Oh, you're the new person, right?  You ever worked in a place like this before?  A human body?  I'll tell you, this is a pretty easy job.  Most things around here run all by themselves.  They even run without the big boss upstairs, y'know, the big brain, even thinking about it.  We can just kick back while everything runs itself.  Still, it's good to know how everything operates . . . just in case anything goes wrong.  


So let's say our human body gets hurt.  Don't worry, that's not going to happen.  This body we're in does not want to get hurt, so it has a system in place to try and stop that from happening.  Let's travel through the muscles right up to the skin on the bottom of the foot.  You can imagine something sharp cutting through the skin here . . . or something hot burning it.  A stimuli is anything from the outside that can make the body react or change what it's doing.  Oh . . . Oh no.  A nail is coming through the shoe!  It's really happening!  I know I said it wouldn't!  I was wrong!  The body is being hurt!  Well, I hope you're ready, new person.  We need to follow the pain to make sure everything goes the way it needs to.


Everything around us that was so quiet before is now coming to life!  The body knows something is wrong and it needs to do something about it!  All kinds of different things can happen to a body's skin, so it has cells to feel them all.  Sensory neurons are nerve cells that are found in the body's skin to help it feel pressure, hot, cold, vibration, and pain.  They send messages to the brain and spinal cord when they feel something.  In this case, they send pain and pressure from the nail going through the skin.  These are the cells that send messages into the body to let us know what's going on outside. 

Something tells me things are getting a little hot around here.


There are different kinds of nerve cells in this body and they have special jobs.  They are all connected to each other and all help each other to do their jobs.  After the pain comes into the body, we have to decide what to do about it.  The cells that feel the pain do not make decisions, their job is just to feel things and send messages to the brain and spinal cord.  Messages can be sent all the way up to the brain, but serious things (like this nail in the body's foot) need a very fast decision to be made.  This signal doesn't have to go all the way up to your head.  An interneuron is the kind of nerve cell found in your brain or spinal cord that makes decisions about what to do with the information that comes in from the outside.  Once the pain starts, the message is sent to these cells so they can choose what needs to happen.  Grab the nail?  Scream?  Jump?  All of the above?  We had better hold on to something.

Ouch, I felt that!


Once the interneuron chooses what needs to be done, a signal is sent back out.  This time, it is sent to a group of cells that can do something about the nail.  A motor neuron is a kind of nerve cell that sends signals away from your brain or spinal cord to make your muscles move.  The message shoots all the way back to the foot - "Jump!"  It might not seem like a long way from the foot to the spine and back again, but it sure feels like it when there's a nail sticking into the body's skin.  It's times like these that you wish the brain was in the feet . . . or maybe not. 


Are you holding on?  Good.  Now that the signal has been sent that something bad is happening, the body can make a move.  A reflex is the process of the body reacting to something right away without the brain having to think about it.  The message is sent from the foot to the spinal cord and back to your leg.  Hold on!  Oof!  The knee jerks up, trying to get the foot away from the nail as fast as it can!  Oof!  Oof!  All this other jumping around is just the body being really unhappy about this.  It looks like the nail is being pulled out.  Things should calm down now.


Now that the nail is out, the body is starts to heal right away.  Stimuli came in from the outside, like this nail that caused pain.  The sensory neurons felt that something was wrong and the interneurons helped them decide what to do about it.  The motor neurons to get the foot to lift up off of the nail really fast.  It did this without even letting the brain know.  It probably knows by now, but it the body was able to decide what to do without asking the brain.  What does this all add up to?  You just saw a reflex, something happening without the brain having to think about it.  Phew.  We are safe now.  Well, nothing like getting thrown right into it on your first day.  You're just lucky the body does this all by itself.  Otherwise, we might have been in trouble.  


References:

Kids Health.  "What Are Reflexes?"  Kids Health, 2013.  <http://kidshealth.org/kid/talk/qa/reflexes.html>

Neuroscience for Kids.  "Reflexes"  Neuroscience for Kids, 2011.  <https://faculty.washington.edu/chudler/chreflex.html>