Therefore, only MCV and mean fiber diameter demonstrated a consistent relationship with recovery time. Scatter plots of axon diameter against g -ratio revealed a significant correlation within each group Fig. At each time point after transection, the g -ratio to axon diameter relation was best fit by the following logarithmic equation:. Each group included five rats and the whole nerve fivers from five different consecutive fields were measured and stored for each nerve specimen. The g -ratio of each measured fiber is indicated by a single red circle.
Note that red circles to the lower left represent small diameter axons with relatively thick myelin sheaths low g -ratio that are likely to be nonconducting. Correlation between axon diameter x 1 and g -ratio y 1 is expressed by the correlation coefficient r 1 of the logarithmic regression curve.
F Logarithmic regression curves of axon diameter against g -ratio for the control black and regenerated nerve fibers 50 days, yellow; days, green; days, blue; days, red.
Lines are nearly superimposed, indicating that this relation is a poor index of recovery. The correlation coefficients r 1 ranged from 0. Time after transection was associated with a rightward shift in these plots, indicating more numerous axons with large diameters and higher g -ratios.
By — days Fig. The tail region at the lower left side of the plots indicates the presence of very thin fibers with excessively thick myelin sheaths low g -ratio. Many axons in the and day groups exhibited these characteristics, while few such axons were found in the control sciatic nerves Fig. We then investigated the quantitative relationship between fiber diameter and internodal length Fig. Following nerve transection, internodal length varied considerably among fibers, as evidenced by the higher scatter of diameter versus internodal length points Fig.
As shown in Figure 3 , the linear regression lines for the transected nerves at 50, , , and days have significantly flatter slopes than those for the control nerves. Furthermore, the correlation between fiber diameter and internodal length was weaker at every posttransection time point coefficients ranging from 0.
Each group included five rats, and about nerve fibers from each specimen were measured. Lines are plotted using different colors: control with black, 50 days with yellow, days with green, days with blue, and days with red.
New nerve repair techniques should only be introduced into general clinical practice if they can be conclusively proved efficient in improving the results obtained from previous techniques. To reach this goal, evaluation methods that provide an objective measure of recovery are required. Animal models also provide objective measures of functional recovery in a manner not presently obtainable in clinical studies. Morphological and electrophysiological measures reflect the inherent variability in the rate of nerve regeneration, myelination, and functional recovery; therefore, a combination of electrophysiological and morphometric measures may yield the best indication of recovery, especially over multiple time points.
We demonstrated that recovery of MCV and mean fiber diameter were well correlated with time after sciatic nerve transection. Although mean myelin thickness, axonal diameter, and g -ratio decreased after transection, they were not well correlated with time or MCV recovery. Conventional MCV measurements tend to reflect primarily upon the faster conducting fibers and provide little information about the conduction properties of the entire population of regenerating fibers Rosen and Jewett ; Dorfman The present study showed that MCV progressively increased through 50— days after transection, although it did not return to normal by days.
These observations reflect the recovery process of the regenerated fibers. Conduction velocity increases in appropriate proportion to fiber diameter Rushton ; Moore et al. Indeed, the histograms plotted in our study revealed a substantial increase in the number of fibers with large diameters during recovery.
Moreover, the histograms for fiber diameter in the transection group revealed a unimodal distribution at all time points up to days, while the fiber diameter distribution for the control group was bimodal, with a significantly higher proportion of fibers with large diameters. Dissociation between MCV recovery and mean fiber diameter recovery, which was calculated from the whole fibers, is therefore expected.
This may simply imply that many nonfunctional regenerating fibers could not be eliminated morphologically, or that there were no significant differences in MCV between the various groups. Many of the fibers with small diameters may in fact be nonconducting and degenerating.
As the nerve fibers regenerate distally and reach the appropriate target organ, fiber diameter increases and the myelin sheath grows Weiss et al. If sprouting axons do not make an appropriate connection with the target organ, they are denied vital growth factors and degenerate. It has been demonstrated that in rat sciatic nerves, there is an initial increase in the number of fibers distal to the site of transection, followed by a gradual decrease Mackinnon et al.
The initial increase can last for approximately six months before axonal number slowly decreases back to pretransection levels over the following two years. It may be difficult to distinguish smaller, successfully regenerated fibers from atrophic, dying fibers, especially during the early phase of regeneration.
Therefore, if studies on the morphological evaluation of rat sciatic nerves are completed within six months, their results may be considered inappropriate. Regenerated fibers have thinner myelin sheaths than those of normal fibers, although axonal diameters may approach normal values. There is an optimal myelin thickness relative to fiber diameter as measured by the g -ratio to maximize conduction velocity Rushton The scatter plots of g -ratio against axon diameter and their regression curves showed that larger fibers had higher g -ratios, whereas smaller fibers had excessively low g -ratios.
The mean axon diameter increased between 50 and days; however, it decreased at days. This serves as your time marker of when you flipped the worm, and now you know which spikes belong to the posterior end and which spikes belongs to the anterior end. The figure below shows a recording of electrode 1 on the bottom and electrode 2 on the top.
You can now zoom in on your spikes and measure the conduction velocity. Take readings of spikes. Repeat the experiment several times with some worms. This will give you a good data set to work with. Don't forget to clean your electrodes with some alcohol or water and a paper towel after each worm. You now need to run a statistical test, namely the T-test, to examine whether the conduction velocities are different for the two nerves.
If you do not yet know how to do this you can take your data set and follow along in our statistics lesson plan. If you have done this lesson plan or have some experience in statistics then you can go ahead and perform the calculations below.
Finally, let's calculate our t-statistic and p-value. What did you find? Are the two conduction velocities different from each other? Discussion If your experiment was successful, you should have found that the MGN anterior end conduction velocity was indeed significantly faster, but not 1. Why is this? You may recall that the earthworm neurons are actually myelinated! Some invertebrates, such as some shrimp and some worms, actually do have myelin.
Typically, as axon increases its diameter, its myelin thickness also increases. Perhaps the MGN has a thicker myelin sheath as well. This would make for an excellent histology project to find out. Let us know if you are up to the challenge, and let us know what you find! If you have an idea about what causes this unexpectedly large difference, we would love to hear about it.
Maybe your professor knows? Welcome to biology and unexpected findings! Also, if you understand why having a longer time constant increases conduction velocity, let us know that as well. This can sometimes be a difficult experiment, because the worm may not produce spikes depending on the amount and time of anesthetic used as well as the general health of the worm. You may also want to try touching the worm with more or less pressure. Sometimes a very small tap will work, other times a stronger press might be needed.
Some worms respond better to a stimulus at the very end of their bodies, while other respond better to a stimulus a few centimeters inwards. Finally, sometimes you will cause an artifact when you touch the worm.
Looking closely on the artifact waveforms, the artifacts will appear at exactly the same on both channels. This is a fake spike and not physiological! Sometimes, drying your probe periodically helps; also do not rehydrate the worm in water too much though also be careful not to dry the worm out.
It is a careful balance, and you will develop your own style and technique as you gain experience. A linear relation was found between axon diameter and fibre diameter, but the slope decreased as atrophy continued. This indicates that the axon cross-sectional area decreases relatively more than the total fibre area.
The ratio of the inner axon perimeter to the outer myelin perimeter remains constant at or near the optimal value of 0. However, in most of our patients aged 20 months, myelination in the peritrigonal areas appeared complete. How do nodes of Ranvier speed up conduction?
Nodes of Ranvier. Nodes of Ranvier are microscopic gaps found within myelinated axons. Their function is to speed up propagation of action potentials along the axon via saltatory conduction. The Nodes of Ranvier are the gaps between the myelin insulation of Schwann cells which insulate the axon of neuron.
Why Saltatory conduction is faster? Electrical signals travel faster in axons that are insulated with myelin. Action potentials traveling down the axon "jump" from node to node. This is called saltatory conduction which means "to leap. Why are Unmyelinated axons slower? This means that unmyelinated axons are slower in the conduction of electric signals, and therefore information, than myelinated axons.
This is important because there is a disease whereupon the body's own immune system attacks the myelin sheath around the axons in the central nervous system. How do I increase myelin? Exercise and Myelin Repair Scarisbrick, which showed that a high-fat diet combined with a sedentary lifestyle can reduce myelin-forming cells, contributing to demyelination and associated cognitive decline.
Adding exercise to this high-fat intake, however, has been proven to increase myelin production.
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