Supplemental Instruction: Neural Transmission

Big picture...First off, this process is very circular.  To suggest a pure starting point in the grand scheme, it's difficult to determine.  Neural transmission is necessary for everything we do - walk to class, processing thinking and feelings, talking, and processing incoming sensations from our environment, controlling heart rate.  All of these go on simultaneously, involving millions of neurons transmitting signals (firing) up to 1,000 times per second.  It's a phenomena process.

Smaller picture...consider that you are talking to someone and you are leaning against a brick wall.  As the touch receptors in your hand receive stimulation, they must transmit a signal to the brain (specifically the somatosensory cortex) to process that raw sensation...to make sense of it.  The brain then interprets the signal as pressure and maybe pain (if there are sharp surfaces in the brick).  While you place pressure on your hand, signals are constantly being sent to the brain so that it interprets the change in the sensation.  If your hand cramps or you push on a particularly sharp surface, you want to remove your hand from the wall.  Thus, the brain sends a signal to the motor neurons to move your hand away.  All of this communicating is done via neurons.

Zoom in...for this process to work, the signals must get from point A (touch receptors) to point B (somatosensory cortex).  Each neuron specializes in a certain function.  Motor neurons in your hand cannot "make decisions"; they must consult the neurons in the brain for action.  Nevermind how that works...I just want to show you why a signal needs to travel to the brain.  Remember, the brain is the ultimate director of the nervous system.

Zoom in closer...There may be hundreds or thousands of neurons between your hand's touch receptors and the neurons in the somatosensory cortex.  Each neuron must pass this information to the next.  For a neuron to send a signal, it has to be "coerced".  Let's say it wants to stay at rest unless there's a reason to fire.  If a message needs to be sent, the neuron needs to fire.

So the touch receptors receive a change in sensation (pressure).  They must tell the brain about this to make sense of the incoming energy.  The first neuron in the line receives a signal to fire and transmit the "pressure" sensation.  The signal is in the form of positive molecules, or excitatory potentials.  Because the internal environment of the neuron is slightly more negative than outside of the neuron, this influx of positive molecules causes a disruption.  When the environment reaches a treshold, it is destablized to the point that it must fire.  The action potential is like a moment of change.  

Think of a plastic bottle where you have sucked all the air out of it.  It crumples and caves in.  The pressures outside and inside the bottle are different, much like the negativity of the environment inside and outside of the neuron.  But when you blow air back into that bottle, the pressures are equalized and the bottle retains its shape.  Blowing in the air is like those gates opening up and letting positive molecules inside.

So the depolarization forces axon gates open, allowing positive molecules to rush in, aiding the signal down the axon and then to release a neurotransmitter to the next neuron in line.  After if fires, the neuron must pump the positive molecules out through those gates so that it retains its resting state of polarization.  Think of flushing a toilet...you can't continually flush it...the tank needs to fill up with some water before it can flush again.  Or think of the bottle.  In order to blow the bottle into shape again, you have to suck the air out first.  Otherwise, it stays the same and no change takes place.  That dynamic change in the neuron is what causes it to send signals and release neurotransmitters.

So once the neuron fires, it cannot fire again for a brief period of time.  Now this refractory period is insanely minute because neurons fire up to 1,000 times per second.  So in real time, we can't comprehend it.  But no sooner does the neuron send a signal for the sensation of touch to its neighbor, it will likely send it again and again as long as the sensation exists.  In that case, the process is very much circular and continuous (and for good reason).  

Neurons are busy players in our everyday life.  In one hour alone, a single neuron may fire over 3 million times.  And there are about 100 billion neurons in the body!

Note to students:  I hope that in being comprehensive, I haven't been more confusing.  I just think that providing a clearer big picture to accompany what's going on at the micro level is very important.  If it's still confusing, I think you should read the book on neural transmission.  In concert with my examples, it should be clearer to you how this process works.