Tools for the Toolbox: _. . ._. …_ . …

In honor of the fact that I’m rotating through Neurology at the moment, I’m proud to present…

NERVES!

(In case you don’t read Morse Code and were too lazy to google it, that’s what the title says.)

So, nerves are super-cool.

Here’s a picture of one, so you can join me in basking in the coolness of nerves.

Are you basking?

What’d I tell you? Coolness personified.

Well, maybe not personified….but if you squint closely, you may find yourself glancing at your neighbor and having a shift in perception, suddenly seeing his shape described only by a network of white, filamentous strands, as if he were a sculpture of spiderwebs….

Only me?

Darn.

Well, that’s okay. I also subconsciously judge people based on how easily I could start an IV in the veins on their hands.

(The study of medicine engenders its own special flavors of crazy.)

Anyhoo. NERVES!

A nerve cell is called a neuron. That’s the cell in whose coolness we were basking, up there a minute ago.

The job of a neuron is to pass information along, in the form of an electrical signal. (The way this happens is super-cool, and warrants its own post, so stay tuned. For right now, we’ll just talk about how they’re arranged.)

There are three parts to any neuron.

The dendrites (from “dendron”, which means “tree”) are the tree-branch-like projections off of the central cell body, also known as the soma or perikaryon. The dendrites collect impulses from surrounding neurons and send them to the perikaryon. There are usually a lot of dendrites on a neuron, making it easier to collect a LOT of information.

(Dendrites are probably the coolest things in the human body, because they like to rearrange themselves like crazy, making a dynamic structure that allows us to learn and remember things, and adapt to changes and all kinds of other important things like that.)

So, a neuron receives signals through its dendrites. But what if it wants to say something to the other nerves in the area? That’s where the third part of a neuron comes in, the axon.

Axons are incredibly cool structures, too! There’s a conical process on one end of a perikaryon called the axon hillock, and that’s the staging platform that shoots off the axon, a long, cylindrical filament that keeps its diameter for practically its entire length (whereas the dendrites tend to taper off). Axons are responsible for carrying information away from the perikaryon, toward whatever it’s supposed to connect with and talk to. Wherever it is.

(Think about how far away the tip of your toe is away from the end of your spinal cord [it’s a little above the level of your iliac crests, if you remember from this post. ] A nerve axon has to run that entire distance to carry its signals! They’re LONG, man!)

Okay, a little more terminology, then I’m calling this post a wrap and going to bed. And then we’ll get to the REALLY incredibly cool stuff in the next couple of posts.

The connection where nerves can talk to each other (or to muscles or gland cells or whatever) is called a synapse. Axons can form synapses with cell bodies, dendrites, or even other axons. For clarity purposes when describing nerve connections, a neuron sending a signal toward the synapse is called “presynaptic”, and a neuron that receives the signal from the synapse is called “postsynaptic”.

Makes sense, right?

Synapses are oases of awesome in a desert where the sand is made of unabashed coolness. They might get their own post. Or a couple. We’ll see.

Anyhoo. NERVES!

Axons are covered with a nifty stretch of jointed insulation, called a myelin sheath. More about that, too. Later.

I know you can’t wait. I hardly can either.

But I’m pushing bedtime as it is.

Man, I’m a geezer already.

But that’s okay. At least you can read this at three in the morning, if you want.

Because your nerves make it possible.

NERVES!!!

Pictures:

Resources:

Junqueira, Luis Carlos; Carneiro, Jose. Basic Histology: text and atlas. 11th edition. McGraw-Hill, 2005.

The contents of this site, such as text, graphics, images, and other material contained on the Site (“Content”) are for informational purposes only. The Content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this Site!

If you think you may have a medical emergency, call your doctor or 911 immediately. This blog does not recommend or endorse any specific tests, physicians, products, procedures, opinions, or other information that may be mentioned on the Site. Reliance on any information provided by this blog, or other visitors to the Site is solely at your own risk.

The Site may contain health- or medical-related materials that are sexually explicit. If you find these materials offensive, you may not want to use our Site. The Site and the Content are provided on an “as is” basis.

If you use this as if it were real medical information, I’ll bask in the cool awesomeness all by myself. Didja hear me? ALL BY MYSELF! (So there.)

Sympathy. And…parasympathy?

Ever wonder how our body knows how to do things and when to do them? There’s an incredibly complex signaling system in our bodies, working ALL THE TIME, without us having to do anything about it. Isn’t that cool?

Well…

Before you answer, I should inform you. It’s cool. It’s really cool.

If you say so.

I do.

Today I want to talk about the autonomic nervous system. “Autonomic” can be broken down into the roots auto = self, and nomos = arrangement or law. So, it’s the branch of the nervous system that is responsible for self-regulation. And when we’re talking about self-regulation, we’re talking about regulation of all kinds of bodily functions: from how quickly you breathe, to how much you salivate, to how big the pupils of your eyes are.

The autonomic nervous system is divided into two opposing forces: the sympathetic influence, and the parasympathetic influence. Both forces are acting upon every bodily system at all times. It’s like playing tug-of-war between two evenly-matched sides. There’s a balance point between the two opposing forces, and a dynamic system that keeps the balance point in place.

You can see an example of this kind of dynamic equilibrium in the first 25 seconds of this video:

So, what do the sympathetic and parasympathetic systems actually do?

I’ll break it down for you.

The sympathetic system is the “fight or flight” system. I like to remember it as, “everything you want to be doing while you’re running away from a lion.”

(well….hopefully more successfully…..)

So, what would be helpful when you’re running away from a lion?

You want your muscles to work really, really well. So you’re going to make sure they get a really good oxygen supply. You’ll dilate your muscular blood vessels, and your heart will beat faster and stronger to make sure enough oxygen is getting to your muscle cells. You’ll also breathe harder and deeper, making sure you have a lot of oxygen in your blood for maximum delivery.

What else? Well, you’ll want to see really well while you’re running away. So your pupils will dilate to a bigger size to let more light in.

And you’ll also want to inhibit a couple of parasympathetic functions, which… well, you’ll see why in a minute.

The parasympathetic system is responsible for the “rest and digest” functions.

Basically, it’s “everything that your body needs to do…unless you’re running away from a lion!”

The parasympathetic system lowers your heart rate and blood pressure. It slows down your breathing. It constricts your pupils and narrows your blood vessels to direct blood flow from your muscles to other important organ systems. It lets your eyes make tears.

The parasympathetic system increases salivation, which helps in the digestion process. It also increases the movement of the digestive tract, allowing you to digest food and excrete waste.

(Helpful tip: Avoid excreting waste while running away from a lion.)

Usually, these systems find an equilibrium point and stick with it, dynamically pulling against each other to maintain it. And they also react to stimuli, changing the equilibrium point as necessary to adapt to daily needs.

Let’s talk about heart rate as an example. A normal heart will contract about 60-100 times per minute. This represents the physiological balance between the sympathetic and parasympathetic influences. At rest, (such as when you’re sleeping), the influence of the parasympathetic function will increase, and your heart rate will slow. When you’re exercising, the influence of the sympathetic function will increase, and your heart rate will speed up.

Another example: your eyes aren’t completely dilated or completely constricted at any one time; they usually hang out around a particular size (that changes slightly according to age). They use the sympathetic and parasympathetic systems to adapt the diameter of the pupil to the amount of light the eye is receiving.

Then, when the particular condition that’s pulling the system in one direction (say, toward the sympathetic side of things) disappears, the other system (the parasympathetic side, in this case) will exert enough influence to bring the systems back into their favorite physiological set point.

How is that not cool?

Your operational definitions are problematic.

It’s still not cool enough? Fine. I hear sex sells.

Fun with autonomics!

The sympathetic and parasympathetic systems are both involved in sexual activity. The parasympathetic system is responsible for arousal, and the sympathetic system takes care of things such as male ejaculation. The mnemonic to remember which does which? “Point and Shoot”. See, doctors are funny!

“Funny”… “Cool”… “Fun”…. You see, this is what I’m talking about. I’m getting you a dictionary for your birthday.

::Sigh:: Another one?

The other ones clearly haven’t helped.

Okay, fine. It’s not cool and doctors aren’t funny. But I still think it’s pretty awesome. So there.

Oh, Dr. G. We really need to get you a life.

It’s going to have to wait till after residency.