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(Controlling the body, part 1)
[This is the May 12 post delayed by Blogspot maintenance issues – sorry for the delay]
Many of the entries in The Lab Rats Guide to the Brain deal with structure of the brain, and contain only limited information about interaction with the rest of the body. Sure, there's mention of motor cortex and sensory inputs, but nothing really about how the body is *controlled.* In fact, there is much more information about the inputs to the brain (senses) than the outputs.
So this post and the one to follow are about specifically about how the brain outputs commands and instructions to the body. Today's blog is about direct neural control – that is, neurons that project down to the spinal cord, out to various muscle and organs, and cause an action. Sunday's blog (May 15) will be about indirect control via hormones and neurotransmitter modulators.
The connections of brain to muscle should be reasonably obvious by now. First we have the "Motor Homunculus" (right) which shows how the motor cortex is organized with respect to outputs to the major muscle groups. In addition to the obvious muscle connections (which we will get back to in a minute), there are connections to the arteries, to regulate the diameter (and hence pressure) of the blood vessels. There are connections to the skin, to change the thickness, control sweat, control temperature and cause "piloerection" (goosebumps) which is a remnant of a reaction to raise hairs and increase the insulating properties of the fur which humans don't have! Less obvious are the connections to the organs to speed up or slow down activity, cause release of chemicals (hormones) and prepare the body for different conditions.
To list off the types of outputs from brain: we have (1) visceral control, (2) neuroregulatory control, (3) hormonal release control, (4) involuntary motor control, and (5) voluntary motor control. By visceral control (1), I mean direct control of the primary organs - for example, the heart will beat even without direct input from the brain- however, that input will cause the heart to slow down or speed up its rate. Likewise, the muscles that line blood vessels and the stomach & intestines (what we call "smooth muscle") have local circuits that control them, but inputs from the brain will change the speed and amount of contraction of the muscle, thus changing pressures and rate of flow (of blood or intestinal contents). Neuroregulatory control (2) is the control of the neural signals to and from the body. We all know of the ability to control or ignore certain sensory signals: pain, hunger, full bladder, etc. This ability is actually due to neural commands from the brain to the nuclei that relay signals to the brain. A certain amount of control of inputs (just like the control of eardrum or iris to limit visual or auditory range) is built into the sensory system to allow the brain to override signals when necessary. Hormonal release control (4) relates to the various *glands* throughout the brain and body, most notably the pineal, pituitary, adrenals, sweat, salivary and genitalia. These glands all release (or change) their chemical contents when stimulated by neural commands. Involuntary motor control comprises all of the reflexes and automatic functions controlled by the brain that a person does not have to think about. The heart beats, the diaphragm and ribcage create the breathing cycle, the stomach and intestines squeeze and relax to promote digestion, and muscles in the pelvic region tighten to limit the flow of fluids and solids when not needed. Virtually all of the neurons that control the above four functions originate in the mesencephalon (e.g. medulla) or lower (spinal cord or ganglia near the appropriate organs).
Finally, voluntary motor control (5) consists of all of the muscles that we can move just by thinking about them: Arms, legs, tongue, fingers, with additional voluntary overrides of involuntary functions (eyeblink, deep breath, bladder retention) etc. These functions (and neurons) are the ones that arise from the motor cortex.
To further understand how all of this works together, there is one further key piece of information: Muscles can only pull. They cannot push.
So how does limb movement work? Clearly there is some sort of a push-pull system going on?
Well, there is an oppositional system, but it is more appropriately a "pull-pull" system. Consider Da Vinci's famous "Vitruvian Man" drawing (left). This illustrates the best coordinate system for talking about anatomical positioning. In the drawing, all limbs are extended, the joints are straight, and the opposing muscle groups are at equal lengths. To bend an arm from this position, the elbow must be bent, changing the angle between upper and lower arms to something less than 180 degrees. This is accomplished by muscles contracting on the *inside* of the arm (inside the elbow joint). To re-straighten the arm, muscles on the outside of the arm contract, pulling the elbow and bones back into alignment.
Even where the brain has *delegated* certain aspects of control to local neural circuits, there is still a provision for descending control directly from brain. Sexual arousal is a perfect example, but so is anxiety, the "fight or flight reflex" and meditation. As we will see with tomorrows concluding blog, ultimately, the brain is in control of all processes of the body!
So stay tuned for the next entry in The Lab Rats' Guide to the Brain!