How an Impulse Works Inside of the Human Body
Have you ever wondered what goes on inside of your body that informs you
to pull your hand back whenever you touch a hot stove? How about what makes
you instantly draw your hand backwards before you even realize the stove is hot?
Well, it is quite simple really. As soon as your hand touches that hot stove, an
impulse in your body goes through multiple steps in a fraction of a second to inform
you to draw your hand away from the stove to prevent severe burning. To help
clarify how quickly this process occurs, an impulse moving through the nerve
network of the human body travels anywhere between three hundred and fifty feet
per second to three hundred and ninety feet per second. Considering the average
human height for males is roughly 5’10” and females 5’4”, this impulse travels
rather quickly.
An impulse is a progressive wave of electric and chemical activity along a
nerve fiber (also known as a neuron) that can either stimulate or inhibit the action
of a human’s body part. The neuron itself is made up of four different sections that
each serve an important purpose. Without getting into too much detail, when an
action potential is triggered, the axon terminal of one neuron sends a message to the
dendrites of another. The message then moves through the dendrites to the nucleus
and through a myelin sheath, which insulates part of the axon and increases the rate
of transmission of signals. Finally, the action potential moves through the axon of
this neuron until it reaches the dendrites of another. The process repeats several
times until an action potential occurs such as drawing your hand back from a hot
stove.
Before the incident with the stove occurs, the neurons inside your body are
not stimulated, though have polarized membranes surrounding each of them.
Putting it simply, this means that the outside of the membrane has a positive
electrical charge because it has a surplus of sodium ions, while the inside of the
membrane has a negative charge because is has a surplus of potassium ions. This
current state is known as a resting potential, and until a certain impulse it triggered
the neuron will remain in this state.
Movement of ions through gated channels
Once a hand touches a hot stove, a nerve impulse, is triggered. This impulse
travels from neuron to neuron through the dendrites. To allow this to happen, gated
ion channels located on the neuron’s membrane open suddenly to allow the
positively charged sodium ions to rush into the cell, thus depolarizing the neuron.
Now depending on the severity of the heat, no impulse may occur. If the stove is
merely warm, the gated ion channels may not open, therefore not triggering an
impulse to pull your hand away. A threshold level determines whether or not this
process will occur. Once a stimulus reaches the threshold level, more gated ion
channels will open thus allowing more positively charged sodium ions into the cell
until the neuron is completely depolarized. This is referred to as an action potential.
After the inside of the cell becomes flooded with positively charged sodium
ions, the gated ion channels on the inside of the membrane (these gated ion
channels are different from the sodium ion channels) open up to allow the
negatively charged potassium ions to migrate outside of the membrane. As soon as
these potassium gates open, the sodium gates close to ensure that no more sodium
ions will transfer across the membrane. This process is known as repolarization.
Repolarization occurs in order to restore an electrical balance throughout the
neuron.
Hyperpolarization
Less than two milliseconds after the potassium channel has opened, once
enough potassium has crossed through the gated ion channel, the potassium gates
close. At this moment, there are slightly more potassium on the outside of the
membrane than sodium on the inside. This causes the membrane potential to drop
slightly lower than the resting potential (where it was prior to touching the stove),
which is a process called hyperpolarization. Over a very short amount of time,
however, after the impulse has traveled through the neuron and onto the next
neuron, the action potential is officially over and the cell membrane returns to its
resting potential.
Refractory Period
The final step that occurs is referred to as the refractory period. Throughout
this process, the sodium and potassium ions return back to their respective sides of
the membrane (remember they switched sides in order to depolarize and repolarize
the neuron). During the refractory period, the neuron is sort of ‘on hold’ in a way
that it will not respond to any incoming stimuli. Luckily, humans have
approximately one hundred billion neurons merely located in the brain so
numerous other neurons remain free to receive incoming stimuli. After a couple
more milliseconds once the sodium and potassium ions return to their respective
sides, the neuron returns to its polarized state. At this point, the neuron is
considered at its resting potential, where it remains until another impulse comes
along and the process repeats.
Conclusion
The human body is magnificent. It can take a multi step function in which
thousands of molecules move from one membrane to another across hundreds of
neurons and complete this process in a matter of a fraction of a second without ever
messing up. Imagine what would happen if a person’s body did not react to a severe
burn on their hand one time because a neuron was unable to trigger a response. It
is clear as time goes on that we, as humans are an absolutely incredible species.