The
speed of your reactions play a large part in your everyday life. Fast reaction
times can produce big rewards, for example, like saving a blistering soccer
ball from entering the goal. Slow reaction times may come with consequences.
Reaction
time is a measure of the quickness an organism responds to some sort of
stimulus. You also have "reflexes" too. Reflexes and reactions, while
seeming similar, are quite different. Reflexes are involuntary, used to protect
the body, and are faster than a reaction. Reflexes are usually a negative
feedback loop and act to help return the body to its normal functioning
stability, or homeostasis. The classic example of a reflex is one you have seen
at your doctor's office: the patellar reflex.
This
reflex is called a stretch reflex and is initiated by tapping the tendon below
the patella, or kneecap. It was first independently described in 1875 by two
German neurologists, Wilhelm Heinrich Erb and Carl Friedrich Otto Westphal. In
their original papers Erb referred to the reflex as the "Patellarsehnenreflex"
while Westphal denoted it as the "Unterschenkel phenomena".
Thankfully, we now refer to it as the patellar reflex.
This
reflex is also known as a "reflex arc". It is a negative feedback
circuit that is comprised of three main components:
· A sensory component or afferent
neuron. These neurons take in information and translate it to an electrical
signal that gets sent to the central nervous system, much like the spikes you
hear when doing the cockroach leg experiments.
· Integrating center or interneuron.
These neurons act as sensory processing centers that determine the magnitude of
the response to the incoming stimulus. They are located in the central nervous
system (your spinal cord).
· The efferent portion or motor
neuron takes the information from the interneuron and sends it to the effectors
which activate a response. The effectors are usually muscle fibers as in the
patellar reflex or a gland such as the salivary gland.
The knee
reflex arc is a spinal reflex, and the circuit is drawn above. This picture
shows how the sensory (afferent) neuron sends information through the dorsal
root ganglion into the spinal cord; where the signal splits into two different
paths. The first is the motor neuron (efferent) leading back to the quadriceps.
When your quad muscle's motor neuron receives the information it fires and
causes your lower leg to spring forward up in the air. The second signal from
the sensory neuron travels to an interneuron which sends a signal to the motor
neuron (efferent) leading to the hamstring. This signal tells your hamstring to
relax so there is no negative force acting on the quadriceps muscle when it
contracts. Both signals work together and all of this happens in the spinal
cord without going to the brain. It never needs the brain.
You may be
asking how a knee reflex arc and a soccer player dealing with an oncoming ball
are different. Are both not reflexes? While it may seem that a soccer player
negotiating an oncoming ball is a simple fast reflex, it is actually a symphony
of hundreds of thousands of neurons working together to produce a conscious
decision. Does the player catch, dodge, or bat away the ball? This choice is
what makes a reaction.
When a
soccer player realizes the ball is blistering towards him, there is visual
information that has to be processed and decisions regarding a correct course
of action. The brain then needs to send many signals to various muscles. Feet
begin to move, hands might travel in front of the face, and eyes may close
shut, along with many more processes. This is the work of many neurons as well
as numerous systems and circuits in the brain, and what's more, and you can
train and enhance your skill through practice. This is how you get better at
sports over time.
Like all
science, the history of the reaction time discovery is peculiar. Dutch
physiologist F.C. Donders in 1865 began to think about human reaction time and
if it was measurable. Prior to his studies scientists thought that human mental
processes were too fast to be measured. This assumption was proved incorrect
with the help of Charles Wheatstone, an English scientist and inventor. In 1840
Wheatstone invented a device, much like his early telegraph system invention,
that recorded the velocity of artillery shells. Donders used that device to
measure the time it took from when a shock occurred on a patient's foot until
when that patient pressed a button. The button had to be pressed by the left or
right hand matching the left or right foot that was shocked. His study tested 2
conditions: in the first, the patient knew in advance which foot was to be
shocked; in the other condition, the patient did not know. Donders discovered a
1/15 second delay between patients who knew which foot was to be shocked versus
patients that did not know. Notably, this was the first account of the human
mind being measured!
These
efforts continue today, with the improvement of "non-invasive"
imaging technologies like fMRI, PET, EEG, etc... You may have had one of these
scans in the hospital.
How quickly
neurons move information is called the "speed of neural
transmission"; we studied it in experiment 11 when we measured the
conduction speed of axons in earthworms. This is only one of the speed
bottlenecks though. You also have to deal with the synapse (which we studied in
experiment 8). Furthermore, the quickness of reaction times can differ
depending upon what type of stimulus you are reacting to and what kind of task
you are doing.
In this
experiment you and a friend will be testing each other's reaction times using a
simple 12 inch ruler. You will be testing not only visual stimulus, but also
auditory and tactile stimuli.
Procedure
This
experiment will be broken into two phases. The first test will use one ruler,
while the second test will use two.
Experiment
1: In this phase you and your partner will test visual, auditory, and tactile
reaction times using one ruler.
1. Have your friend sit at a table with
their dominant hand over the edge.
2. First we will test visual response.
Hold the ruler at the 30 cm mark so that the 0 cm end is just at your friend's
index finger.
3. Tell your friend that when you
release the ruler they are to grab it as fast as possible. Do not make any
sounds or gestures that you are releasing the ruler. They have to react to the
visual stimulus of seeing the ruler being released. Record the centimeter mark.
4. Repeat the experiment three more
times. Then switch places with your partner and redo it.
5. Now you will record auditory
reactions. Have your partner sit at the table as before, also be sure your
partner puts on the eye shades.
6. Again testing the dominant hand,
tell your partner that you will say the word "Release" as you release
the ruler. Once they grab it record the centimeter mark and repeat 3 times.
Switch places with your partner again.
7. For the last test, have your partner
sit at the table wearing the eye shades again. This time you will test the
tactile response. Tell your partner that you will touch the shoulder of their non-dominant
arm as you release the ruler.
8. Give you partner no auditory cue
that you are releasing, just a simple touch. Record the measurement and like
before, repeat three times, then switch places and redo.
Here is the table for
the first experiment:
Experiment
2: In this phase you and your partner will test visual and auditory reaction
times using two rulers.
1. For the Visual portion of this
experiment have your partner sit as the table, like before, but have both of
their hands over the edge.
2. You will hold both rulers this time
instead of just one.
3. Tell your partner that you will
release just one ruler and they must pick the correct one and grab it as fast
as possible€¦Tell them they must not squeeze both hands, only one.
4. When you are ready to begin,
randomly decide one ruler to drop. It does not matter which one, you will
perform this test 3 more times, but never tell your partner which ruler you
will drop.
5. Again as before switch roles and
redo.
6. Finally, we will test the auditory
reaction again. This time using both rulers.
7. Get in the same position as before
with both rulers. Make sure your partner has the eye shades on.
8. You will then proceed to say
"left" or "right". As you say it you will drop the
corresponding left or right ruler. Your partner must decide which ruler to
grasp based on the auditory cue you give: "left" or
"right". As before, your partner must only squeeze one hand.
9. Record the measurement and repeat 3
more times, remember to randomly decide which ruler to drop. Switch roles and
repeat.
Here is the table for
the second experiment:
Math
In your
chart above you are going to take all the centimeter measurements you have
collected and convert the measurement in centimeters to seconds. This will tell
you how long it takes, in seconds, an object (the ruler) to fall a certain
distance. The formula below is comprised of three variables.
· Y = the distance you measured in
centimeters
· g0 = the acceleration due to
gravity constant (980 cm/sec2 )
·
t = time in seconds
Here is an
example of the equation being used:
It may seem
tedious to convert by hand each number you recorded so instead you will be
provided with a quick chart to convert your centimeter measurement to seconds.
However, there are several values missing in the table. You will need to fill
them out to complete the table. Use the equation above to fill out the
remainder of the chart. If you are savvy you can also design a computer program
to do this.
After using
the chart and converting your centimeter measurements into seconds you will
have your ruler reaction time in seconds. Looking at your data you might be
thinking how you compare to the human average reaction time. Here it is! The
average reaction time for humans is 0.25 seconds to a visual stimulus, 0.17 for
an audio stimulus, and 0.15 seconds for a touch stimulus.
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