Today we'll be talking
about reflexes. Reflexes are often thought of in terms of
muscular movements. For example, when someone knocks a glass
off the table but is able to catch it before the glass hits the ground. Another
example is in sports
when we refer to a goalie in hockey. They have the reflexes of a cat.
In reality reflexes extend beyond movements of our arms and our legs.
There are a ton of different types reflexes out there
for controlling breathing, blood pressure, defecation,
and urination just to name a few. All reflexes
do have something in common however this is known as the reflex arc.
A reflex arc is the neural pathway
involved in accomplishing a reflex and it has five basic components.
The first one: receptors, the second
is an afferent neuron, number three
is an integrating center, number four
is an efferent rneuron, number five
as an effector. Remember these five components
as they can be applied to all reflexes. Today we'll be talking specifically
about two types the reflexes
the first will be a monosynaptic reflex and the second will be a polysynaptic
reflex.
To begin a monosynaptic reflex
is a relatively simple reflex as it consists of only one synapse
as the name suggests. The most common monosynaptic reflex
is the patellar tendon reflex. This is a stretch reflex.
Before we continue, I'll remind you of the five basic components
of a reflex arc. They are: number one
receptors, number two afferent neuron,
number three integrating center, number four
efferent neuron, number five effector.
So for a patellar tendon reflex
imagine you are in a doctor's office. The doctor will sit you down on the bed
and take a small reflex hammer and tap you just below the knee cap.
The result is that your knee responds by kicking out. If you've ever wondered
why this is done, it is essentially performed to assess nervous system
function.
They can see how well a bunch of things are working including muscle spindles,
afferent neurons, motor neurons, and neuromuscular junctions.
They can also see how the excitatory and inhibitory input to motor neurons from
the brain
are working and make sure they are all in balance. This is basically determined
by a depressed or exaggerated kick after performing the test.
So, let's go through the steps of the patellar tendon reflex.
The first step: activation of a receptor by a stimulus.
Firstly the stimulus is the hammer hitting your patellar tendon just below the
knee cap.
The receptors are actually found on the quadriceps muscle.
They are activated because tapping the tendon stretches the muscle spindles
found in the quadriceps muscle.
This changes their shape and thus activates them.
Number two: activation
of a sensory or afferent neuron. This step involves the activation of a neuron
that starts from the receptors in your muscle ends in the spinal cord.
The activation of this neuron causes the information to travel the length
of the neuron's axon. Number three:
This is the integrating center and information-processing.
Now, the signal has traveled to the spinal cord.
In a monosynaptic reflex remember there is only one synapse
so the integration center and information processing
occurs only in the cell body the postsynaptic cell which in this case
is the motor neuron. Activation of a motor
or efferent neuron. The signal, in the form of an action potential,
travels down the motor neuron carrying information from the spinal cord
to the effector. Number five:
Response by the effector organ. The last step of a reflex
is for the effector to respond. The information comes from the motor neuron
which innervates the quadriceps muscle or the effector.
The response is for the knee to extend and this causes the kicking motion.
So that is a monosynaptic reflex.