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The next topic we're going to discuss is the polysynaptic reflex
a very common example is the withdrawal reflex. We will apply the withdrawal
reflex
to the act of touching a hot stove with your hand. Imagine you're cooking
pasta
in boiling water you accidentally touch the hot element with your hand.
What do you do? You immediately pull your hand away without even thinking.
Similarly to the patellar tendon reflex
the actual act withdrawing your hand from a stove is also accomplished solely at the
level
of the spinal cord, however, signals are still sent to the brain
in order to tell you that you're in pain and to aid in the formation of memories
about what just happened. Remember
the five basic components of a reflex arc.
Receptors 2. Afferent neuron
3. Integrating center
4. Efferent neuron and 5.
Effector. This occurs in a withdrawal reflex
but with a few modifications. So 1.
Receptors. Touching a hot stove activates pain receptors
in your hand. These send signals
through an afferent neuron to the level of the spinal cord
so this is step 2. Now step 3 is where some of the modifications take
place.
The integrating centers are found in the spinal cord.
In contrast to a monosynaptic reflex the afferent neuron does not synapse onto
the motor neuron.
It actually synapses onto what's called
and interneuron. This interneuron then diverges
onto the motor neuron which will then send a signal to the biceps brachii muscle
to cause a withdrawal of your hand from the stove. Interestingly,
however, the interneuron in the spinal cord synapses on to other neurons as
well.
One of these is an inhibitory neuron. The inhibitory neuron
will actually send signals to your triceps muscle in the back of your arm
and ensure that it does not contract which normally would cause the extension
of your arm.
So basically it's a backup plan to make sure you withdraw your hand
quickly and smoothly. Lastly, the interneuron synapses on neurons
traveling to your brain.
This tells you that your hand is in pain and also works in the formation of memories
so that you can think about what just happened
and remember not to touch that stove again. The withdrawal reflex
is very important and helpful for maintaining the safety of an individual.
Interestingly the greater the stimulus,
meaning the hotter the stove is, the greater the response. If the stove is
somewhat warm
a person is slightly remove their hand whereas if it is extremely hot
the individual would likely jerk their hand away much more quickly and possibly
even twist their entire body away from the stove
which would involve the activation of many more motor neurons.