Tuesday, January 30, 2007

 

Thinking with the spinal cord?

Two scientists from the University of Copenhagen have demonstrated that the spinal cord uses network mechanisms similar to those used in the brain. The discovery is featured in the current issue of the journal Science.

The research group behind the surprising results consists of Professor Jorn Hounsgaard and Post.doc Rune W. Berg from the University of Copenhagen, and Assistant Professor and PhD Aidas Alaburda from the University of Vilnius. The group has shown that spinal neurons, during network activity underlying movements, show the similar irregular firing patterns as seen in the cerebral cortex.

New approach

Our findings contradict conventional wisdom about spinal cord functions, says Rune W. Berg from Department of Neuroscience and Pharmacology at the Faculty of Health Sciences. Until now, the general belief was that the spinal networks functioned mechanically and completely without random impulses. The new discovery enables researchers to use the theory on cortical networks to explore how spinal cords generate movements.

Still puzzled by movement

How humans are able to move at all remains a puzzle. Our muscles are controlled by thousands of nerve cells in the spinal cord. This entire, complex system must work as a whole in order to successfully create a single motion. The new research shows that even if we repeat a certain motion with high accuracy, the involved nerve cells never repeat their activity patterns. This particular observation reflects the organisation of the nerve cells of the cerebral cortex.

Source: University of Copenhagen

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Based on the paper:

Balanced Inhibition and Excitation Drive Spike Activity in Spinal Half-Centers

Berg et al.
Science 19 January 2007: 390-393
DOI: 10.1126/science.1134960

Many limb movements are composed of alternating flexions and extensions. However, the underlying spinal network mechanisms remain poorly defined. Here, we show that the intensity of synaptic excitation and inhibition in limb motoneurons varies in phase rather than out of phase during rhythmic scratchlike network activity in the turtle. Inhibition and excitation peak with the total neuron conductance during the depolarizing waves of scratch episodes. Furthermore, spike activity is driven by depolarizing synaptic transients rather than pacemaker properties. These findings show that balanced excitation and inhibition and irregular firing are fundamental motifs in certain spinal network functions.

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Other papers of interest:

The evolution of the spinal cord in primates: evidence from the foramen magnum and the vertebral canal
MacLarnon A
Journal of Human Evolution, Volume 30, Number 2, 1996, pp. 121-138(18)

Based on evidence from the foramina magna of two Eocene adapid species, Adapis parisiensis and Leptadapis magnus, it has previously been suggested that the spinal cord increased in size during primate evolution (Martin, 1980). However, it is shown here that foramen magnum size is not related simply to spinal cord size. The adapid group as a whole had a wide range of relative foramen magnum size, which is very difficult to explain. Unlike the foramen magnum, vertebral canal dimensions provide quite good indication of spinal cord dimensions.

The canal dimensions of two other adapid species, Notharctus tenebrosus and Smilodectes gracilis , are compared with the vertebral canal and spinal cord dimensions of modern primates. The cervical and thoracic spinal cords of these notharctine species were relatively smaller than those of any modern primate, and a spinal cord of modern relative dimensions could not have fitted through the fossil thoracic vertebral canal. On the other hand, the lumbar spinal cord of the notharctines was within the relative size range of modern species. The increase in the size of the cervical and thoracic regions of the spinal cord that has occurred during primate evolution is probably related to locomotor control of both the forelimb and the hindlimb. Forelimb innervation apparently increased as well as the flow of neural information to and from the hindlimb and the brain.

Size increase in the corticospinal tract and dorsal columns may have been important, affecting the agility and coordination of movement, including fine digital movement. Evidence from the vertebral canal therefore supports the suggestion from previous work that spinal cord size has increased during primate evolution in association with increasing sophistication of locomotor control.

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The scaling of gross dimensions of the spinal cord in primates and other species
MacLarnon A.
Journal of Human Evolution, Volume 30, Number 1, 1996, pp 71-87(17)

Previous work on quantitative aspects of the evolution of the spinal cord suggests either that the cord has undergone considerable evolutionary changes, or that it is invariable and somatic. Variation in gross dimensions of the spinal cord is investigated here, mostly in mammals, including new data on 12 primate species, plus some bird and amphibian species.

Allometric analyses demonstrate that spinal cord size varies little relative to body size in mammals, including primates. Some gross locomotor differences, but not others, are associated with small differences in relative cord dimensions. Birds, on average, have slightly smaller spinal cords than mammals relative to body size; amphibians may have much smaller spinal cords. On the basis of present evidence, spinal cord weight in mammals scales to body weight with an exponent of 0?69. This is significantly different from the scaling exponent of 0?75 for brain weight to body weight. Therefore, metabolic explanations developed to explain the scaling of brain size are not applicable to the spinal cord.

However, the scaling exponent calculated for spinal cord weight is compatible with its scaling to body weight as a surface area to a volume, which may be important for functional interpretation of the scaling of somatic innervation. Simple brain:cord ratios should not be used as measures of relative brain size or intelligence, as the two parts of the central nervous system scale to body size with different exponents, and, at least between classes, because relative spinal cord size also varies. Analysing brain weight relative to spinal cord weight allometrically gives a functionally meaningful assessment of brain size relative to the level of neural communication between the brain and body.

Mammals have both large relative brains and cords compared with other classes, which may be required for the sophisticated control of their mode of locomotion. Some species, including the haplorhine primates, have expanded their brain sizes beyond this common level for increased cognitive capacity.

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Proceedings of the National Academy of Sciences U S A. 1983 October; 80(19): 5936-5940.
Ontophyletics of the nervous system: development of the corpus callosum and evolution of axon tracts.
M J Katz, R J Lasek, and J Silver

The evolution of nervous systems has included significant changes in the axon tracts of the central nervous system. These evolutionary changes required changes in axonal growth in embryos. During development, many axons reach their targets by following guidance cues that are organized as pathways in the embryonic substrate, and the overall pattern of the major axon tracts in the adult can be traced back to the fundamental pattern of such substrate pathways. Embryological and comparative anatomical studies suggest that most axon tracts, such as the anterior commissure, have evolved by the modified use of preexisting substrate pathways.

On the other hand, recent developmental studies suggest that a few entirely new substrate pathways have arisen during evolution; these apparently provided opportunities for the formation of completely new axon tracts. The corpus callosum, which is found only in placental mammals, may be such a truly new axon tract. We propose that the evolution of the corpus callosum is founded on the emergence of a new preaxonal substrate pathway, the "glial sling," which bridges the two halves of the embryonic forebrain only in placental mammals.

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Recent posts include:

"How many genes does it take to learn? Lessons from sea slugs"

"Complexity Constrains Evolution of Human Brain Genes"

"Neuron Cell Stickiness May Hold Key To Evolution Of The Human Brain"

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