Muscle Spindle Sensory Receptors

Spindle-Runway Model(For an in-depth study of the muscle spindle, click here, or go to the ARTICLES section on this website)

Muscle spindles are small nerve bodies
 scattered throughout skeletal muscle; one of the jobs these sensory receptors perform is to detect changes in the length of their muscle, and then convey that length information to the central nervous system via sensory neurons; from this information the brain determines the position of body parts. Spindles also regulate muscle contraction via the stretch reflex, and most importantly, they, together with the cerebellum, monitor and maintain the system of muscle tone.

LOCATIONMuscle spindles are  attached to the outside of the perimysial wall surrounding fascicles (bundles of muscle fibers); so they are actually in the corridors between fascicles, along with lymph ducts, nerves, capillaries, and other structures. The spindle, thus buried in the fascia surrounding bundles of muscle cells, is able to take an average of the activity of all the cells within, and thereby monitor the contraction of the fascicle. In sampling a whole group of muscle fibers through the perimysial wall, we could say it operates on hearsay, without a direct witness. Then the spindle reports the activity of these muscle fibers to either the cerebellum or spinal cord. Spindles are not placed inside the fascicles, attached to the fascia of individual fibers, because there’s no room for them in there ~ too much goes on inside the fascicle; nevertheless, the spindles do manage to take accurate information from the perimysial wall. The common (although little-known & widely-overlooked) malfunction that takes place in this system occurs not in the information the spindles gather from their fascia, but in relaying that information to the brain.

STRUCTUREMuscle spindles are fusiform-shaped organs, encased in a connective tissue capsule in parallel with a fascicular group of muscles.  Each muscle spindle cell is built around 6 to 14 small intrafusal muscle fibers – actual muscle cells within these nerve bodies. Like extrafusal muscle fibers (the working forest of regular skeletal muscle), intrafusal muscles are encased in endomysium, and capillaries run between them. There are three kinds of intrafusal fibers: bag 1 (dynamic), bag 2 (static), and chain fibers. Only the ends of the intrafusal fibers contract, as the central portions contain no actin or myosin filaments, only nuclei, so the central fibers function as sensory receptors, not contractile units. Little gamma efferent (motor) nerves also feed into the spindle, attaching to both ends of the intrafusal spindle fibers. The alpha motor nerve branch to the extrafusal fibers gives off gamma 1 efferent nerves to the dynamic bag 1 fibers, whereas gamma 2b to the static bag fibers and gamma 2c to the chain fibers come directly from the cerebellum. They all excite mechanical contraction of these intrafusal fibers, just as the alpha motor nerves excite contraction to the extrafusal fibers.

 There are also gamma afferent (sensory) nerves leaving the spindle, traveling to either the cerebellum or to the spinal cord through the posterior nerve route.  Each of these gamma afferents begins with a nerve end organ attached to its intrafusal muscle fiber within the spindle. These end organs are called flower spray and annulospiral end organs.

 And lastly, the muscle spindle cell has one lymph duct attached to it, designed to draw fluid, i.e. lactic acid, out of the spindle.  Skeletal muscles become inundated with lactic acid after strenuous use, and if this metabolic toxin reaches a concentration level in the muscle, it will spill over and become trapped in these spindle organs, where it does great damage.

Although conventional thinking is that once a muscle stops contracting, blood flow resumes and eventually flushes the lactic acid out of the tissues, the reality is that not all of the lactic acid gets flushed out. Some acid remains trapped in the spindle cells lying out between the fascicles, primarily  because of the action of the solitary lymph duct on each spindle.  Lymph ducts utilize skeletal muscles to pump lymph fluid back towards the heart; once the level of lactic acid has receded in the surrounding muscle tissue, the ducts then act to help flush the spindle and dilute the acid. Nevertheless, in people with hypertonic muscle, during intense activity, the levels of acid can accumulate to the point that muscle contraction becomes set in ~ almost locked ~ for a period of time. This contraction both continues to produce lactic acid and to block circulation in the veins; the veins are susceptible because they have such thin walls and so little blood pressure. The lymph duct is actually trying to remove the lactic acid, but when the surrounding muscle is so full of it, the duct ends up drawing it into the spindle. You wouldn’t, for example, want to have the air circulator intake on your car next to the exhaust ~ but that’s sort of how we’re designed! And, once the acid has been drawn into the spindle, the spindle membrane traps it, acting as an accumulating repository for lactic acid.

 NeuroSoma® practitioners know we are releasing trapped lactic acid because that release is accompanied by tiny bursts of extreme heat (people think a nerve has been fired, or a lit match held to the skin), or by a histamine reaction of redness, whealing, or welts on the skin. The amount of skin irritation depends on 2 factors: 1] the level of sickness in the subject’s suboccipital muscles (very spastic suboccipitals produce large amounts of histamine), and 2], how far away from the surface ~ how deep into the belly ~ the release is occurring. The deeper it is, the less likely we are to see the reaction at the surface; so only in the early stages of treatment will we see a very rapid or large surface response. Then as more layers of muscle are softened, we see less and less reaction. It’s still happening, but it’s not reaching the surface.

FUNCTIONMuscle spindles act as both mechanical & electrical synapses.  They receive electrical, efferent signals through the gamma efferents, which they convert into mechanical muscle contraction.  Then the annulospiral nerve end organs take that mechanical reaction (i.e. contraction) of the spindle, summate it with the action of the main muscle fibers, and convert it all back into an electrical signal that is fed out through the gamma afferents to the spinal cord; that signal then reflexly fires the alpha motor nerves back into the working forest of main muscle fibers within the fascicle, causing them to contract; this is a reflex arc.  The flower spray nerve end organs also summate mechanical contraction, but they feed through their gamma afferents directly back to the cerebellum with a report on the amount of contraction taking place in their fascicle. Synapses ordinarily only feed forward from the CNS, and only allow electrical information to be summed. However the spindle synapse summates both mechanical and electrical signals.

The spindle cell has 4 assignments, which are 1) control and maintain muscle tone; 2) activate the dynamic stretch reflex mechanism; 3) maintain muscle contraction against the constant force of gravity (the static stretch reflex mechanism); and 4) control fine motor movements. ‘Static’ means in equilibrium, steady, balanced, constant. ‘Dynamic’ and ‘myotatic’ mean the same thing – constantly changing.

 NeuroSoma® focuses predominantly on 2 of these spindle functions ~ correcting muscle tone and avoiding activation of the stretch reflex. The stretch reflex mechanism is well known and is acknowledged to be of great significance; even so, it is widely ignored by bodyworkers. This mechanism instantly contracts skeletal muscle tissue as a guarding action when the muscle is stretched past a certain set limit, whether that stretch be active or passive. In other words, the stretch reflex activates when one overstretches one’s own muscles, or when another overpresses into one’s muscle fibers, regardless of how slow the approach. The NeuroSoma® myotherapist therefore applies a limited pressure not deep enough to activate the stretch-reflex mechanism, and so avoids contraction and irritation of the muscle.

NeuroSoma’s primary consideration is maintenance of muscle tone by the spindle. Although this is an essential, key spindle function, it is nevertheless not widely understood. It is only rarely that reference connecting the spindle cell to the maintenance of muscle tone can be found, and usually physiologists who are scientists rather than doctors make the connection. In order to understand muscle malfunction, one must first understand muscle function.  When the function/physiology of muscle tonus is comprehended in its entirety, the malfunction and correction of the system of muscle tone becomes clear. NeuroSoma® is the product of this knowledge, and although hundreds of therapies promise to “soften” muscle, this bodywork modality actually does turn hard, hypertonic muscle back into healthy soft tissue by utilizing cutting edge science.