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

Wednesday, March 9, 2011

…a little man who wasn't there…

The poem by William Hughes Mearns observes: "Last night I saw upon the stair/a little man who wasn't there/He wasn't there again today/Oh, how I wish he'd go away…"  While commonly attributed as a ghost story, the little man in the context of this blog is a homunculus.  The "little human" often grossly misshapen, is a common feature of functional descriptions of the brain.

We have seen over the past weeks, how each lobe of the brain often has one or two key functions:  Occipital contains vision, temporal contains memory and hearing, parietal combines senses into associations and frontal is serves executive function.  However, one of the most astounding features of the brain lies directly at the junction of the Parietal and Frontal lobes.  In keeping with the general scheme of Occipital/Temporal/Parietal as input structures, and Frontal as output structures, the Parietal side of the Central Sulcus (Figure right) is the somatosensory cortex and represents the tactile sensory inputs from the body – pretty much everything that isn't vision or hearing.  The Frontal side of the Central Sulcus contains the primary motor cortex – the region responsible for enabling the movement of limbs and joints.

The organization of this "Sensorimotor Complex" as it is sometimes called, is fascinating.  Our old friend, Dr. Wilder Penfield, created the idea of the cortical homunculus after determining that electrical stimulation in particular regions of motor cortex caused specific muscle movements, while stimulation of the corresponding somatosensory cortex was reported by the patients as a tactile sensation in the relevant part of the body.  Penfield's motor (right) and sensory (left) homunculi are diagrammed below.

The positions and proportions of the homunculi are distorted (a feature which was critical in the original use of the term) because of the density of nerve endings and sensory neurons.  Regions with very precise, fine sensation: face, lips, tongue, fingertips have large representations proportional to both the number of sensory neurons (left) and independent muscle control nerves (right).  Regions with low sensitivity (large "dermatomes," as will be discussed in a later blog) or just a few large muscles, have small representations.  It should be noted that any ear, eye, nose and tongue representations are for the muscles and surrounding tissues, and not for vision, hearing, smell or taste.  As mentioned in yesterday's blog, the regions involved in speech and language map quite closely adjacent to tongue and pharynx motor regions.

This simplified structure is one region why groups such as the John Hopkins University Consortium, under the direction of the Defense Advanced Research Agency, has been successful in developing a complete arm, wrist and hand prosthetic limb with movement controlled strictly by recording the brain signals from the appropriate region of motor cortex.  Providing sensory/tactile feedback is taking a bit longer, so we are not quite at the level of "The Six Million Dollar Man," but science is progressing, and we are learning a lot about what it takes to map the outside world onto the brain.
Continuing to move forward from the Sensorimotor Complex, we will start to combine the multiple senses with motor control to allow for tracking of touch, sight and sound and decision-making of *when* to do so, in the Frontal Lobe.  Of course I would be remiss if I didn't remind the readers that all of the signals – both motor and sensory – are relayed through the thalamus.  In fact, even the projections to other regions of the brain for association with other information, are routed through cortico-thalamic and thalamocortical projections.   Other key contributors include the cerebellum which receives and provide information relevant to coordinated, smooth muscle movement, and the basal ganglia which maintain the signals required to keep the muscles ready to move on command.  These areas will be mentioned much more extensively in the sections on diseases and disorders which are yet to come in The Lab Rats' Guide to the Brain.

Thanks for tuning in. and welcome to any new readers that may have been referred from other blogs and via contacts made at Stellarcon!   This would be a good time to remember to take care of your brain, for now, it's the only one you've got!

1 comment:

  1. Excellent, as always. And, as always, quite riveting! I'm sorry I don't post feedback as often as I should. Because I am so enjoying this -- and *learning* so much!

    Thank you!



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