NOTICE: Posting schedule is irregular. I hope to get back to a regular schedule as the day-job allows.

Monday, April 4, 2011

Brainstem, brainstem...

Yes, it's time for Keith's favorite.*

In the last blog I showed the diagram at right - which purports to be the cerebellum and brainstem.

Yet, the diagram below comes from Gray's Anatomy:

I've blown it up to show the fine detail with which Gray describes the various parts of the brainstem - and how it appears to include many more structures than the simple diagram favored by most illustrators.

In the lettered, color diagram, A is the midbrain (portions of thalamus and hypothalamus); B is the pons, C is the medulla, and D is the top of the spinal cord.  Gray's diagram doesn't pay much attention to the medulla and spinal cord, but does show an important feature which is the exit points of many of the cranial nerves - the ones serving many of the primary sensory inputs and essential controls for face, throat, heart, lungs, diaphragm, etc.

The medulla is the site of many of the "automatic" functions for the body - sweat glands, heart rate, blood pressure, respiration.  Spinal cord is obviously the conduit for neuron connections between brain and body, but it serves as so much more - damage to the spinal cord alone will leave a patient paralyzed, but still alive - damage to medulla may require that the patient be on total life support - with pacemaker, respirator, and a central line to deliver nutrients.  Damage *above* the medulla will leave the body intact, heart beating, breathing, muscle reflexes intact - just with no higher brain function.

The pons is the location where many of the nerve fibers cross-over to the opposite side such that the right side of the body is connected to the left side of the brain, and vice versa.  The bulge in B actually results from the crossing fibers.  

Above the medula and pons termed "mesencephalon" (middle brain), is the beginning of "brain" proper with the "diencephalon" (inter-brain).  The diagram at right compares the  human brain with that of a shark.  The major developmental change leading to *higher* brain development is in the "telencephalon" (top brain).   The diencephalon includes the thalamus, hypothalamus, and the many structures associated with thalamus that act as relays for the sensory and motor regions of the brain.  As seen in Gray's diagram, this includes the colliculi and geniculate bodies.

The "primitive" brain of the shark includes all of the functions for controlling the body.  As long as the diencephalon and lower structures are intact, the body is fully function - what then makes the difference in the human brain?  More sensory detail, more more detail, all of the association cortices - and conscious control of all of the above.  The need for memory storage also contributes to enlargement of the telencephalon. 

So the brainstem is more than just a vestigial piece of spinal cord sticking out of the bottom of the brain.  Rather, the brainstem is the *original* brain - designed for control of reflexes, and automatic (the technical term is "autonomic") processes.  We will talk further about the "Control Center" aspects of the diencephalon when we discuss Hypothalamus in the next blog post.


Administrative note:  Blog posts for the forseeable future will return to a 3-4 per week schedule.  Unfortunately, last week's glitch threw me off of the "every even date" plan, although I will strive to return to that schedule by mid-month.


Until next time, remember the immortal words of Pinky and the Brain:

"Neocortex, frontal lobe.  Brainstem! Brainstem! Hippocampus, neural node, right hemisphere..."


1 comment:

  1. Something I've never seen adequately explained. WHY is the brain crossed up like it is? What is the benefit of having the halves of the brain connected to the opposite halves of the body? Is this true of all mammals? What about reptiles, fish, insects etc?

    Since the right half of the brain controls the left half of the body, only left handed people are in their right mind! ;)


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