Andrew Huberman

You've got motor neurons that can cause muscles to contract and shorten.

And we have these spindles within the muscles themselves that wrap around the muscle fibers.

And that information is sent from the muscle back to the spinal cord.

It's a form of sensing what's going on in the muscle.

Now, why would that be useful?

Well, what this does is it creates a situation where if a muscle is stretching too much because the range of motion of a limb is increased too much, then the muscle will contract to bring that limb range of motion into a safe range again.

So just to clarify, this whole thing looks like a loop and the essential components of the loop are motor neurons, contract muscles, sensory neurons that we call spindles are sensing stretch within the muscles.

And if a given muscle is elongating because of the increased range of motion of a limb, those sensory neurons send an electrical signal into the spinal cord such that there is an activation of the motor neuron.

which by now should make perfect sense as to why that's useful.

It then shortens up the muscle.

It actually doesn't really shorten the muscle, but it contracts the muscle that brings the limb back into a safe range of motion.

So that's one basic mechanism that we want to hold in mind, this idea of a spindle that senses stretch and can activate contraction of the muscles and shorten the muscles.

The next mechanism I want to describe,

once again, there are only two that you need to hold in mind for this episode, has to do with sensing loads.

So at the end of each muscles, you have tendons typically, and there are neurons that are closely associated with those tendons.

that are called Golgi tendon organs, right?

These are neurons that are sensory neurons that sense how much load is on a given muscle, right?

So if you're lifting up something very, very heavy, these neurons are going to fire, meaning they're going to send electrical activity into the spinal cord.

And then those neurons have the ability to shut down, not activate, but shut down motor neurons and to prevent the contraction of a given muscle.

So for instance, if you were to walk over and try and pick up