Everyone learned about muscle spindles in their basic training, but they are such a fundamentally important part of the neurological system that reviewing them is worthwhile.

During muscle contraction, the central nervous system (CNS) sends signals to make muscles contract. It is the muscle spindles that convey more specificity to the CNS, essentially providing more proprioceptive sensory information about muscle length, contraction rate, and effort, as well as they body’s position in space. The spindles are made up of several types of fibers and have more complexity than one might think.

One of the more complex elements of the muscle spindle’s function is the gamma efferent system, which involves the gamma motor neurons. We focus here on the gamma system, but know that the neurological system involved in muscle contraction is quite complex. Let’s first take a brief look at the anatomy of the muscle spindle.

Anatomy of the Spindle

Muscle spindles are embedded within a muscle’s main belly and have two primary functions. First, they relay proprioceptive information to the central nervous system about changes in muscle length (how much a muscle elongates), called the tonic response. Second, they convey information about the rate of change in length (how fast a muscle elongates), referred to as the phasic response.

Inside the muscle are three types of fibers involved in muscle contraction: alpha, beta, and gamma motor neurons. The gamma motor neurons innervate specialized contractile fibers called intrafusal fibers, of which there are two main types — the nuclear chain fiber and the nuclear bag fiber. The nuclear bag fibers are divided into two types — type 1 or dynamic nuclear bag and type 2 or static nuclear bag.

These fibers get their name from the arrangement of their nuclei which are gathered in a large bundle near the center of the muscle belly (thus the name bag). The nuclear chain fibers have their nuclei spread out in a longer string, like a chain. They all have a central region that does not contract and fiber ends that do.

The type 1 or dynamic nuclear bag fibers are most responsive to the rate of change in length of the muscle. The type 2 or static nuclear bag fibers are partially responsive to the rate of change, but also to the amount of change in length. The nuclear chain fibers are exclusively responsive to the amount of change in length.¹

Therefore, you can say that the phasic response of the muscle spindle is generated by the dynamic nuclear bag fiber with some help from the static nuclear bag fiber. The tonic response is generated by the nuclear chain fiber with some help from the static nuclear bag fiber.

There are two types of afferent (sensory) nerve fibers that supply the intrafusal fibers of the muscle spindle. The annulospiral endings convey information on the phasic response (rate of length change). The flower spray endings convey information on the tonic response (amount of length change). Now let’s look at the gamma system.

The Gamma Efferent System

One of the most unique factors of the muscle spindle is that they are a sensory receptor, but they also receive efferent (motor) input from the central nervous system. Thus they are not only responsive to pure mechanical changes in the muscle, but their sensitivity can be modified by the central nervous system. This is done by the gamma efferent system.

A specialized motor fiber goes to the muscle spindle called a gamma fiber, represented by the Greek letter gamma (insert symbol for Greek letter gamma). The gamma efferent fibers are connected to the ends of the nuclear bag and nuclear chain fibers. As stated earlier, the ends of these fibers can contract, but the central portions do not.

Therefore, when a stimulus is sent along the gamma efferent fibers it will cause the ends of the nuclear bag and nuclear chain fibers to contract. When the ends contract the central portion that doesn’t contract will be stretched.

The afferent (sensory) fibers of the intrafusal fiber are located in this non-contractile central portion and when stretched, they will increase their rate of firing increasing the contraction stimulus sent to the main muscle. An example of how this works is the stretch reflex.

The Stretch Reflex

If there is a very rapid stretch of the muscle, the central portion of the intrafusal fibers will be mechanically stretched. This will increase the rate of firing of the sensory cells located in the central portion of the intrafusal fiber. As a result of this very strong stimulus, the central nervous system will initiate a muscle contraction to offset the perceived danger of this rapid stretch. This process is known as the stretch reflex.

The other way that muscle spindles increase their firing rate and increase muscle tone is if there is an increase in activity in the gamma efferent fibers. When the gamma efferents fire they contract the ends of the intrafusal fibers, stretching the central portion and increasing the rate of firing of the intrafusal sensory fibers, heightening the muscle tonus.

Since an increase in the firing of the gamma fibers can lead to an increase in muscle tone, an overactive gamma fiber can create excess muscle tension. Excess gamma activity can come from elevated nervous system activity like stress, or from myofascial trigger points, muscle hypertonicity, injury, pain, nutritional imbalance, or a variety of other factors.

The increase in gamma activity does not need to be very large in order to have significant effects on the muscle’s overall function. Increased levels of gamma efferent activity are a significant cause of muscular dysfunction in the body. We often hear about how increased tension or stress in our lives creates tight and dysfunctional muscle structures. Excess activity in the gamma system is a big part of this tension-creating process.

Understanding the gamma efferent system will help you choose better treatment methods for certain pain problems. There are a number of treatment techniques such as muscle energy technique, trigger point therapy, muscle stripping or other facilitated stretching methods that are helpful to reduce excess gamma efferent activity.

Reference

1. Leonard C. Neuroscience of Human Movement. St. Louis: Mosby; 1998.