Muscle spindles are found in all skeletal muscles. They are more highly concentrated
in muscle utilizing fine delicate control and less so in the large antigravity support
muscles. The greatest percentage of spindles are located in the belly of the muscle.
Spindles contain two types of intrafusal fibers. Both types are multinucleated contractile
Nuclear bag fibers receive their name from the fact that their nuclei are clustered
together in a bag like enlargement near the center of the fiber. Nuclear chain fibers,
on the other hand, have no central enlargement, and their nuclei are spread out in
a chainlike fashion in the equatorial region of the fiber. Both types are able to
contract as contractile myofilaments are present in their striated peripheral portions.
Nuclear bag fibers typically have greater diameters and are longer than chain fibers.
A typical muscle spindle might contain up to eight chain and one or two bag fibers.
The shorter chain fibers are often attached to the bag fibers, which in turn attach
to the endomysium of the extrafusal muscle fibers. Extrafusal fibers are the large
contractile fibers of the muscle, while the intrafusal fibers are the nuclear bag
and chain fibers within the encapsulated muscle spindles.
Innervation of the Spindles
Before examining the role of the muscle spindle in regulating and responding to changes
in muscle tone. let's first begin by looking at its neural connections . Each nuclear
bag fiber has both motor and sensory innervation. One or two gamma motor neurons
form several distinct motor end plates, or plate endings, with the contractile portions
of the fiber. Firing of the gamma fibers contracts and shortens the bag fibers, a
feature which we will see is important in setting the sensitivity of the spindle.
Stretch of the nuclear bag fibers is detected by specialized stretch-sensitive endings
of both group Ia and group II nerve fibers. The Ia fibers form primary endings (annulospiral
endings) by wrapping around the central region of the bag fibers. Group II fibers
form secondary endings (flower-spray endings) over the striated portions of the bag
fibers. The nuclear chain fibers also have both motor and sensory innervation.Very
small gamma motor neurons form rather nondistinct trail endings on the contractile
portion of the chain fibers rather than the more distinct plate endings of bag fibers.
Group Ia and II nerve fibers also form primary and secondary endings with the chain
The Myotatic (Stretch) Reflex
When a muscle is stretched, the spindles in that muscle are also stretched. Stretch
of the nuclear bag and chain fibers in the spindles stimulates the primary and secondary
endings of the Ia and II afferent fibers, causing them to send impulses into the
cord. Many of these fibers (particularly the Ia fibers) synapse directly on alpha
motor neurons supplying the same muscle which was initially stretched. This causes
the muscle to contract and shorten, relieving the initial stretch. Such neurons are
called homonymous alpha motor neurons. This "stretch-resulting-in-relieved-stretch"
is known as the myotatic or stretch reflex. Once the muscle contracts and the stretch
is relieved, the firing rate of the spindle afferents returns to the resting level
Skeletal muscles are attached to the skeleton in order to bring about movements of
the body. It is usually necessary for muscles opposing a reflex movement (antagonists)
to relax while those producing the movement (agonists) contract. This reciprocal
action requires the incorporation of inhibitory interneurons in the spinal cord.
Branches (collaterals), typically from the Ia spindle afferents, synapse in the posterior
horn of the spinal cord gray matter. Here they stimulate inhibitory interneurons
which depress activity in the alpha motor neurons to those muscles antagonistic to
the desired movement. The patellar tendon or knee jerk reflex illustrates this point
When the tendon is tapped with a reflex hammer, the anterior thigh (quadriceps) muscles
and many of its muscle spindles are stretched. Accordingly, volleys of impulses are
sent into the spinal cord over the spindle afferents. Those fibers synapsing directly
on homonymous alpha motor neurons bring about contraction of the quadriceps, causing
the leg to kick in the classic response. Of course the posterior thigh muscles (hamstrings)
must relax in order to allow this to happen. This is accomplished by spindle afferent
stimulation of inhibitory interneurons (Renshaw cells). Once activated, they depress
firing in the alpha motor neurons to the antagonistic muscles. Renshaw cells release
the inhibitory neurotransmitter GABA at their synapses. Notice that the same spindle
afferents which increase the firing rate in the homonymous alpha motor neurons decrease
activity in the antagonistic motor neurons. The latter is accomplished through "feed-forward"
inhibition. Keep in mind that the spindle afferents are excitatory neurons releasing
ACh at their synapses. The desired inhibition of the antagonistic alpha motor neurons
is "fed forward" through the inhibitory interneuron, the Renshaw cell.
The Gamma Efferents and Spindle Sensitivity
Up to this point we have only been concerned with the action of the muscle spindle
afferents on alpha motor neurons. Now let's examine how the sensitivity of the spindles
can be adjusted to maintain a preset level of muscle tone. Recall that the spindle
afferents are stimulated whenever the intrafusal fibers are stretched taut. Now if
the intrafusal fibers are already partially contracted, only a slight amount of stretch
is needed to pull them taut, increasing the firing rate of the spindle afferents.
On the other hand, if the intrafusal fibers are relaxed and slack, a considerably
greater stretch of the muscle is needed in order to pull them taut and fire the spindle
afferents. In other words, the muscle spindle is more sensitive to stretch when its
intrafusal fibers are partially contracted then when they are not. The degree of
contraction of the intrafusal fibers and thus the sensitivity of the muscle spindle
is controlled by the activity of the gamma motor neurons. The greater the firing
rate of the gamma efferents, the greater the degree of intrafusal contraction, and
the greater the sensitivity of the spindle.
Spindle Maintenance of a Preset Muscle Tone
Recognize that when muscles isotonically contract they shorten. Similarly, relaxation
causes them to lengthen. Now let's assume that a given muscle is set to maintain
a certain degree of contraction or tone. If the muscle relaxed too much it would
lengthen and its spindles would stretch, initiating the stretch reflex. This would
cause the muscle to contract, thereby relieving the stretch brought on by the initial
relaxation. Similarly, if the muscle contracted too much, it would shorten and its
spindles would become increasingly slack. This would decrease the stimulation of
the spindle afferents, thereby decreasing the stimulation of the homonymous alpha
motor neurons and causing the muscle to partially relax. As a result of this "servomechanical"
nature of the muscle spindles, muscle tone remains very constant at any preset level.
Increases in tension are reflexly countered by relaxation, while decreases in tension
are countered by contraction.
It is important to recognize that tone is regulated by the stretch reflex and is
not a characteristic of the muscle itself. This can be demonstrated by the immediate
loss of muscle tone which occurs when the reflex arc is interrupted at any point.
For example, sectioning either the anterior or posterior roots of spinal nerves results
in the immediate loss of tone to all those muscles involved.
"Tuning" the Muscle Spindles
In order to remain sensitive to the slightest change in muscle tone it is important
that the spindles not be allowed to go completely slack. Under normal conditions
intrafusal spindle fibers are partially contracted. In this state, a slight relaxation
or stretch of the muscle will be detected by the spindles as will a slight contraction
or shortening. The firing rate of the spindle afferents will increase or decrease
accordingly, and the spindles are said to be "in tune" with the muscle.
One of the important roles of muscle spindles is to keep the brain and particularly
the cerebellum continually informed of even slight changes in muscle tone. This is
accomplished via collaterals from the spindle afferents which synapse on neurons
of the spinocerebellar tracts. The second-order neurons of these tracts conduct information
concerning the state of muscle tone and movement to this important coordinating center
of the brain . Now consider what would happen if the motor cortex of the brain directed
a particular muscle to maintain a higher level of contraction (tension). Without
a simultaneous contraction of the spindle intrafusal fibers in that muscle, the spindles
would go slack and the firing rate of the spindle afferents would drop off to zero,
producing a "silent period." Consequently, the spindles would no longer be able to
detect slight increases or decreases in muscle tone and they would be "out of tune"
with the muscle (. If, as neurophysiologists suspect, detecting slight changes in
muscle tone is an important feature of muscle spindles. these would no longer be
contributing, and the cerebellum would be out of touch with tension changes in the
muscle. Fortunately, activity in the gamma efferent nerve fibers prevent this from
happening by increasing the degree of intrafusal fiber contraction at approximately
the same time that the alpha motor neurons contract the extrafusal fibers. By this
"coactivation" of alpha and gamma motor neurons, spindles are kept "in tune" with
their muscles .
The role of the gamma efferents in adjusting the sensitivity of the muscle spindles
has already been discussed. The basal rate of firing of the gamma efferents and,
through them, the contractile state and sensitivity of the spindles are regulated
by the brain through pathways descending in the spinal cord. The principal route
is the medial reticulospinal tract. This tract, which originates in the reticular
formation of the brainstem, receives input from many areas of the brain, including
the cerebral and cerebellar cortexes.
Cerebellar "Awareness" of Muscle Tone
The cerebellum is an important center for the central coordination of muscle activity.
As such, it is necessary for the cerebellum to be continually informed of progressing
body movements and changes in muscle tone. As previously mentioned, this is accomplished
by collaterals from the spindle afferents which synapse in the nucleus dorsalis of
the spinal cord. Some of the second-order nerve fibers from this nucleus ascend the
cord in the posterior spinocerebellar tract (PSCT) to enter the cerebellum via the
interior cerebellar peduncle on the same (ipsilateral) side of the body as the entering
spindle afferents. They terminate in the cerebellar cortex of the vermis . Other
second-order nerve fibers from the nucleus dorsalis cross over to the opposite (contralateral)
side of the spinal cord and ascend to the brainstem in the anterior spinocerebellar
tract (ASCT), where they cross back to enter the cerebellum via the superior cerebellar
peduncle and terminate in the vermal cortex.
By "tapping off " the signals from the spindle afferents and conducting them cranially
over these pathways, the cerebellum is continually kept informed of the ever-changing
status of muscle tone. Electrophysiological studies indicate that group II fibers
appear to be concerned with relaying information concerning changes in muscle length,
while Ia fibers are concerned with changes both in length and contraction velocity.
It is important to recognize that the cerebellum functions as a coordinator examining
the performance of a muscle during a given movement and comparing it with the intended
movement directed by the cerebral cortex. If the intended performance and the actual
performance don't match up exactly, the cerebellum can take corrective action to
synchronize them through its own output to the motor system. Therefore it is important
for the cerebellum to continually receive input from the muscle spindles on the progression
of any given movement. Input from Golgi tendon organs and joint receptors is also
necessary for movement coordination.
THE GOLGI TENDON ORGAN
The tendons of skeletal muscle contain special receptors called Golgi tendon organs.
These receptors are sensitive to the changes in tension generated by muscles as they
contract. Little is known about their structure except that they are in intimate
contact with the peripheral endings of group Ib afferent fibers. It is through impulses
generated in these afferent fibers that changes in muscle tension detected by the
tendon organs are relayed to the spinal cord and brain. As muscles contract and tension
is applied to their tendons, the tendon organs are stimulated, which in turn propagate
impulses over group Ib fibers into the cord, where they take several divergent routes
Function of the Golgi Tendon Organ
The sensitivity of the tendon organs is considerably less than that of the muscle
spindles. As little as 1 or 2 g of tension is sufficient to increase the firing rate
of the spindle afferents. On the other hand, the group Ib afferent fibers from the
tendon organs don't register impulse conduction until the tension reaches as high
as 100 g. When tension in the tendons begins to exceed this level, the tendon organs
become sufficiently stimulated to produce impulse firing in the group Ib fibers.
Like the spindle afferents, the group Ib fibers send collaterals into the nucleus
dorsalis of lamina VII of the spinal cord gray matter. Subsequently, both ASCT and
PSCT second-order neurons conduct information from the tendon organs to the cerebellum.
If the tension developed in a strongly contracting muscle becomes excessive, it is
not inconceivable that the tendon could pull free from the bone, certainly an undesirable
situation. However, before this can happen the tendon organs become sufficiently
stimulated to send large volleys of impulses into the cord to directly stimulate
the alpha motor neurons to antagonistic muscles and inhibitory interneurons to homonymous
alpha motor neurons. The resulting feed-forward inhibition to the strongly contracting
muscle causes it to suddenly relax, relieving the strain on the tendon and preventing
possible damage. This sudden relaxation of a muscle in the face of dangerously high
tension is called the lengthening reaction or the "clasp-knife" reflex because of
its similarity to the way a pocketknife suddenly snaps closed when the blade is moved
to a certain critical position.
It was originally thought that little if any information from the tendon organs or
the muscle spindles reached the conscious level in humans. The vast majority of the
signals from these receptors which ascend the cord were thought to be directed exclusively
to the cerebellum for subconscious evaluation. However, recent evidence now indicates
that input from muscle spindles, tendon organs, and joint receptors is also relayed
to the cerebral cortex and is probably responsible for the conscious sensation associated
with the position and movement of limbs.