Corpus Callosum : The Brain Bridge

List of images in Gray's Anatomy: IX. Neurology

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In the two preceding posts, I’ve mentioned how marsupials, monotremes, and reptiles lack a corpus callosum, so I’d like to take the time to explain what this structure is. The corpus callosum, or colossal commissure, is a part of the brain that connects the right and left cerebral hemispheres and allows for communication between the two. It is the largest white matter structure in the brain. It also happens to be absent in fish, birds, and amphibians. Some groups have a different form of cerebral connection, like marsupials with their anterior commissure. The corpus callosum aids in functions such as brachiation (swinging through tree limbs using their arms) in arboreal primates by allowing coordination of the limbs.

Agenesis of the corpus callosum is a rare congenital disorder in which the corpus callosum is partially or completely absent. Common symptoms include:

  • vision impairments
  • low muscle tone (hypotonia)
  • poor motor coordination
  • delays in motor milestones such as sitting and walking
  • low perception of pain
  • delayed toilet training
  • chewing and swallowing difficulties
  • cognitive disabilities
  • social difficulties (possibly due to impaired facial processing)
There is currently no treatment, but therapy may be beneficial. The corpus callosum is not capable of regeneration.

If you’d like to see a pretty neat episode of House MD that involves “split brain“, check out Season 5 Episode 24, Both Sides Now.

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Monotremes – The Egg-laying Mammals

A Short-beaked Echidna on the move

Monotremes are egg-laying mammals indigenous to Australia and New Guinea. The name refers to the “single opening” they have (the cloaca) through which excretory and reproductive functions occur. There are two families, Ornithorhynchidae (“bird nose” ; platypus) and Tachyglossidae (“rapid tongue” ; 4 species of echidna).  Though they lay eggs, they do produce milk for their young. No defined teats means the milk simply oozes from their mammary glands. They have low reproduction rates (eggs are retained in the mother’s pouch for some time) and lengthy parental care. Lower metabolic rate means monotremes have relatively lower body temperature than placental mammals. As with reptiles and marsupials, they lack a corpus callosum (to be discussed in a later post). They have no teeth in adulthood, but need an egg tooth to hatch. Baby echidnas are commonly referred to as “puggles”.

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