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Brain mass and cranial nerve size in shrews and moles.

Leitch DB, Sarko DK, Catania KC - Sci Rep (2014)

Bottom Line: We found that moles and shrews have tiny optic nerves with fiber counts not correlated with body or brain size.Trigeminal nerve number was by far the largest and also increased with increasing brain and body size.Despite this hypertrophied cranial nerve, star-nosed mole brains were not larger than predicted from body size, suggesting that magnification of their somatosensory systems does not result in greater overall CNS size.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, Vanderbilt University, Nashville, TN, USA.

ABSTRACT
We investigated the relationship between body size, brain size, and fibers in selected cranial nerves in shrews and moles. Species include tiny masked shrews (S. cinereus) weighing only a few grams and much larger mole species weighing up to 90 grams. It also includes closely related species with very different sensory specializations - such as the star-nosed mole and the common, eastern mole. We found that moles and shrews have tiny optic nerves with fiber counts not correlated with body or brain size. Auditory nerves were similarly small but increased in fiber number with increasing brain and body size. Trigeminal nerve number was by far the largest and also increased with increasing brain and body size. The star-nosed mole was an outlier, with more than twice the number of trigeminal nerve fibers than any other species. Despite this hypertrophied cranial nerve, star-nosed mole brains were not larger than predicted from body size, suggesting that magnification of their somatosensory systems does not result in greater overall CNS size.

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Examples of microscopy of cranial nerves.(A) The optic nerve (CN II), the smallest of those examined, was imaged used transmission electron microscopy to clearly define individual myelinated axons. The example shown is a cross-section from a water shrew. (B) Example of a semi-thin cross-section stained with toluidine blue, as employed in vestibulocochlear and trigeminal nerve preparations. Example shown is a section from a hairy-tailed mole's cochlear nerve (CN VIII). (C) Example of a completed montage of a trigeminal nerve assembled from light microscopy of semi-thin sections as seen in (B). This is a cross-section of the trigeminal nerve (CN V) from the star-nosed mole.
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f4: Examples of microscopy of cranial nerves.(A) The optic nerve (CN II), the smallest of those examined, was imaged used transmission electron microscopy to clearly define individual myelinated axons. The example shown is a cross-section from a water shrew. (B) Example of a semi-thin cross-section stained with toluidine blue, as employed in vestibulocochlear and trigeminal nerve preparations. Example shown is a section from a hairy-tailed mole's cochlear nerve (CN VIII). (C) Example of a completed montage of a trigeminal nerve assembled from light microscopy of semi-thin sections as seen in (B). This is a cross-section of the trigeminal nerve (CN V) from the star-nosed mole.

Mentions: Optic nerve axons were readily distinguished in transverse sections viewed through transmission electron microscopy (TEM). Myelinated fibers were typically 1 to 2 μm in diameter (mean = 1.53; n = 52, Std dev. = 0.39) (Fig. 4A). The layers of the perineurium as well as collagenous connective tissue that ensheathed the entire nerve could be seen in complete TEM montages of each optic nerve. The total number of myelinated axons within the optic nerve showed little relationship to average body (R2 = 0.003, p = 0.914) or brain mass (R2 < 0.0001, p = 0.999) of each species. The fewest optic nerve fibers were noted in the masked shrew (mean = 1483, Std dev. = 115), and the most were found in the water shrew (mean = 6342, Std dev. = 703) (Table S2).


Brain mass and cranial nerve size in shrews and moles.

Leitch DB, Sarko DK, Catania KC - Sci Rep (2014)

Examples of microscopy of cranial nerves.(A) The optic nerve (CN II), the smallest of those examined, was imaged used transmission electron microscopy to clearly define individual myelinated axons. The example shown is a cross-section from a water shrew. (B) Example of a semi-thin cross-section stained with toluidine blue, as employed in vestibulocochlear and trigeminal nerve preparations. Example shown is a section from a hairy-tailed mole's cochlear nerve (CN VIII). (C) Example of a completed montage of a trigeminal nerve assembled from light microscopy of semi-thin sections as seen in (B). This is a cross-section of the trigeminal nerve (CN V) from the star-nosed mole.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4150104&req=5

f4: Examples of microscopy of cranial nerves.(A) The optic nerve (CN II), the smallest of those examined, was imaged used transmission electron microscopy to clearly define individual myelinated axons. The example shown is a cross-section from a water shrew. (B) Example of a semi-thin cross-section stained with toluidine blue, as employed in vestibulocochlear and trigeminal nerve preparations. Example shown is a section from a hairy-tailed mole's cochlear nerve (CN VIII). (C) Example of a completed montage of a trigeminal nerve assembled from light microscopy of semi-thin sections as seen in (B). This is a cross-section of the trigeminal nerve (CN V) from the star-nosed mole.
Mentions: Optic nerve axons were readily distinguished in transverse sections viewed through transmission electron microscopy (TEM). Myelinated fibers were typically 1 to 2 μm in diameter (mean = 1.53; n = 52, Std dev. = 0.39) (Fig. 4A). The layers of the perineurium as well as collagenous connective tissue that ensheathed the entire nerve could be seen in complete TEM montages of each optic nerve. The total number of myelinated axons within the optic nerve showed little relationship to average body (R2 = 0.003, p = 0.914) or brain mass (R2 < 0.0001, p = 0.999) of each species. The fewest optic nerve fibers were noted in the masked shrew (mean = 1483, Std dev. = 115), and the most were found in the water shrew (mean = 6342, Std dev. = 703) (Table S2).

Bottom Line: We found that moles and shrews have tiny optic nerves with fiber counts not correlated with body or brain size.Trigeminal nerve number was by far the largest and also increased with increasing brain and body size.Despite this hypertrophied cranial nerve, star-nosed mole brains were not larger than predicted from body size, suggesting that magnification of their somatosensory systems does not result in greater overall CNS size.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, Vanderbilt University, Nashville, TN, USA.

ABSTRACT
We investigated the relationship between body size, brain size, and fibers in selected cranial nerves in shrews and moles. Species include tiny masked shrews (S. cinereus) weighing only a few grams and much larger mole species weighing up to 90 grams. It also includes closely related species with very different sensory specializations - such as the star-nosed mole and the common, eastern mole. We found that moles and shrews have tiny optic nerves with fiber counts not correlated with body or brain size. Auditory nerves were similarly small but increased in fiber number with increasing brain and body size. Trigeminal nerve number was by far the largest and also increased with increasing brain and body size. The star-nosed mole was an outlier, with more than twice the number of trigeminal nerve fibers than any other species. Despite this hypertrophied cranial nerve, star-nosed mole brains were not larger than predicted from body size, suggesting that magnification of their somatosensory systems does not result in greater overall CNS size.

Show MeSH
Related in: MedlinePlus