Anatomy and Physiology series: The Peripheral Nervous System

Hey, hey, hey...wait a minute here!  Where's the post on the physiology of the articulation system?  Did I seriously just post several posts full of boring old artic. muscles and skip the physiology?!  How dare I?!  If any of my regular readers (if I have regular readers) are currently feeling that way, have no fear!  I'm gonna get to it.  I'm taking a detour and going over the nervous system, peripheral and central, first because the physiology of the articulation system is actually still a bit of a mystery.  What we do know is that how the articulatory system all works together has a lot more to do with the nerves that control speech than it does the muscles themselves, so to understand the mystery, we've got to understand the nervous system...at least to some extent.  So welcome to the post on the peripheral nervous system.

The peripheral nervous system consists of all the nerves and nervous system connections that lie outside of your brain and spinal cord.  (The brain and spinal cord make up the central nervous system, which will be the topic of my next A&P post.)

I've decided to not go into the super small details of how neurons talk to one another...their axons, dendrites, terminal buttons and neurotransmitters...but if any of you happen to feel jipped and would really like me to post on that, let me know.  (Geeks are always willing to geek out even more if given the opportunity. :p)  But I do want to talk about some terms of the PNS and its basic divisions:  The somatic system, autonomic system and also the sympathetic and parasympathetic systems, which are both part of the autonomic system.

The nerves in peripheral nervous system are divided into two basic groups by their function:  afferent and efferent.  Afferent nerves carry their signal up the body back to the brain, and efferent nerves carry their signal out of the brain and down to the rest of the body.  Afferent nerves are considered sensory nerves, because all sensory information is sent to the brain for processing (think of them like the "paper work" of the body being sent of to the central office for processing and filing).  Efferent nerves are classically classified as motor nerves, because motor impulses are what go out of your brain to your muscles so you can complete the tasks your brain says  you should do.  (And I'm going to rip off the little mnemonic my professor taught us:  "A" stands for arriving, so "afferent" nerves are arriving at the brain, "E" stands for "exiting", so efferent nerves are exiting the brain.  And I'm going to let that go unaccredited because I don't know if she would want me including her name here, and also because I'm pretty sure she wasn't the only one to think of that one.)  Now, I'm going to stick to the traditional classification of afferent being sensory and efferent being motor, but you should know that in reality, there are mixed nerves that do both jobs, but they are usually classified by how the nerve function can be assessed (motor or sensory).

The somatic nervous system is the easiest one to talk about.  It's associated with the motor control of your voluntary skeletal muscles.  So that means that for every time you want to raise your arm or walk around, your somatic nervous system is handling those motor tasks.  What the somatic system is not involved in is any sensory information, such as someone touching your arm or leg, or any reflexes that occur, such as when you touch a hot surface.  Any muscle movement under your conscious control involves the somatic system.  So does that mean that the autonomic has to do with involuntary?  Well...not completely.  In fact, the autonomic system can still play a part in the function of skeletal muscle, but only when it's not involved in conscious control.

So how does the autonomic system work?  Well, this system does control involuntary muscles, like your heart and digestive muscles, but it does also impact the background tonicity of the skeletal muscles involved in things like keeping you erect when sitting up.  For example:  When you're maintaining a certain posture, you don't maintain it through conscious thought, but nerve impulses are still sent via the autonomic system to maintain partial-contraction of the proper muscles to keep you upright.  Any voice teacher or voice student has probably experienced this when a singer starts out a vocalise with great posture when instructed to think about it, but the posture slowly returns to that person's typical "steady state" posture as soon as they stop focusing on it.  (This is probably why my teacher just stopped working on posture with me a few months after I started a regular yoga workout.  My background tonicity was strong enough to maintain a healthy posture without my thinking about it...I guess.)  Breathing is usually a part of the autonomic system, but it can be put under conscious control, (and all singers out there say "duh") which would involve the somatic system.  But breathing rate is usually autonomic, and is determined in part by either the sympathetic or parasympathetic systems.

A lot of folks have heard about the sympathetic and parasympathetic systems, and I've met a few folks who seem to think of one as "bad" and the other as "good."  I suspect this is mostly related to reports about the effects of chronic stress on our systems, but in general, these two nervous systems compliment one another.  And when you get down to it, these systems are not as cut-and-dry as we usually talk about them...but I'll talk about them that way anyway just cause it's easier that way.

The sympathetic nervous system is in charge of our "fight or flight" response.  When activated, like say...when you're nervous before that big audition, this system raises your heart and breathing rate, makes you sweat, and raises your blood pressure.  The only thing it doesn't "turn on" is your digestion;  digestion is actually turned off by this system, which is why eating before a performance is a bad idea...for me, at least.  This system also turns on the more primal parts of your brain that assess a threat, (which I believe might be why at an audition, you remember every detail of every judge...they are the "threats" your brain detects, so they're the ones you notice the most.)  (This one also causes a bit of our endocrine (hormonal) system to kick in, which is why calming down after kicking it in can take a while:  Those hormones can still be in the blood stream for a while.)

The parasympathetic nervous system is in charge of our "rest and repose."  This system "turns off" everything that the sympathetic nervous system turns on:  it slows our breathing and heart rate, lowers our blood pressure, and turns on our digestive system.  When you're just hanging out at home, this system is maintaining your resting state.  Deep breathing and other relaxation techniques can help to kick in the parasympathetic system during a nervous state, but they might not work totally if you're blood is flooded with adrenaline and such...hence why it might take you a few phrases into your performance before you calm down a bit.  (But practicing relaxation before a performance is always a fruitful and useful thing to do, in my opinion.)

Alrighty, so moving on to the central nervous system next.  If you have any questions, please feel free to post them, and I'll do my best to answer.

*Seikel, J. A., King, D. W., & Drumright, D. G. (2010). Anatomy and physiology for speech, language, and hearing. Clifton Park, NY: Delmar.