Let's first start talking out by which particular nerve fiber
properties are associated with which kind of nerve.
Muscle afferents. These are the ones leaving the muscles
going out towards the spinal cord and eventually the brain.
In the muscle, we have a couple of classifications.
The first are muscle spindles.
Muscle spindles have two classes associated with them.
There are primary nerve endings or Ia sensory fibers.
And there are secondary endings or type II sensory ending.
Their conduction velocity is different between these two.
The primary ones has a faster conduction velocity, somewhere between
80 to 120 m/s, versus the slower 35 to 75 with the type II.
Golgi tendon organs.
These are classified as Ib sensory muscle afferents.
They have a fast conduction velocity, 80 to 120 m/s.
Therefore they are very similar in their speed to the Ia's.
If we now look at muscle efferents,
so these are now going to do something
like the contraction going from the brain
to the spinal cord to the muscle.
Alpha motor neurons also have a
very fast conduction velocity, 80 to 120 m/s.
Gamma motor neurons, also known as intrafusal fibers,
these are slower, only 15 to 30 m/s.
So that brings up there's a difference between
extrafusal and intrafusal fibers for muscle efferents.
Extrafusal fibers are the traditional alpha motor neurons
that are going to cause muscle contraction.
These intrafusal neurons, gamma mediated class.
They require us to talk through a little bit more
cause they are associated with muscle spindles.
Muscle spindles. Muscle spindles are located
within the belly of the muscle.
There are a couple different types.
One are nuclear chain and the other are nuclear bag fibers.
These nuclear chain and bag fibers
respond a little bit differently to stretch.
Muscle spindles stretch. All of these stretch.
Here what we want to think about is stretching the fiber,
the primary nerve endings.
These little ones that are the Ia's, they were fired very fast
and response to a change in length.
If you look at the secondary fibers,
these ones will also increase their firing frequency
but they do so, not related to the speed of change,
but rather to the new length they're at.
So the Ia's response to rapid changes by
having a volley of spikes right when the change happens.
And the secondary ones,
these will change in regard to length changes.
So you can see how stretch has multiple components.
There's a speed at which the muscle stretches
and then the end amount of stretch the muscle has undergone.
So if you think about these being protective fibers,
when there be a time when the muscle could stretch too much.
Well if it's stretching too fast or it stretch too far, you may need
to adjust the muscle contraction to make sure damage doesn't happen.
That is what the myotatic or stretch reflex does.
If you use a reflex hammer to percuss a tendon,
this tendon causes a very rapid stretch of the muscle.
This rapid stretch of the muscle causes an effect.
It's sensed by muscle spindles.
So this is all a muscle spindle response.
It sense their information via the muscle spindle
through the sensory afferent nerve,
back to the spinal cord
At the level of the spinal cord, you'd get a reflex that happens.
That reflex involves two things.
One is you'll get a contraction of the muscle being stretched,
and the second thing is you'll get
a relaxation of the opposing muscle group.
So if we were using this example that you see here,
you'll see that the quadricep will undergo a contraction,
so this will be a shortening. And then the flexor,
which is the hamstring, or semimembranosus, or tendinosus
that will be relaxed.
Another important part about this reflex is the one
that's occuring in the spinal cord with the individual nerves.
Here we want to highlight that there's a direct synapse
between the sensory nerve and the alpha motor neuron
for the particular component that will be stimulated
in our example here of the quadricep.
If we're stimulating the alpha motor neuron,
we're inhibiting it from the flexor
such as the semimembranosus or tendinosus.
This is done by an inhibitory interneuron.
So there are two synapses for the inhibitory response,
only one synapse for the excitatory response.
Golgi tendon organs. Golgi tendon organs are located
in the muscular tendinous junctions.
Now, in the tendon or muscular tendinous junctions,
there's not much stretch that happens.
Stretch is what's been sensed by muscle spindles.
Tendons sense tension. So, tendons sense tension.
How much pull is being applied to that particular
tendon or muscular tendinous junction.
They work via a class Ib afferent.
So they move very fast back to the spinal cord.
The reflex that's engaged by GTOs or Golgi tendon organs
involves a protective mechanism.
So if a muscle is applying too much force, and that is
pulling too hard on the tendon or muscular tendinous junction,
there is a reflex that happens to reduce the muscle contraction
and therefore prevent the tendon from rupturing.
So this response is sensed by the Golgi tendon organ,
sent by the Ib fiber back to the spinal cord.
The spinal cord then synapses with an inhibitory neuron
to the alpha motor neuron to decrease forced output.
The other thing of this particular reflex does
is also strengthen or contract the opposing muscle group.
So the opposing muscle group is what is
on the opposite side of the bounding liber.
So in this case, you might have
a contraction of a muscle like the biceps.
And therefore your Golgi tendon organ
is sensing too much strain or tension.
It is going to reduce the amount of constriction to the bicep.
But at the same time, it is also strengthening
or engaging contraction in the triceps
so again, to prevent that tendon from becoming
too stressed and potentially rupturing.