeneities at branch points are well-known as factors of reliable propagation e. This leads to several different scenarios about temporal features consequent to collateralization. First, there can be synchronous activation throughout the daughter branches.
The auditory brainstem pathways project via a single bifurcating axon to ipsi- and contralateral targets respectively, short and longer physical pathways.
Isochronic transmission is achieved by differential myelination and axon caliber of the two daughter branches i. Since most branched axons, in comparison with brainstem auditory pathways, cover a larger territory and subserve less well defined functions, data are largely incomplete or lacking for other systems. One might predict, however, in the case of synchronicity, that proximal branches i. As noted above, this simple prediction does not seem to hold.
Further investigation will entail sampling from identified axons over long distances, and would not be easy to achieve. Second, branch-specific activation may be asynchronous. Models of cortical circuits describe distinctive routing states of short-lived transient synchrony that could dynamically shape the flow of information Palmigiano et al. The major conclusions of this review are 1 the mechanisms of axon extension and branching are not identical; 2 active suppression of protrusive activity along the axon negatively regulates branching; 3 the earliest steps in the formation of axon branches involve focal activation of signaling pathways within axons, which in turn drive the formation of actin-based protrusions; and 4 regulation of the microtubule array by microtubule-associated and severing proteins underlies the development of branches.
Linking the activation of signaling pathways to specific proteins that directly regulate the axonal cytoskeleton underlying the formation of collateral branches remains a frontier in the field. Acute axon damage is serious and life changing. Severe and diffuse axonal injuries can explain why people with head injury may be limited by a vegetative state.
Axonal tears have been linked to lesions responsible for loss of consciousness in people who experience mild head injuries or concussions. Axon damage can result in axon degeneration loss and can eventually kill the underlying nerve. Head trauma can occur from different types of injury, including:. Axon loss is an early sign of neurodegenerative diseases like:. Some disease states can cause this myelin breakdown. While the sheath can technically repair itself, damage can be severe enough to kill the underlying nerve fiber.
These nerve fibers in the central nervous system cannot fully regenerate. A demyelinated axon transmits impulses up to 10 times slower than a normal myelinated axon, and a complete stop of the transmission is also possible.
Conditions that can cause demyelination include:. In the nervous system, the axon hillock is a specialized location on a cell body soma where the neuron connects to an axon. It controls the firing of neurons. Axon terminals are located at the end of an axon. This is where messages from neurotransmitters are received. Myelin insulates an axon by surrounding the thin fiber with a layer of fatty substance protection.
This layer is located between the axon and its covering the endoneurium. An axon is a thin fiber that extends from a neuron, or nerve cell, and is responsible for transmitting electrical signals to help with sensory perception and movement. Each axon is surrounded by a myelin sheath, a fatty layer that insulates the axon and helps it transmit signals over long distances. Sign up for our Health Tip of the Day newsletter, and receive daily tips that will help you live your healthiest life.
Myelin and axon abnormalities in schizophrenia measured with magnetic resonance imaging techniques. Biol Psychiatry. The University of Queensland. Axons: the cable transmission of neurons. National Cancer Institute.
0コメント