06/11/2026
Thursday nerd out
How can a horse remain chronically tense long after the original cause is gone?
To answer that, we have to talk about something called the gamma loop.
The gamma loop is one of the nervous system's mechanisms for regulating muscle tension and movement. Its job is to help the brain monitor what's happening in the muscles so it can maintain posture, coordinate movement, and respond to changes in the environment.
To understand how it works, we need to define two types of motor neurons.
Alpha motor neurons connect to the muscle fibers that produce force and create movement. These are called extrafusal fibers.
Gamma motor neurons connect to specialized sensory fibers inside structures called muscle spindles. These are called intrafusal fibers.
Muscle spindles are stretch receptors. They constantly provide information to the brain about muscle length and changes in muscle length. They are one of the primary ways the nervous system knows where the body is in space, a sense known as proprioception.
Wrapped around each spindle are sensory nerve endings. When the spindle detects a change, those sensory neurons send information back into the nervous system, helping regulate the activity of the alpha motor neurons that control the muscle itself.
In other words, muscle length is not determined solely by commands coming down from the brain. It is also influenced by the ongoing conversation happening within the gamma loop itself.
When a movement occurs, alpha and gamma motor neurons fire together. This process is called alpha-gamma co-activation.
As the muscle contracts, the spindle contracts as well. This allows the spindle to continue monitoring changes in length throughout the movement.
Under normal circumstances, this system works beautifully.
A muscle changes contracts, the spindle detects the change, and the nervous system responds appropriately then the muscle returns to a neutral resting baseline.
The system continually updates itself based on incoming sensory information.
Problems arise when the gamma motor neurons become overactive creating an increased level of muscle tension or chronic contraction. The original cause can be repetitive movement, pain, injury, emotional stress, compensation, or a combination of factors.
Over time, the shortened muscle adapts to its new resting length. The muscle spindle also adapts and becomes increasingly sensitive to stretch.
At that point, even small amounts of lengthening can trigger the stretch reflex, causing the muscle to contract again.
The result is a self-perpetuating cycle: the muscle is shortened, the spindle becomes hypersensitive to stretch, stretch triggers contraction, the muscle shortens again, and the cycle repeats.
Eventually, the nervous system begins accepting this shortened state as normal. The brain's internal map of the body adapts to the new resting length, making the pattern increasingly automatic and unconscious.
What may have begun as a useful protective response gradually becomes the horse's normal.
This is how chronic tension can become habituated.
It's also important to understand that muscles don't exist in isolation.
Pain, immobility, and visceral discomfort can influence this system.
Sensory information from internal organs also feeds into this system. The nervous system does not completely separate information coming from muscles, joints, and organs. These signals converge and influence one another.
A horse experiencing gastric discomfort, for example, may develop protective muscular contractions around the affected region. Visceral sensory input can increase the activity of gamma motor neurons, which increases muscle spindle sensitivity and raises baseline muscle tone in surrounding tissues.
From the horse's perspective, this is a protective strategy. The nervous system is attempting to guard and stabilize an area it perceives as vulnerable.
The challenge is that these protective contractions can persist long after the original problem improves, gradually becoming part of the horse's habitual posture and movement patterns.
When we talk about restoring movement, we're talking about helping the nervous system update its map of the body.
** image from clinicalpub.com
