No matter how behind you are on your to-do list, you should still be proud: You're getting a lot more accomplished right now than you're probably giving yourself credit for. In fact, you're directing, supervising and improvising many complicated functions, and you're making split-second decisions about all of them simultaneously. On top of that, you're not giving any of it one second's conscious thought.
The nervous system is a giant network of signal-transmitting neurons and the connections between them. Most of your neurons -- 100 billion or so -- are in your brain (the runner-up, your spine, has 1 billion). There's not just one type of neuron, but thousands, each with a special shape, function or purpose.
The nervous system has two parts, which both ultimately serve to distribute signals among neurons throughout the body:
- The central nervous system consists of the brain and spinal cord and acts as the primary interstate along which messages go to and from the brain.
- The peripheral nervous system serves as the highways, streets and alleyways of this transportation system. It connects the central nervous system network to the rest of the body. The peripheral nervous system has two parts: somatic (which sends messages back to the central nervous system) and autonomic (which spurs or calms the actions of your organs).
We're going to look at 10 things your nervous system handles automatically, but first, you'll have to make a conscious effort to check out the next page.
Nobody thanks the nervous system after sweating straight through a work outfit during a summertime lunch break. However, if it weren't for your body's ability to take action when the heat turns up, each one of those summertime lunch breaks would end with fatal heat stroke.
Without any help from you, your body carefully maintains the water content of all your cells, as well as the temperature of your body. When it needs to release some heat, it pushes sweat out of approximately 2.6 million sweat glands. You're always sweating, though sweat -- a mixture of water, chloride, sodium and potassium -- is often absorbed back into the sweat glands before it reaches the surface of the skin.
It's not necessarily the act of sweating, but rather the water evaporating from the surface of your skin, that removes excess heat.
But what if there is no excess heat? Nervous sweat is your body readying itself for an increase in temperature once you choose between the fight or flight reactions. As you sweat it out taking the mic in that karaoke bar, for example, both you and your nervous system are boldly announcing, "I Will Survive."
Allow your eyes to focus before reading the next section.
9: Pupil Dilation
If you've ever looked directly at a light bulb before dimming the lights, you've undoubtedly experienced a brief period of visual disorientation while your eyes adjusted to the darker environment. After a minute or so, you're able to see with much less light than when the lights immediately went off.
We take a lot of things our nervous system does for granted, and one of them is our ability to see.
In order to see, light must first enter the pupil (the black center of your eye), which is about 3 to 5 millimeters in diameter.
The amount of light that enters the pupil makes a big difference to our quality of vision. Letting in too much light on a sunny day will temporarily blind us; too little in dark conditions, and we won't see a thing.
Our nervous system is constantly monitoring the amount of light entering our pupils. It also monitors how effective that amount of light is when it comes to making heads or tails of the image. When it's dark, our pupils dilate to allow in as much light as possible, improving our sight; when it's bright out, our pupils shrink, limiting the amount of light allowed to enter.
If you want to see your autonomic nervous system at work, face a mirror, then cover and uncover your eyes to see the changes in your pupils. While your pupils adjust, your body's also doing something much more important. To find out what, see the next page.
8: Breathing and Lung Function
If you think it's hard remembering to remove the cup of coffee from the top of your car before you get in and drive away, be glad you're not responsible for remembering to take a breath every couple of seconds for the rest of your life.
Not that your nervous system thinks you're completely irresponsible -- breathing is one task handled by your nervous system that you can, to a degree, override for periods of time. However, the boredom involved in doing so will quickly lead you to pass this duty back off to your nervous system.
You can try to suppress your breathing, with limited success. Your nervous system has posted lookouts to monitor levels of carbon dioxide in the blood. If these specialized cells, known as peripheral chemoreceptors, don't like what they see, they'll fire a report off to the brain stem. Receiving this, the brain stem (specifically, the medulla oblongata) signals your diaphragm and other related muscles to contract. Satisfied that carbon dioxide levels have decreased, the peripheral chemoreceptors will take an oxygen reading and let the medulla know how quickly it needs the next diaphragm contraction.
Another set of lookouts is posted in your lungs. If these stretch receptors think the lungs are overexpanding, they'll report to the medulla to get the OK to bail out on the current breath. Honestly, sometimes all the back-and-forth messages turn into a garbled game of "telephone." These mix-ups in signaling can cause a benign form of breathing disarray: hiccups.
7: Adrenaline and Stress (Fight-or-flight Response)
While your autonomic nervous system gets a lot of things right, occasionally it overshoots the mark. Such can be the case with its go-to response in a time of stress: the fight-or-flight response. It enables us to perform superhuman feats, but it also frequently perceives things like public speaking as existential threats.
When we experience stress or alarm, we feel a heightened sense of anxiety that may include trembling, dry mouth, sweating or even distortions in our vision. While we associate this feeling with being scared or vulnerable, it's actually what it feels like to become something akin to a superhero -- you're fully ready for drastic action.
At the core of this state are the adrenal glands -- two tiny glands that rest just above the kidneys. Certain neurons located within these glands work as a part of the sympathetic nervous system. When faced with emergency, the nervous system prompts the adrenal glands to release adrenaline into the bloodstream. This hormone causes many changes to take place quickly:
- Heartbeat quickens and strengthens, which increases blood pressure
- Blood sugar increases
- Resources are reallocated, enabling blood to clot faster
- Pupils dilate, allowing you to take in the full scope of a threat
- Bronchi dilate, preparing you to maximize oxygen intake/turnover
When the threat (or public speech) passes, your parasympathetic nervous system tells the adrenal glands to stop releasing adrenaline and sends out another chemical to counteract adrenaline's effects.
Now, belly up to the next section and dig in.
In order to use it, food must be digested -- broken down into smaller components that can then be used for the body's needs. It's a surprisingly complex process, especially considering that all we consciously do is a little chewing and swallowing. The rest just seems to take care of itself.
Nerves originating in the brain and spinal cord called extrinsic nerves deliver messages to the digestive system on behalf of the nervous system. These nerves can call for acetylcholine or adrenaline:
- Acetylcholine ramps up your digestive tract's squeezing, which moves food along. This chemical also prompts the stomach and pancreas to produce more digestive juices.
- Adrenaline calms everything down, relaxes digestive muscles and ends production of digestive fluids. This seems counterintuitive, but remember that adrenaline gets you ready for fight-or-flight, and the ability to stop digestion aids in that.
Intrinsic nerves located directly in the tissue of the esophagus, stomach, small intestine and colon react to stretching and release chemicals that regulate the speed of digestion and the release of digestive juices. These intrinsic nerves form a nervous system all their own, separate from the greater nervous system and containing a similar number of neurons as the spinal cord.
The next section may (not) make your mouth water.
It would make for an awkward first date if, before every bite of food, both you and your date had to pause to concentrate on cuing salivary glands to activate. Of course, this ability would come in handy later on when both your mouths go bone-dry as the anticipation of the first kiss builds.
Salivation is important to digestion because it helps lubricate your mouth and esophagus for swallowing. It also initiates the breakdown of food once it's in your mouth. Saliva production occurs in your salivary glands. You have three sets of salivary glands, each producing a different mix of saliva -- some more watery, some with more mucus. Your autonomic nervous system controls the amount of saliva produced, and which type.
If you're anxious or scared, you may experience an extremely dry mouth. Instead of assisting you in crying for help, your autonomic nervous system diverts fluid from wherever it can -- including from your mouth -- to reallocate to more pressing needs during fight-or-flight.
The nervous system anticipates the need for saliva, first demonstrated in Pavlov's famous experiments with dogs. And when you're anticipating an anxiety-provoking situation, your saliva (what amount there is, at least) contains higher amounts of the stress hormone cortisol as other nervous system directives are being put into action [source: Martel et al].
Next: What's all that feverish dancing outside the locked bathroom door about?
As the nervous system instructs the digestive system to break down food, a waste product called urea is left behind in the blood. All blood is constantly circulated through the kidneys, which separates out urea and other waste products. This waste, along with water, is diverted to your bladder.
You produce up to 1.5 quarts (1.4 liters) of urine a day [source: National Kidney and Urologic Diseases Information Clearinghouse]. Your bladder, at maximum capacity, can hold less than a third of that total amount (around 13.5 fluid ounces or 400 milliliters), though commonly less, depending on your physical size.
Your nervous system monitors just how full your bladder is throughout the day. Nerves connected to your spinal cord reach the detrusor muscle in the bladder's wall, as well as the urethral sphincter muscle. In order for urination to occur, the detrusor muscle must be contracted, and the urethral sphincter muscle must be relaxed. The "potty dance" is your attempt to keep this from occurring.
As it fills, the walls of the bladder stretch, and this stretching is dutifully reported to the medulla. Fortunately, for decorum's sake, your nervous system temporarily restricts spinal-cord reflexes that would cause immediate, involuntary urination. Instead, it makes you consciously aware of the need to urinate. As with breathing, you can override the nervous system and suppress this function entirely -- just not for too terribly long. As time marches on and demands increase for more space in your bladder, your central nervous system will ease the restriction on these reflexes.
Next, we'll learn what's responsible for most of the trouble you'll encounter in life.
3: Sexual Arousal and Orgasm
The parts of your brain responsible for emotions, sensory input and thought play a large role in sexual arousal. Once triggered, they'll (normally) send a message to the hypothalamus, which then activates the autonomic nervous system. The sympathetic and parasympathetic nervous systems then send forth a host of signals to your body, increasing heart rate, contracting muscles and increasing blood flow to the penis or clitoris. Increased blood flow to the vagina increases pressure on the vaginal walls, leading to increased lubrication. At all times, the nervous system regulates the temperature of the scrotum by tightening or loosening the scrotum in relation to the temperature of the body and the external environment.
The sympathetic nervous system controls climax by increasing respiration, blood flow, perspiration and muscle contractions. After orgasm, the parasympathetic nervous system restores the body to a normal (albeit more relaxed) state.
Interestingly, other things stimulate the sympathetic nervous system, including exercise. One study showed that women became more aroused by watching an erotic film when the viewing occurred after a period of exercise [source: Meston]. Anxiety also increases genital arousal in women, but decreases subjective arousal, meaning that while the body is more prepared for a sexual encounter, the mind has less interest than usual.
While you may promise a lover your heart, your nervous system may have something to say about that, as we'll learn next.
2: Heartbeat, Heart Rate and Blood Pressure
If you measured your resting heart rate, then stood and walked across the room, your heart rate would increase. But why?
Your nervous system controls the beating of your heart -- about 100,000 beats a day -- as well as the rate at which it beats and the force with which blood is sent coursing through every artery and vein in your body. With a tip of the hat to the nervous system, it's a full-time job.
And its efforts are put to good use -- with each beat of the heart, oxygenated blood is delivered throughout your body. When you're resting, your cells need less oxygen, and your nervous system knows to conserve those precious heartbeats. It slows down the rate of beating and the force with which it beats.
Your nervous system closely monitors how much oxygen the cells are getting and makes split-second adjustments to ensure they get enough. While you can alter your heart rate by altering your activity level, your nervous system is actually doing the adjustment. You're just providing a prompt (or a demand) for more or less oxygen.
Your nervous system also accepts situational cues. Accidentally kick a hornets' nest while out hiking and your heart rate will skyrocket because your nervous system knows you'll soon be taking flight.
Earlier, we talked about pupil dilation. Next, we'll learn about other ways your nervous system handles everything in the world that's coming at you.
1: Sensory Adaptation
Your five senses -- touch, sight, hearing, taste and smell -- deliver a lot of sensory data to the brain for processing. And your nervous system is there to allocate cerebral resources to sensory information that matters most.
Specialized cells called mechanoreceptor cells play a large role in this. If you place and leave your finger on a piece of clothing you're wearing right now, it won't take long before you lose all sense of how it actually feels. Since it's not causing pain, your nervous system stops sending or reading signals about it. Your nervous system has calmed down since the initial sensory input, and reports, "All's well -- nothing much here to feel." However, move your finger again across the garment, the new sensory input will be acknowledged, and you'll know what that garment feels like again.
Your nervous system doesn't place much value in meaningless, repetitive noises, such as the buzz of a fluorescent light bulb or your laptop computer. Should a new noise crop up, it'll come to your attention immediately because it represents a change from the aural status quo. And if you stop by your friend's home in Paper Mill City for a short visit, your nervous system will allow you to perceive the mill's full nasal assault while your friend insists he or she can't smell a thing. They have the merciful nervous system to thank for that.
For lots more information on the nervous system, check out the next page.
Lots More Information
- 10 Amazing Advances in Neuroscience
- Nervous System Pictures
- Cognitive Neuroscience Puzzles
- Body Works: Brain Quiz
- Fact or Fiction: Brain Myths Quiz
- American Heart Association. "Autonomic Nervous System." 2010. (Dec. 1, 2010) http://www.americanheart.org/presenter.jhtml?identifier=4463
- Bowen, R.A., DVM, PhD. "Pathophysiology of the Digestive System." July 5, 2006. (Dec. 1, 2010) http://www.vivo.colostate.edu/hbooks/pathphys/digestion/index.html
- Candler, Rob. "Human Sensors." (Dec. 1, 2010)http://www.stanford.edu/class/me220/data/lectures/lect01/outline01.html
- Carter, J. Stein. "Nervous System and Senses." University of Cincinnati Clermont College. Nov. 2, 2004. (Dec. 1, 2010) http://biology.clc.uc.edu/courses/bio105/nervous.htm
- Evans-Martin, F. Fay. "The Human Body and How it Works: The Nervous System." Infobase Publishing. 2010.
- Farabee, Michael J. Ph.D. "The Nervous System." Online Biology Book. June 22, 2001. (Dec. 1, 2010) http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookNERV.html
- Greenberg, Jerrold S. et al. "Exploring the Dimensions of Human Sexuality." Jones and Bartlett Publishers. 2011.
- Kimball, John W. "The Adrenal Glands." Kimball's Biology Pages. Sept. 15, 2005. (Dec. 1, 2010) http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Adrenals.html
- Martel F.L. et al. "Salivary Cortisol Levels in Socially Phobic Adolescent Girls." Depression and Anxiety. October 1999. (Dec. 1, 2010)http://www.ncbi.nlm.nih.gov/pubmed/10499186?dopt=Abstract
- Meston, Cindy M., PhD. "Sympathetic Nervous System Activity and Female Sexual Arousal." The American Journal of Cardiology. July 20, 2000. (Dec. 1, 2010) http://homepage.psy.utexas.edu/homepage/group/MestonLAB/Publications/snsarousal.pdf
- National Digestive Diseases Information Clearinghouse (NDDIC). "Your Digestive System and How It Works." April 2008. (Dec. 1, 2010)http://digestive.niddk.nih.gov/ddiseases/pubs/yrdd/
- National Kidney and Urologic Diseases Information Clearinghouse (NKUDIC). "Your Urinary System and How It Works." August 2007. (Dec. 1, 2010) http://kidney.niddk.nih.gov/kudiseases/pubs/yoururinary/
- National Panic & Anxiety Disorder News. "Anxiety & Panic Attacks Symptoms." 2010. (Dec. 1, 2010) http://www.npadnews.com/anxiety-symptoms.asp
- The Merck Manuals Online Medical Library. "Autonomic Nervous System Disorders." November 2006. (Dec. 1, 2010)http://www.merckmanuals.com/home/sec06/ch098666/ch098666a.html