In order to do the forward bend position during your yoga class, your nervous system has to do a number of things.
First, an upper motor neuron from your brain - specifically your cerebral cortex - has to send a signal down to a lower motor neuron that’s in the anterior horn of the spinal cord.
This lower motor neuron is also called an alpha motor neuron, and it relays an action potential through an axon that goes to muscles in your legs, which enable you to extend them.
Now, when you stretch or flex your muscles, proprioceptors that detect the position and movement of the muscles initiate reflexes that prevent you from damaging the muscles from overstretching or over contracting.
These proprioceptors are scattered throughout your skeletal muscles, and operate on a subconscious level so you never even notice them.
Now a muscle looks like it’s made of a bundle of muscle fibers with extrafusal muscle fibers on the outside and intrafusal muscle fibers on the inside.
Extrafusal muscle fibers provide most of the force during a muscle contraction, and are innervated by lower motor neurons which are also called alpha motor neurons.
Extrafusal muscle fibers attach to bones with tendons which are a specific type of connective tissue.
These tendons have proprioceptors called golgi tendon organs which lie at the ends of these extrafusal fibers.
Now, if we pull apart the extrafusal fibers, there’s another proprioceptor called the muscle spindle that lies within the extrafusal fibers.
Each muscle spindle contains multiple intrafusal muscle fibers.
Just like extrafusal muscle fibers, intrafusal muscle fibers have contractile proteins like actin and myosin. However these contractile proteins don’t extend through the entire length of intrafusal muscle; instead they’re only present at each end of a intrafusal muscle fiber.
Therefore, the central region of a intrafusal muscle doesn’t contract, even though the ends do.
The central portion of the intrafusal muscle fiber contains the muscle fiber’s nuclei, and the arrangement of the nuclei, determines whether the intrafusal muscle fibers are considered nuclear bag fibers or nuclear chain fibers.
Nuclear bag fibers are ones that have their nuclei concentrated in a wide, central portion of the fiber, like a sack full of oranges.
On the other hand, nuclear chain fibers, are half as long as the nuclear bag fibers, and have their nuclei arranged in series like peas in a pod through the central region.
Coiled around the central region are two kinds of sensory neurons that depolarize when the intrafusal fiber is stretched.
Type Ia neurons fibers coil around the central region of both the nuclear bag and the nuclear chain fibers, and relay information about how far and how fast the muscle is being stretched.
Type II neuron fibers branch out and attach to the ends of the central region of the nuclear chain fibers and relay information about how far the muscle is being stretched.
In addition, gamma motor neurons innervate both ends of the intrafusal muscle which contain actin and myosin, and when stimulated, the gamma motor neurons cause these end regions to contract which shortens the intrafusal muscle and keeps it taut.
First let’s look at the stretch reflex which prevents overstretching of the muscles.
So let’s say you’re sitting on a high stool with your legs comfortably dangling. In that position, the extensor muscles in your legs, like the quadriceps femoris are at their resting length, and the type Ia and type II neuron fibers are firing off action potentials at a normal rate.
These action potential travels through the peripheral nerves to enter the dorsal ganglia where the cell bodies of both types of fibers are located.
Each cell body also gives off another axon that continues on into the spinal cord.
Type Ia fibers go to the anterior horn and directly synapse with alpha motor neurons of the muscles being stretched.
Type II fibers also go to the anterior horn to synapse with inhibitory interneurons, which synapse with alpha motor neurons of the antagonist muscles, or muscles that serve the opposite function, in this case they’re the leg flexors like the hamstrings.
Now if you tap the area below your patella, where the quadriceps femoris muscle attaches, it stretches the muscle tendon a bit. This also stretches the extrafusal muscle fibers and the muscle spindles.