Voiceover: In this video, I'm gonna talk about the muscle stretch reflex. The nervous system performs many reflexes. And a reflex is a response
to a stimulus that doesn't require the involvement of consciousness. All reflexes have two parts. The first part is called
the afferent part, afferent. And that involves bringing
information about a stimulus in to the central nervous system. So there'll be some sort
of receptor somewhere in the body that can detect the stimulus. And then there'll be some
sort of neuron that brings that information in to the
central nervous system. The other part of a reflex is called the efferent part of the reflex. Efferent, which carries information away from the central nervous system to cause a response somewhere in the periphery. So there will be some sort
of neuron that'll carry that information away from
the central nervous system out into the periphery to
cause some sort of response. Now, some reflexes, like
the muscle stretch reflex that I'm just about to describe
happen on the same side so that the afferent
part of the reflex brings information in from one side of the body. And the efferent part of the
reflex brings information back to that same side of the
body to cause the response. Other reflexes, particularly
those up in the brain stem, have an afferent limb
that comes in on one side, and then efferent responses
that come out to both sides. So there's some variety
for how the information travels in reflexes,
depending on the reflex. One of the simplest reflexes
that's a good example and that happens to be one
of the most medically useful is called the muscle stretch
reflex, muscle stretch reflex. If a skeletal muscle,
like in this drawing, here's a skeletal muscle in the arm. If this is rapidly stretched,
the muscle stretch relfex will cause it to contract very
quickly after it stretched presumably as a protective
response to prevent injury to a muscle from being
stretched too rapidly. But let's go over the one
that happens around the knee. And that's also called the knee-jerk. Because most of us are probably
familiar with this one. Because a lot of us, when
we're in our doctor's office, have had the experience where
we're sitting in a chair. So here, I've drawn a person. And we're looking at their right side. And here's their trunk and their leg. And if you're in the
clinic, often your doctor will take a little, rubber hammer. And they'll take that
little, rubber hammer. And they'll hit you
right below the kneecap. And to your surprise, when
they hit you below the kneecap with the little, rubber hammer,
your leg will often kick out without you telling your leg to kick out. There's this involuntary
response of the leg kicking out to the stimulus of the rubber hammer hitting you just below the kneecap. So why does this happen? Well, the place that your doctor's
hitting you at the little, rubber hammer is not actually
in the kneecap, itself. But it's in the tendon that's
just below the kneecap. So let me draw that here in orange. And that tendon hooks onto
the bones in the lower leg. And connected to the
kneecap on the other side from the tendon is a large group of muscles in the front of the thigh. And when your doctor
hits you in that tendon, it actually stretches this
large group of muscles. Because for just a moment,
the little, rubber hammer bends this tendon, and that
pulls on the kneecap like this. And that pulls on this
muscle, and it stretches it. Now, it doesn't stretch it very far. But it does stretch it rapidly. And there are receptors in skeletal muscle that can detect muscle stretch. I'll just write a big "R" here to represent one of the receptors. And there are lots of
these receptors spread out throughout all of the
skeletal muscle in the body. And these receptors are
called muscle spindles. Muscle spindles, and here's a
drawing of a muscle spindle. So here's a skeletal muscle. And they've magnified
this little receptor. And we won't go into the details. But there are these
specialized, little fibers inside the muscle spindle
that gets stretched when the rest of the
muscle gets stretched. And then there are neuron
axons that are wrapped around these special
fibers that can detect that stretch of these fibers
and send that information back into the central nervous system. So that these axons that are
leaving the muscle spindle here will travel back through nerves of the peripheral nervous system. And then they'll enter either the spinal cord or the brain stem. And these are somatosensory
neurons that tend to have their somas and ganglia close to the spinal cord or the brain stem. And since these are neurons
carrying information into the central nervous system, we can call them afferent neurons. And they make up the afferent part of the muscle stretch reflex. Let me just write that out. That for the muscle stretch
reflex, the afferent part or the somatosensory neurons,
somatosensory neurons inside the central nervous system. Like here in the spinal cord,
these somatosensory neurons carrying that muscle stretch information, that information about the stimulus, are gonna form an excitatory synapse. So let me just draw a
little plus sign here to represent an excitatory
synapse with another neuron, whose soma is in
the central nervous system. And this neuron is gonna send an axon out through nerves of the
peripheral nervous system back to the same muscle
that was stretched. And it's gonna synapse on and excite skeletal muscle cells in that same muscle. Causing the muscle to
contract, causing the response. And the neurons that
synapse on and control skeletal muscle cells
are lower motor neurons. Lower motor neurons. And for the muscle stretch reflex, the lower motor neurons
make up the efferent part of the reflex that causes the response of contraction of the
muscle that was stretched. In the video where we
went over the motor unit, we talked about lower motor
neuron signs that can appear with abnormalities of
the lower motor neurons. And we talked about hyporeflexia, meaning a decrease in the
muscle stretch reflexes. And I think you could see
why that would happen. If the lower motor neuron
is not able to communicate with the muscle, then it
can't tell it to contract in response to the
stimulus of muscle stretch. But it turns out that you
can also have diminished muscle stretch reflexes
if there's a problem with these somatosensory
neurons, bringing information about the muscle stretch back
to the lower motor neurons. So that if there's a problem with either the lower motor neurons or
the somatosensory neurons, you can have a diminished
muscle stretch reflex. And it turns out this is
true for all reflexes. If there's a problem with
either the afferent part of the reflex, bringing
stimulus information into the central nervous
system, or if there's a problem with the efferent part of
the reflex carrying response information out to the periphery. A problem with either
the afferent or efferent part of a reflex can cause a
diminished or a lost reflex. Because both parts have to be working for the reflex to occur. Now, one important thing
to notice about reflexes is how all of this just occurs down here. In this case, in the spinal cord. Or it could occur in the brain stem if it was a brain stem reflex. But the higher parts of the
nervous system, the cerebrum, where a lot of the higher
functions of the nervous system like cognition, emotion and
consciousness, they don't have to get involved for a
reflex like this to occur. And this is the reason we say reflexes are responses to stimuli
that don't require the involvement of consciousness. Because the wiring tends to
occur at these lower parts of the central nervous system
and peripheral nervous system without having to involve the higher parts of the nervous system
way up in the cerebrum. Now, this is all you need for
the muscle stretch reflex. But there is another part
to it that isn't necessary. But does add something,
because it turns out that while this muscle is
contracting, in response to the stretch of the muscle,
the muscle on the opposite side, in this example, the
muscle on the back of the thigh that bends the knee when it contracts. This muscle actually relaxes. So while the muscle on
the front of the thigh is contracting, the muscle
on the back is relaxing. The way this occurs is that
this same somatosensory neuron that's exciting the
lower motor neuron back to the muscle that was
stretched is also sending axon terminals to other neurons. And it's gonna excite those neurons. So I'll draw a little plus sign. But these neurons are
actually inhibitory neurons. So they're gonna form a
synapse that's inhibitory. So I'll draw a little minus sign to represent that they're inhibitory. And what they inhibit
are lower motor neurons to the muscles on the opposite side. So these lower motor
neurons would normally be exciting the muscles here
in the back of the thigh that would cause the knee to bend. But when they're being
inhibited by this other neuron, these lower motor neurons
aren't exciting that muscle in the back of the thigh, so it relaxes. Now, this isn't necessary
for the reflex to occur. You just need the
afferent and the efferent part of the reflex for it to occur. But because this muscle,
when it's contracting, isn't fighting against this
muscle in the back of the thigh since it's relaxing, that
does increase the response. So there's more straightening of the leg at the knee and kicking outward. And a lot of the reflexes
in the nervous system have some similarities
to this sort of setup. Where you can almost
think of a balance between responses that the nervous
system can choose from. And that the reflex tips
the balance in favor of a response in one direction.