So, let's look at how muscle spindles and Golgi tendon organs might operate in a
coordinated fashion during muscle activity.
And let's begin by considering what happens when a muscle is stretched
against some kind of load that's being applied.
So here the muscle is lengthening, and so there's going to be an increase in
activity in our muscle spindle afferents as this muscle is stretched, okay?
Now, while that stretch is happening, there's really not much change and the
afferent activity derive from the Golgi tendon organ.
Because again, the Golgi tendon organ is not so much sensitive to a change in
muscle length, it's sensitive to the active contraction of the muscle.
And so far, this is a passive stretch of a muscle.
So that's going to activate the muscle spindle afferent and it's not going to
activate much at all, the Golgi tendon afferent.
However, if we were to now actively contract that muscle by stimulating the
alpha motor neuron, what we find is that the muscle is now contracting against
that load, it's getting shorter. And if we didn't fill-in that muscle
spindle afferent by also activating the gamma motor neuron, notice that this
muscle spindle now relaxes. And as the muscle is getting shorter,
there is a drop in afferent activity derived from the muscle spindle.
So, this active contraction is going to unload the muscle spindle.
However, this active contraction provides exactly the stimulus that will drive
afferent activity in the Golgi tendon organ.
So, as this muscle is getting shorter, there is now a dramatic increase in the
output from the Golgi tendon organ. So, we can see how during muscle stretch
and muscle contraction, the spindles and the Golgi tendon organs are tuned to
complimentary sources of information about the movement.
The muscle spindle is sensitive to length but not tension.
The golgi tendon organ is sensitive to tension but not length.
We need them both and they both operate to allow that muscle to function within
the context of the goal that's been established by our descending upper motor
neuronal systems. And lastly, in this part of our tutorial,
let's consider yet a third reflex, and this is the most complicated of the ones
that we've considered. But it's really not so bad once we think
about what has to happen here. So, this reflex is the one that would be
engaged if we were to step on a tack, or perhaps we were to extend our hand and
inadvertently touch a hot surface. the immediate response is to withdraw the
limb from the damaging stimulus. So, this is called deflection withdrawal
reflex or sometimes, it's called the crossed extensor reflex.
And while the circuitry involved here looks rather daunting at first glance,
I'm going to suggest that it really makes perfect sense if we consider the behavior
and what must be coordinated in the central nervous system in order to
produce the desired result, which is the withdrawal of the limb from the
potentially harmful stimulus. Now let's consider, as is illustrated
here, the example of stepping on a tack, okay?
So, the cutaneous receptor in question here is going to be a free nerve ending
supplied by, let's say, our A delta fiber.
So, this is going to be an example of a first pain signal, coming from an A delta
fiber that's transmitted into the central nervous system and notice what happens.
There is the distribution of this incoming sensory signal to a set of
interneurons. Well, why does this need to be so
complicated? well, for very good reason, think of what
would be a desired outcome of this sensory stimulus being transmitted into
the central nervous system. What we would want to do is we would want
to increase the activity of the flexor muscles on the side that encountered the
tack. And in order to facilitate the withdrawal
of the limb, we are going to want to decrease the activity of the extensor
muscles on that very same limb. Now, if we have any hopes of maintaining
a standing posture while we withdraw our foot from the tack, we're going to want
to do exactly the opposite on the other side of the body.
That is, we’re going to want to increase the activity to the extensor muscle and
decrease activity to the flexor muscle. Now, if you just understand what’s going
on with the limbs, then I think you can infer what the circuitry must be like in
the spinal cord to achieve these goals. Knowing that there is what we call,
polysynaptic activation in the circuitry of the spinal cord, that simply means we
have a number of synapses involving several different kinds of neurons that
are interposed between the incoming sensory signal via the A delta fiber and
the outgoing motor signal being derived from the alpha motor neurons.
And these interneurons need to be some of them need to be excitatory, some of them
need to be inhibitory. In order to activate the appropriate
muscles, there needs to be excitatory interneurons linking the sensory signal
to the alpha motor neuron. We need to excite the flexor muscles in
the ipsilateral limb, that is the limb that encountered the attack.
Meanwhile, we need to excite the extensor muscles that are on the opposite side of
the body to maintain posture. So, that must mean that some of these
interneurons must actually expand the midline and allow incoming information
from one side of the spinal cord to impact the output on the opposite side of
the spinal cord. Now, let's look at the actual circuitry
that we find here and see what elements that we can detect.
So, with respect to the flexor muscle on the ipsilateral side of the body, what we
have is basically disynaptic excitation. So, we have one interneuron, a second
interneuron, and then the activation of the alpha motor neuron that supplies the
flexor muscle. So disynaptic excitation to the
ipsilateral flexor. With respect to the extensor muscle that
we want to suppress, we have disynaptic inhibition that is, we have two synapses
first an excitatory interneuron. And then, the signal is inverted by
virtue of the fact that we now have an inhibitory interneuron that is
responsible for the second synapse between the sensory signal and the motor
output. So, with that disynaptic inhibition, the
output to the extensor muscle is suppressed.
Now, on the opposite side of the body, we have polysynaptic excitation, again just
meaning multiple synapses perhaps at least two or three are present here, that
convey an excitatory signal, eventually to the alpha motor neuron that supplies
the extensor muscle. And that allows us to maintain our
posture even while we are disturbing posture by withdrawing the limb from the
tack. Meanwhile, we have polysynaptic
inhibition, that is applied to the antagonistic muscle on the opposite side
of the body. So, we have some number of interneurons,
maybe two or three, that ultimately lead to an inhibition of the alpha motor
neuron that innervates the flexor. So, this is a beautiful example of how
circuitry can coordinate the output of our alpha motor neurons.
And that circuitry can do the job of either increasing the output of the alpha
motor neuron, or decreasing it depending upon if the circuitry interposes an
exitatory interneuron or an inhibitory interneuron between the incoming sensory
signal, which is always going to be an excitatory signal.
And then, the output from the alpha motor neuron, which likewise, is always going
to be an excitatory signal. So, the way that we increase or decrease
the output of the spinal cord to our muscle is by either exciting the alpha
motor neuron or inhibiting it through the activity of an interneuron.
In the next part of this tutorial, we're going to consider one particular kind of
circuitry that will be similar in nature to what we've just described here.
It's one that will coordinate the activity of flexor and extensor muscles
on both sides of the spinal cord, and it's the kind of circuitry that's
essential for generating the rythmical behavior that supports locomotion.
We call this kind of circuit essential pattern generator.
So, that's the topic that we come to next.