Neuroscience

Articles and news from the latest research reports.

1,942 notes

Pulse propagation and signal transduction in the hydraulic brain
When Descartes turned his critical eye to the nervous system, he reasoned that the nerves must transduce hydraulic power to control the musculature. In the circulatory system, blood is pushed comparatively slowly through the aorta, perhaps around 0.3 meters per second. Superimposed on that flow, however, is an arterial pulse wave which propagates much faster, both through the blood and the walls of the vessel. For compliant and healthy vessels that speed might be around 10 meters per second, while for more hardened arteries, it could be 15 or higher. Modern day electrophysiologists have since replaced the hydraulic model with the idea that nerves really only transmit information—electrical information no less. Yet when looking at the power supply to the leg, for example, it is still hard to ignore the fact that the main femoral artery, at a diameter scarcely a half of an inch, looks rather meager next to the “information-supplying” sciatic nerve, which may actually be more like three-quarters of an inch. A conflux of ideas from a variety of disciplines has recently led to a critical re-emergence of the more mechanical side of the nervous system. To that point, two German scientists have just published a paper in the journal, Medical Hypotheses, where they suggest that the pulse wave is the main event in nervous conduction, while the electrical show is mere epiphenomenon.
We recently discussed the increasingly popular idea that action potentials may actually be soliton waves which propagate in the membranes of axons as phase transitions with minimal loss in energy. Convincing biologists that these subtle creatures could exist in the chaotic and varied conditions inside neurons has been a challenge. However, it is harder to argue against the fact that any kind of electrochemical spike based on the rapid influx of ions will be accompanied by a significant pressure pulse. The idea that the German researchers have supported, is that these as the pressure pulses naturally decay in the viscoelastic medium of the nerve, they are refreshed by ionic input at the nodes between myelinated axon segments, or continuously in unmyelinated axons.
If you have ever been absent-minded enough to grab a live wire, or even brush up strongly against one, the sensation is unforgettable. It is not such a stretch to acknowledge that when you slam your funny bone, or more precisely the Ulnar nerve (largest unprotected nerve in your body), the resultant vibe and decay feels almost identical to a real electrical assault. Similarly, the so-called “stingers” that run down the limbs after a sharp blow to the head are familiar to most footballers, and can give one quite a shock. Unfortunately these (albeit very simplistic) macroscopic intuitions don’t hold up so well when extended to the microscopic domain. Granted, when the electrochemical mechanisms that are assumed to underlie nervous conduction are looked at in detail, it becomes more difficult to disentangle the mechanical from the electric. However, as the authors observe, at some point, an attentive electrophysiologist must ask his or herself, “why are so many ion channels mechanosensitive” ?
One unexpected finding of the patch clamp recording technique was that the dilation of the membrane caused by local tension leads to considerable increase in transmembrane ion flow. Impulse waves causing short extensions in the membrane can directly induce opening and closing of both voltage and ligand gated channels. The idea that the pore in these channels is a rigid tube isolated from larger membrane events is difficult to support in this context. According to the authors, it is quite likely that common mechanoreceptor devices, like the pressure- or vibration-sensitive Vater-Pacinian corpuscles of the skin, conduct signals to initiate high-speed polysynaptic muscle reflex circuits without any classical intermediary electrical conversion.
The exact conduction velocity of mechanical impulses in nerve fibers remains unknown. It is estimated that under physiologic conditions, an unamplified axoplasmic pressure pulse would decay over roughly 1 mm due to viscosity, depending on the distensibility of the axon wall. When compared to the theoretical case of an absolutely rigid wall, a typical myelin sheath may be rigid enough to support pulse speeds up to one-fifth of the estimated maximum. That speed is not to shabby when compared with some rough estimates from previous authors, which put the maximum pulse velocity under an indistensible membrane somewhere upwards of 1500 meters per second. Suddenly, the quicker than life eyeblink response, or speed of the tenderfoot stepping on a sharp shard, become a little more comprehensible.
The theory as it stands is incomplete and needs to be adapted for specific cases with real biology in mind. In different animals, and in different regions of their brains, conduction in neurons goes by different names. For example, in the cerebellum, the unmyelinated parallel fibers pack to extreme densities in a regular crystalline lattice whose reason to be defies physiologic explanation to this day. Just as we currently have no good explanation for how signals could be properly isolated in nerve bundles where seemingly random nodes of Ranvier overlap in extent and influence, it is hard to imagine parallel fibers could maintain their electrochemical, or even mechanical, autonomy within this geometry.
The pressure wave theory wields considerable predictive power when it comes to explaining some of the unique synaptic specializations found throughout the brain. When considered only from an electrochemical point of view, the huge structural synaptic investments, like those found at the neuromuscular junction (NMJ), can hardly be imagined to be driven by local, and weak, current or field effects. One might need look no further than simple-to-recreate Chaldni patterns set up in two dimensions on the surface of a taunt drum, to make the imaginative leap to a three dimensional system, with multiple vibrating players, where more extreme patterns might easily be set up to provide authorship to repeatable complex structure. For the NMJ in particular, the case has been made that at the end-plate, the comparatively enormous efflux of acetylcholine to the deeply-guttered cleft, and propagation of excitation through the transverse tubule system, are all components of a continuous mechanical amplifier.
The apparent ease with which evolving organisms manage to cobble together all manner of sensitive hearing devices becomes infinitely more explicable once we see that nature has apparently been doing this kind of thing all alone inside of neurons. The amplification and transduction through liquid channels, of barely noticeable vibrations against a background of thermal noise much greater in magnitude, is in this light, no evolutionary stumble-upon, but rather the bread and butter of neural systems, and perhaps many aspects of life in general.

Pulse propagation and signal transduction in the hydraulic brain

When Descartes turned his critical eye to the nervous system, he reasoned that the nerves must transduce hydraulic power to control the musculature. In the circulatory system, blood is pushed comparatively slowly through the aorta, perhaps around 0.3 meters per second. Superimposed on that flow, however, is an arterial pulse wave which propagates much faster, both through the blood and the walls of the vessel. For compliant and healthy vessels that speed might be around 10 meters per second, while for more hardened arteries, it could be 15 or higher. Modern day electrophysiologists have since replaced the hydraulic model with the idea that nerves really only transmit information—electrical information no less. Yet when looking at the power supply to the leg, for example, it is still hard to ignore the fact that the main femoral artery, at a diameter scarcely a half of an inch, looks rather meager next to the “information-supplying” sciatic nerve, which may actually be more like three-quarters of an inch. A conflux of ideas from a variety of disciplines has recently led to a critical re-emergence of the more mechanical side of the nervous system. To that point, two German scientists have just published a paper in the journal, Medical Hypotheses, where they suggest that the pulse wave is the main event in nervous conduction, while the electrical show is mere epiphenomenon.

We recently discussed the increasingly popular idea that action potentials may actually be soliton waves which propagate in the membranes of axons as phase transitions with minimal loss in energy. Convincing biologists that these subtle creatures could exist in the chaotic and varied conditions inside neurons has been a challenge. However, it is harder to argue against the fact that any kind of electrochemical spike based on the rapid influx of ions will be accompanied by a significant pressure pulse. The idea that the German researchers have supported, is that these as the pressure pulses naturally decay in the viscoelastic medium of the nerve, they are refreshed by ionic input at the nodes between myelinated axon segments, or continuously in unmyelinated axons.

If you have ever been absent-minded enough to grab a live wire, or even brush up strongly against one, the sensation is unforgettable. It is not such a stretch to acknowledge that when you slam your funny bone, or more precisely the Ulnar nerve (largest unprotected nerve in your body), the resultant vibe and decay feels almost identical to a real electrical assault. Similarly, the so-called “stingers” that run down the limbs after a sharp blow to the head are familiar to most footballers, and can give one quite a shock. Unfortunately these (albeit very simplistic) macroscopic intuitions don’t hold up so well when extended to the microscopic domain. Granted, when the electrochemical mechanisms that are assumed to underlie nervous conduction are looked at in detail, it becomes more difficult to disentangle the mechanical from the electric. However, as the authors observe, at some point, an attentive electrophysiologist must ask his or herself, “why are so many ion channels mechanosensitive” ?

One unexpected finding of the patch clamp recording technique was that the dilation of the membrane caused by local tension leads to considerable increase in transmembrane ion flow. Impulse waves causing short extensions in the membrane can directly induce opening and closing of both voltage and ligand gated channels. The idea that the pore in these channels is a rigid tube isolated from larger membrane events is difficult to support in this context. According to the authors, it is quite likely that common mechanoreceptor devices, like the pressure- or vibration-sensitive Vater-Pacinian corpuscles of the skin, conduct signals to initiate high-speed polysynaptic muscle reflex circuits without any classical intermediary electrical conversion.

The exact conduction velocity of mechanical impulses in nerve fibers remains unknown. It is estimated that under physiologic conditions, an unamplified axoplasmic pressure pulse would decay over roughly 1 mm due to viscosity, depending on the distensibility of the axon wall. When compared to the theoretical case of an absolutely rigid wall, a typical myelin sheath may be rigid enough to support pulse speeds up to one-fifth of the estimated maximum. That speed is not to shabby when compared with some rough estimates from previous authors, which put the maximum pulse velocity under an indistensible membrane somewhere upwards of 1500 meters per second. Suddenly, the quicker than life eyeblink response, or speed of the tenderfoot stepping on a sharp shard, become a little more comprehensible.

The theory as it stands is incomplete and needs to be adapted for specific cases with real biology in mind. In different animals, and in different regions of their brains, conduction in neurons goes by different names. For example, in the cerebellum, the unmyelinated parallel fibers pack to extreme densities in a regular crystalline lattice whose reason to be defies physiologic explanation to this day. Just as we currently have no good explanation for how signals could be properly isolated in nerve bundles where seemingly random nodes of Ranvier overlap in extent and influence, it is hard to imagine parallel fibers could maintain their electrochemical, or even mechanical, autonomy within this geometry.

The pressure wave theory wields considerable predictive power when it comes to explaining some of the unique synaptic specializations found throughout the brain. When considered only from an electrochemical point of view, the huge structural synaptic investments, like those found at the neuromuscular junction (NMJ), can hardly be imagined to be driven by local, and weak, current or field effects. One might need look no further than simple-to-recreate Chaldni patterns set up in two dimensions on the surface of a taunt drum, to make the imaginative leap to a three dimensional system, with multiple vibrating players, where more extreme patterns might easily be set up to provide authorship to repeatable complex structure. For the NMJ in particular, the case has been made that at the end-plate, the comparatively enormous efflux of acetylcholine to the deeply-guttered cleft, and propagation of excitation through the transverse tubule system, are all components of a continuous mechanical amplifier.

The apparent ease with which evolving organisms manage to cobble together all manner of sensitive hearing devices becomes infinitely more explicable once we see that nature has apparently been doing this kind of thing all alone inside of neurons. The amplification and transduction through liquid channels, of barely noticeable vibrations against a background of thermal noise much greater in magnitude, is in this light, no evolutionary stumble-upon, but rather the bread and butter of neural systems, and perhaps many aspects of life in general.

Filed under nervous system action potentials myelin sheath axons nerve cells ion channels neuroscience science

  1. zinethong reblogged this from neurosciencestuff
  2. mind-wondered reblogged this from neurosciencestuff
  3. the-twelfth-dimension reblogged this from vestfigial
  4. mika-h reblogged this from neurosciencestuff
  5. vestfigial reblogged this from neurosciencestuff
  6. divellere reblogged this from neurosciencestuff
  7. pornsince5 reblogged this from cachete7
  8. ethorial reblogged this from neurosciencestuff
  9. theparanoidnerd reblogged this from bucellarius
  10. happyseed37 reblogged this from neurosciencestuff
  11. illuminaughtyhottie reblogged this from neurosciencestuff
  12. bowieandbondage reblogged this from electricalascension
  13. electricalascension reblogged this from neurosciencestuff
  14. imcray-z reblogged this from neurosciencestuff
  15. giaccomozion reblogged this from neurosciencestuff
  16. ether927 reblogged this from neurosciencestuff
  17. dj-phr0 reblogged this from neurosciencestuff
  18. einneinineinemreim reblogged this from burdenofself
  19. phbelow7 reblogged this from neurosciencestuff
  20. thepursuitofselfactualization reblogged this from neurosciencestuff
  21. moodykid reblogged this from moodykid
  22. clovesmokex reblogged this from neurosciencestuff
  23. greychemical reblogged this from earlydecline
  24. livinginthesound reblogged this from neurosciencestuff
  25. seltzer9 reblogged this from kia411
  26. janbird reblogged this from rachaelpotter
  27. rachaelpotter reblogged this from neurosciencestuff
  28. octagonperfectionist reblogged this from neurosciencestuff
  29. thottiemccall reblogged this from neurosciencestuff
  30. tyromedico reblogged this from neurosciencestuff
free counters