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.